Connecting the Tecsun PL-360 to a large air-core loop


dhsatyadhana <satya@...>
 

Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has produced, I decided to see what the performance of the PL-360 would be using a large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19 turns of wire, spaced over about a 10-inch coil depth. It measures out to just over 350 uH, matching the inductance of Gary's ferrite rig. I simply connected both of the coils leads to a 1/8" plug, and connected it to the jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The 7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in at 56 db signal and 24 db SNR, and it really sounded like a huge boost as well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the air-core started having problems, apparently with overloading, as I got into the MW band. Tuning up and down the dial, I often had the dreaded "50-00" reading on the PL-360, which I interpret to mean that it has so much signal that it is desensitized/swamped. DX targets could often be heard, but at much reduced volume. The ferrite, on the other hand, was still behaving well and receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop, even on fairly vacant channels, except again at the far end of the band, away from my various strong locals. It wasn't quite as bad as when my locals were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton e100 - it was simply too much, and near locals the desensitizing was significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a broadband source, at least in an urban area. I know that I cannot plug and untuned active whip or a Super Loop/Wellbrook FLG-100 combination in, either - simply too much input from strong locals. Obviously, based on Guy Atkins' 18" ferrite experiment, at least that amount of signal is acceptable - maybe there is something about ferrites (reduced stray capacitance, etc.) that also is a factor? At any rate, those wanting to plug an air-core loop straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


pianoplayer88key
 

--- In ultralightdx@..., "dhsatyadhana" <satya@...> wrote:

Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has produced, I decided to see what the performance of the PL-360 would be using a large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19 turns of wire, spaced over about a 10-inch coil depth. It measures out to just over 350 uH, matching the inductance of Gary's ferrite rig. I simply connected both of the coils leads to a 1/8" plug, and connected it to the jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The 7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in at 56 db signal and 24 db SNR, and it really sounded like a huge boost as well! I felt my pulse quicken just a bit...
Now that sounds like a sizeable boost. :) I wonder how a 9-foot loop would do... for example if I have a station that's completely unreadable (15dBu, 0dB S/N, 1kHz BW) with the stock stick on my PL-380... I wonder if a sizeable loop might be capable of bringing it in at 63dBu, 25dB S/N in 6kHz mode?


Alas, being in an urban area near several powerful broadcasters, the air-core started having problems, apparently with overloading, as I got into the MW band. Tuning up and down the dial, I often had the dreaded "50-00" reading on the PL-360, which I interpret to mean that it has so much signal that it is desensitized/swamped. DX targets could often be heard, but at much reduced volume. The ferrite, on the other hand, was still behaving well and receiving stations as it should.
Oh, ****, yes, that 50/00 reading. :( I don't quite get that barefoot here at my house, but the only spots on the dial I get 15/00 when I'm not in a car are at 520 and 1680, give or take a channel or two. (I have a locals on 540 and 1700 that are about 48-51/25 or so on the stock stick on my 380) Otherwise, most of the band is about 30/00, with the 1100s being around 36/00 to 45/00 in the blank spots. There's a 10kW station on 1130 about 6-7 miles from me, and a 50kW on 1170 about 9 miles away. At night 1170 powers down to 2.9kW, but 760 (about 7 miles away) powers up from 5kW to 50kW, and reads about 39/00 on 747 and 774, stock. At my grandparents' house, it's even worse. They live about 1/3 mile from a 23kW on 1300 and a 50kW on 1430, and it pretty much reads 50/00 across at least half the band. I was there a couple months or so ago, and too my 380 to somewhere within a few hundred yards or so of their transmitter site, and It was reading 50/00 from around 960 all the way up to 1710 or so, and that's without any type of external antenna. (It was 49/00 from 690 to 950.) I really wish the 380 was better at dealing with that, too, as there's a station on 1290 in San Bernardino that I'd like to be able to listen to when I'm at their house, and according to Radio-Locator, I'm just within the fringe contour of that station.
Also, I've discovered that putting a radio up next to a power pole (not just any - it needs to be one with a ground wire running down the pole) will give a sizeable boost, as it seems to couple the overhead powerlines to the radio as if it was an antenna. In fact... it gives SO much of a boost, that... well, like I said, I live 9 miles away from a 50kW daytime blowtorch on 1170. (Their TX site wasn't always there, though - until a few years ago it was 6 miles away, and I bet if I'd had my 380 then I would have had some 50/00 readings across a portion of the band from that monster signal that was pumping about 300kW or so toward me.) Well, I took my SAT, tuned it to 1170, and put it and my radio up to the power pole.... and the SECOND HARMONIC (2340) was reading 63dBu, and the 3rd (3510) was about 41dBu! Also, it was 50dBu across pretty much the entire AM band, and 45dBu at 153kHz LW, and probably 50dBu quite a ways up into the shortwave spectrum - up to around 3 MHz if I remember correctly. As for what it sounded like... well... check out the link below for some audio files.
http://cid-6bdd1917662288cb.skydrive.live.com/browse.aspx/AM%20radio%20files/1170%20KCBQ%20-%202010-04-16
There's a bandscan from 1170 to 1710, one from 1170 down to 520 (and also similar "bandscans" on my Panasonic RQ-SW20 which is about 15dB less sensitive than the PL-380 and about as selective as the Sony SRF-M37). Also there's demos of a few of 1170's harmonics, and an audio file of 1170 on the PL-380 during which I move the radio from a couple feet away to right up next to the antenna and power pole. http://cid-6bdd1917662288cb.skydrive.live.com/self.aspx/AM%20radio%20files/1170%20KCBQ%20-%202010-04-16/KCBQ%201170%20-%20PL-380%20-%20overload%20demo%20-%202nd%20half%20SAT%20@%20PowerPole.mp3 You should actually be able to hear the distorted audio ON the assigned frequency. No, it wasn't tuned off channel, it was tuned RIGHT ON 1170 where the station broadcasts!! If the dBu reading wasn't capped at 63dB... I wonder what it would read on 1170 under those conditions?
If I remember correctly, driving right by the transmitter site a couple months or so ago (and also when I went close to 1300/1430 when visiting my grandparents last month), I didn't even have anywhere CLOSE to that strong of a signal.
I really wish the PL-380 did better at rejecting overload, too, as there are probably some stations that I need that much gain just to even be able to hear a slight hint of a trace. For example, KSL 1160 comes to mind, but even if the PL-380 did really well at not being overloaded by 1170, there is a 1160 in San Quintin, Baja Cal Norte, which, although inaudible barefoot, can be heard with the SAT here in the daytime, under the right conditions.


At night, everything was still at or above 50 SNR with the air-core loop, even on fairly vacant channels, except again at the far end of the band, away from my various strong locals. It wasn't quite as bad as when my locals were on day patterns, but still quite swamped.
Just curious... how close are you to your locals, and what power do they transmit? Also what are your typical barefoot readings, including a few between channels? Some of mine in the daytime are...
600 KOGO - 63,25 - 5kW DA 8 mi W, IBOC carriers about 42,00 or so.
690 XEWW - 63,25 - 77.5kW DA 32mi SSW
760 KFMB - 63,25 - 5kW ND 7 mi NNW (50kW DA night)
910 KECR - 61-62,25 - 5kW DA 9 mi NNE
1130 KSDO - 63,25 - 10kW DA 6 mi N
1170 KCBQ - 63,25 - 50kW DA 9 mi NNE (diplexes with 910)
1360 KLSD - 63,25 - 5kW ND 8 mi W.
As I said above, across most of the band the readings are 30,00, with them approaching 39,00 to 45,00 in the 1100s, and dipping to 15,00 to 20,00 at the extreme edges.

Now.. I can understand it overloading / desensitizing like that when I'm using a large antenna right next to their tower... but NINE BLEEPING MILES AWAY? That blows my mind.
I wonder how well Scott's doing at the drawing board with a chip that will pull in a 1µV/m signal a few kHz away from a 50V/m signal (or whatever would be the difference between a carrier that's barely detectable with a longwire beverage antenna vs. bordering on being in the proximity of a dangerously high RF field)? ;)


I had similar results when I jacked in a large air-core loop to the Eton e100 - it was simply too much, and near locals the desensitizing was significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a broadband source, at least in an urban area. I know that I cannot plug and untuned active whip or a Super Loop/Wellbrook FLG-100 combination in, either - simply too much input from strong locals. Obviously, based on Guy Atkins' 18" ferrite experiment, at least that amount of signal is acceptable - maybe there is something about ferrites (reduced stray capacitance, etc.) that also is a factor? At any rate, those wanting to plug an air-core loop straight into a PL-360 had better live outside an urban area!
So what would be recommended for those living in an urban area? I'm near San Diego, but visit L.A. fairly regularly. There are some stations I would like to be able to hear that are 10kHz away from nearby 50kW blowtorches, or 20kHz away from some IBOC stations, that even if there was nothing on the air, would be completely unreadable even WITH a select-a-tenna.



Kevin S
Bainbridge Island, WA
Also... do any southern CA stations make it up there at night, and if so how strong are they, typically? I think some likely candidates, either due to their power or directional pattern, would be 640 KFI, 760 KFMB, 1070 KNX and 1090 XEPRS, and maybe 1700 XEPE. As for the WA stations I get here... the only one I can get somewhat regularly is 1000 KOMO, and it's usually mixing with 250w KCEO 38 mi NNW of me. I don't get 690, 950 or 1130, as I have locals on those frequencies, and 710 is dominated by KSPN, which is 10kW nighttime directed toward me.


Gary DeBock
 

Hi Kevin (and Stephen),
 
Thanks for sharing the results of your PL-360 air core loop experimentation, and thanks again for your positive comments on the 7.5" plug-in loopstick.
 
Regarding the overloading issue with air core loops directly connected to the PL-360 (or other Ultralights), my own experience has been very similar to yours. Inductive coupling to the large PVC loops had always provided satisfactory results with the new DSP radios, and avoided any RF overloads from local pests. As you have also documented in your excellent tuned passive loop articles, the extra tuned circuit in the Crate Loop helps for filter out RF overloads from local pests, and provides additional selectivity to enhance DX reception. With the Crate Loop or PVC Loop tuning capacitor optimized on a DX station's frequency, the splatter and spurious signals from local pest stations is significantly reduced. From actual DXing experience I'm sure that we would both agree that this extra tuned circuit is not a hindrance-- it's actually a great advantage.
 
Probably the easiest way to understand the performance challenges in creating Ultralight radio antennas is to remember that all designs have a tradeoff in portability, gain, and size. The very best designs have good performance in all three aspects, although this a subjective judgement. The 7.5" plug-in loopstick was designed to offer good portability and gain with reasonable size (although some would dispute that last point :-)  Your Crate Loop might provide more gain with slightly less portability, and a 9' PVC loop would provide massive gain with almost no portability. Each DXer needs to decide what the objectives are, and one design certainly won't work for everybody.
 
73, Gary DeBock (in Puyallup, WA)   
 
 
 
 
 
  
 
In a message dated 5/12/2010 10:06:21 P.M. Pacific Daylight Time, satya@... writes:

 

Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has produced, I decided to see what the performance of the PL-360 would be using a large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19 turns of wire, spaced over about a 10-inch coil depth. It measures out to just over 350 uH, matching the inductance of Gary's ferrite rig. I simply connected both of the coils leads to a 1/8" plug, and connected it to the jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The 7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in at 56 db signal and 24 db SNR, and it really sounded like a huge boost as well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the air-core started having problems, apparently with overloading, as I got into the MW band. Tuning up and down the dial, I often had the dreaded "50-00" reading on the PL-360, which I interpret to mean that it has so much signal that it is desensitized/swamped. DX targets could often be heard, but at much reduced volume. The ferrite, on the other hand, was still behaving well and receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop, even on fairly vacant channels, except again at the far end of the band, away from my various strong locals. It wasn't quite as bad as when my locals were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton e100 - it was simply too much, and near locals the desensitizing was significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a broadband source, at least in an urban area. I know that I cannot plug and untuned active whip or a Super Loop/Wellbrook FLG-100 combination in, either - simply too much input from strong locals. Obviously, based on Guy Atkins' 18" ferrite experiment, at least that amount of signal is acceptable - maybe there is something about ferrites (reduced stray capacitance, etc.) that also is a factor? At any rate, those wanting to plug an air-core loop straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


Rik
 

I tried my 2 foot air core loop again tonight, and the white noise blocking my reception to the west is gone at the moment. I found a good signal on AM 620, and it turned out to be WVMT from Burlington VT. That is a 5kW I don't recall hearing here before, but I get it when I am in Northern VT. So a 2 foot loop may not be too large for the PL-360. I do have to shut off my CRT computer monitors, and I don't have any local stations to worry about. I tried AM 650 WSM, but just got noise on that frequency. Also a pretty steady signal on AM 540, but it's all in Spanish, so I wouldn't be able to ID it easily. - FARMERIK

--- In ultralightdx@..., D1028Gary@... wrote:

Hi Kevin (and Stephen),

Thanks for sharing the results of your PL-360 air core loop
experimentation, and thanks again for your positive comments on the 7.5" plug-in
loopstick.

Regarding the overloading issue with air core loops directly connected to
the PL-360 (or other Ultralights), my own experience has been very similar
to yours. Inductive coupling to the large PVC loops had always provided
satisfactory results with the new DSP radios, and avoided any RF overloads from
local pests. As you have also documented in your excellent tuned passive
loop articles, the extra tuned circuit in the Crate Loop helps for filter
out RF overloads from local pests, and provides additional selectivity to
enhance DX reception. With the Crate Loop or PVC Loop tuning capacitor
optimized on a DX station's frequency, the splatter and spurious signals from
local pest stations is significantly reduced. From actual DXing experience I'm
sure that we would both agree that this extra tuned circuit is not a
hindrance-- it's actually a great advantage.

Probably the easiest way to understand the performance challenges in
creating Ultralight radio antennas is to remember that all designs have a
tradeoff in portability, gain, and size. The very best designs have good
performance in all three aspects, although this a subjective judgement. The 7.5"
plug-in loopstick was designed to offer good portability and gain with
reasonable size (although some would dispute that last point :-) Your Crate Loop
might provide more gain with slightly less portability, and a 9' PVC loop
would provide massive gain with almost no portability. Each DXer needs to
decide what the objectives are, and one design certainly won't work for
everybody.

73, Gary DeBock (in Puyallup, WA)








In a message dated 5/12/2010 10:06:21 P.M. Pacific Daylight Time,
satya@... writes:




Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has
produced, I decided to see what the performance of the PL-360 would be using a
large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19
turns of wire, spaced over about a 10-inch coil depth. It measures out to
just over 350 uH, matching the inductance of Gary's ferrite rig. I simply
connected both of the coils leads to a 1/8" plug, and connected it to the
jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The
7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in
at 56 db signal and 24 db SNR, and it really sounded like a huge boost as
well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the
air-core started having problems, apparently with overloading, as I got into the
MW band. Tuning up and down the dial, I often had the dreaded "50-00"
reading on the PL-360, which I interpret to mean that it has so much signal
that it is desensitized/swamped. DX targets could often be heard, but at much
reduced volume. The ferrite, on the other hand, was still behaving well and
receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop,
even on fairly vacant channels, except again at the far end of the band,
away from my various strong locals. It wasn't quite as bad as when my locals
were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton
e100 - it was simply too much, and near locals the desensitizing was
significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a
broadband source, at least in an urban area. I know that I cannot plug and
untuned active whip or a Super Loop/Wellbrook FLG-100 combination in,
either - simply too much input from strong locals. Obviously, based on Guy
Atkins' 18" ferrite experiment, at least that amount of signal is acceptable -
maybe there is something about ferrites (reduced stray capacitance, etc.)
that also is a factor? At any rate, those wanting to plug an air-core loop
straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


Rik
 

I guess it is a northern New England night, just logged AM 870 Auburn, ME playing Scott Shannon's oldies show. Mentioned sister station on 1470 on air. That is a new log too. 1kW at night. PL-360 on 2 foot AC loop. - FARMERIK [in CT]

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:


I tried my 2 foot air core loop again tonight, and the white noise blocking my reception to the west is gone at the moment. I found a good signal on AM 620, and it turned out to be WVMT from Burlington VT. That is a 5kW I don't recall hearing here before, but I get it when I am in Northern VT. So a 2 foot loop may not be too large for the PL-360. I do have to shut off my CRT computer monitors, and I don't have any local stations to worry about. I tried AM 650 WSM, but just got noise on that frequency. Also a pretty steady signal on AM 540, but it's all in Spanish, so I wouldn't be able to ID it easily. - FARMERIK


--- In ultralightdx@..., D1028Gary@ wrote:

Hi Kevin (and Stephen),

Thanks for sharing the results of your PL-360 air core loop
experimentation, and thanks again for your positive comments on the 7.5" plug-in
loopstick.

Regarding the overloading issue with air core loops directly connected to
the PL-360 (or other Ultralights), my own experience has been very similar
to yours. Inductive coupling to the large PVC loops had always provided
satisfactory results with the new DSP radios, and avoided any RF overloads from
local pests. As you have also documented in your excellent tuned passive
loop articles, the extra tuned circuit in the Crate Loop helps for filter
out RF overloads from local pests, and provides additional selectivity to
enhance DX reception. With the Crate Loop or PVC Loop tuning capacitor
optimized on a DX station's frequency, the splatter and spurious signals from
local pest stations is significantly reduced. From actual DXing experience I'm
sure that we would both agree that this extra tuned circuit is not a
hindrance-- it's actually a great advantage.

Probably the easiest way to understand the performance challenges in
creating Ultralight radio antennas is to remember that all designs have a
tradeoff in portability, gain, and size. The very best designs have good
performance in all three aspects, although this a subjective judgement. The 7.5"
plug-in loopstick was designed to offer good portability and gain with
reasonable size (although some would dispute that last point :-) Your Crate Loop
might provide more gain with slightly less portability, and a 9' PVC loop
would provide massive gain with almost no portability. Each DXer needs to
decide what the objectives are, and one design certainly won't work for
everybody.

73, Gary DeBock (in Puyallup, WA)








In a message dated 5/12/2010 10:06:21 P.M. Pacific Daylight Time,
satya@ writes:




Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has
produced, I decided to see what the performance of the PL-360 would be using a
large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19
turns of wire, spaced over about a 10-inch coil depth. It measures out to
just over 350 uH, matching the inductance of Gary's ferrite rig. I simply
connected both of the coils leads to a 1/8" plug, and connected it to the
jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The
7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in
at 56 db signal and 24 db SNR, and it really sounded like a huge boost as
well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the
air-core started having problems, apparently with overloading, as I got into the
MW band. Tuning up and down the dial, I often had the dreaded "50-00"
reading on the PL-360, which I interpret to mean that it has so much signal
that it is desensitized/swamped. DX targets could often be heard, but at much
reduced volume. The ferrite, on the other hand, was still behaving well and
receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop,
even on fairly vacant channels, except again at the far end of the band,
away from my various strong locals. It wasn't quite as bad as when my locals
were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton
e100 - it was simply too much, and near locals the desensitizing was
significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a
broadband source, at least in an urban area. I know that I cannot plug and
untuned active whip or a Super Loop/Wellbrook FLG-100 combination in,
either - simply too much input from strong locals. Obviously, based on Guy
Atkins' 18" ferrite experiment, at least that amount of signal is acceptable -
maybe there is something about ferrites (reduced stray capacitance, etc.)
that also is a factor? At any rate, those wanting to plug an air-core loop
straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


Rik
 

Here are a few more stations new to my log. Nothing really distant.

920 WHJJ Providence, RI. 5kW Don't know why I haven't heard this before.

1160 WVNJ Business Talk Radio, Oakland, NJ. They use IRN news, usually on Christian stations, and run an overnight program from GCN Radio network.[SW] 20kW days, 2.5 kW at night.

1300 WGDJ, Rensselaer, NY. Jim Bohannon show, 5kW day/night.

I'd say the 2 foot loop is doing pretty well on the PL-360. The On these weak stations, the dBu is ranging from 50 to 80, and the dB is usually staying in the single digits. - FARMERIK

PS- don't email me at home, it isn't working.

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:

I guess it is a northern New England night, just logged AM 870 Auburn, ME playing Scott Shannon's oldies show. Mentioned sister station on 1470 on air. That is a new log too. 1kW at night. PL-360 on 2 foot AC loop. - FARMERIK [in CT]

--- In ultralightdx@..., "farmerik" <farmerik@> wrote:


I tried my 2 foot air core loop again tonight, and the white noise blocking my reception to the west is gone at the moment. I found a good signal on AM 620, and it turned out to be WVMT from Burlington VT. That is a 5kW I don't recall hearing here before, but I get it when I am in Northern VT. So a 2 foot loop may not be too large for the PL-360. I do have to shut off my CRT computer monitors, and I don't have any local stations to worry about. I tried AM 650 WSM, but just got noise on that frequency. Also a pretty steady signal on AM 540, but it's all in Spanish, so I wouldn't be able to ID it easily. - FARMERIK


--- In ultralightdx@..., D1028Gary@ wrote:

Hi Kevin (and Stephen),

Thanks for sharing the results of your PL-360 air core loop
experimentation, and thanks again for your positive comments on the 7.5" plug-in
loopstick.

Regarding the overloading issue with air core loops directly connected to
the PL-360 (or other Ultralights), my own experience has been very similar
to yours. Inductive coupling to the large PVC loops had always provided
satisfactory results with the new DSP radios, and avoided any RF overloads from
local pests. As you have also documented in your excellent tuned passive
loop articles, the extra tuned circuit in the Crate Loop helps for filter
out RF overloads from local pests, and provides additional selectivity to
enhance DX reception. With the Crate Loop or PVC Loop tuning capacitor
optimized on a DX station's frequency, the splatter and spurious signals from
local pest stations is significantly reduced. From actual DXing experience I'm
sure that we would both agree that this extra tuned circuit is not a
hindrance-- it's actually a great advantage.

Probably the easiest way to understand the performance challenges in
creating Ultralight radio antennas is to remember that all designs have a
tradeoff in portability, gain, and size. The very best designs have good
performance in all three aspects, although this a subjective judgement. The 7.5"
plug-in loopstick was designed to offer good portability and gain with
reasonable size (although some would dispute that last point :-) Your Crate Loop
might provide more gain with slightly less portability, and a 9' PVC loop
would provide massive gain with almost no portability. Each DXer needs to
decide what the objectives are, and one design certainly won't work for
everybody.

73, Gary DeBock (in Puyallup, WA)








In a message dated 5/12/2010 10:06:21 P.M. Pacific Daylight Time,
satya@ writes:




Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has
produced, I decided to see what the performance of the PL-360 would be using a
large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19
turns of wire, spaced over about a 10-inch coil depth. It measures out to
just over 350 uH, matching the inductance of Gary's ferrite rig. I simply
connected both of the coils leads to a 1/8" plug, and connected it to the
jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The
7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in
at 56 db signal and 24 db SNR, and it really sounded like a huge boost as
well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the
air-core started having problems, apparently with overloading, as I got into the
MW band. Tuning up and down the dial, I often had the dreaded "50-00"
reading on the PL-360, which I interpret to mean that it has so much signal
that it is desensitized/swamped. DX targets could often be heard, but at much
reduced volume. The ferrite, on the other hand, was still behaving well and
receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop,
even on fairly vacant channels, except again at the far end of the band,
away from my various strong locals. It wasn't quite as bad as when my locals
were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton
e100 - it was simply too much, and near locals the desensitizing was
significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a
broadband source, at least in an urban area. I know that I cannot plug and
untuned active whip or a Super Loop/Wellbrook FLG-100 combination in,
either - simply too much input from strong locals. Obviously, based on Guy
Atkins' 18" ferrite experiment, at least that amount of signal is acceptable -
maybe there is something about ferrites (reduced stray capacitance, etc.)
that also is a factor? At any rate, those wanting to plug an air-core loop
straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


dhsatyadhana <satya@...>
 

Hey Farmerik:

Good to hear that your PL-360 is not overloading. I have several 50 kw stations within 10-15 miles, plus two 5-10 kw stations only 3 miles away, so my location is probably the extreme. If you have a little more breathing room, perhaps you can continue to rake 'em with your 2 foot loop!

I would be curious to know if you've determined the inductance of your 2-foot loop - is it around 350 uH? Per:
http://www.ourhousenews.com/loopAnt.php4
a 24-inch loop with a 5-inch coil depth should have about 18 turns. If your 2-footer is wound so that it resonates over the whole band with a tuning capacitor, you're probably looking at only about 250 uH, or about 15 turns. So, you may be able to throw some more wire on to get the inductance up to where the PL-360 likes it, and even more gain!

Kevin S
Bainbridge Island, WA

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:


I tried my 2 foot air core loop again tonight, and the white noise blocking my reception to the west is gone at the moment. I found a good signal on AM 620, and it turned out to be WVMT from Burlington VT. That is a 5kW I don't recall hearing here before, but I get it when I am in Northern VT. So a 2 foot loop may not be too large for the PL-360. I do have to shut off my CRT computer monitors, and I don't have any local stations to worry about. I tried AM 650 WSM, but just got noise on that frequency. Also a pretty steady signal on AM 540, but it's all in Spanish, so I wouldn't be able to ID it easily. - FARMERIK


--- In ultralightdx@..., D1028Gary@ wrote:

Hi Kevin (and Stephen),

Thanks for sharing the results of your PL-360 air core loop
experimentation, and thanks again for your positive comments on the 7.5" plug-in
loopstick.

Regarding the overloading issue with air core loops directly connected to
the PL-360 (or other Ultralights), my own experience has been very similar
to yours. Inductive coupling to the large PVC loops had always provided
satisfactory results with the new DSP radios, and avoided any RF overloads from
local pests. As you have also documented in your excellent tuned passive
loop articles, the extra tuned circuit in the Crate Loop helps for filter
out RF overloads from local pests, and provides additional selectivity to
enhance DX reception. With the Crate Loop or PVC Loop tuning capacitor
optimized on a DX station's frequency, the splatter and spurious signals from
local pest stations is significantly reduced. From actual DXing experience I'm
sure that we would both agree that this extra tuned circuit is not a
hindrance-- it's actually a great advantage.

Probably the easiest way to understand the performance challenges in
creating Ultralight radio antennas is to remember that all designs have a
tradeoff in portability, gain, and size. The very best designs have good
performance in all three aspects, although this a subjective judgement. The 7.5"
plug-in loopstick was designed to offer good portability and gain with
reasonable size (although some would dispute that last point :-) Your Crate Loop
might provide more gain with slightly less portability, and a 9' PVC loop
would provide massive gain with almost no portability. Each DXer needs to
decide what the objectives are, and one design certainly won't work for
everybody.

73, Gary DeBock (in Puyallup, WA)








In a message dated 5/12/2010 10:06:21 P.M. Pacific Daylight Time,
satya@ writes:




Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has
produced, I decided to see what the performance of the PL-360 would be using a
large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19
turns of wire, spaced over about a 10-inch coil depth. It measures out to
just over 350 uH, matching the inductance of Gary's ferrite rig. I simply
connected both of the coils leads to a 1/8" plug, and connected it to the
jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The
7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in
at 56 db signal and 24 db SNR, and it really sounded like a huge boost as
well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the
air-core started having problems, apparently with overloading, as I got into the
MW band. Tuning up and down the dial, I often had the dreaded "50-00"
reading on the PL-360, which I interpret to mean that it has so much signal
that it is desensitized/swamped. DX targets could often be heard, but at much
reduced volume. The ferrite, on the other hand, was still behaving well and
receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop,
even on fairly vacant channels, except again at the far end of the band,
away from my various strong locals. It wasn't quite as bad as when my locals
were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton
e100 - it was simply too much, and near locals the desensitizing was
significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a
broadband source, at least in an urban area. I know that I cannot plug and
untuned active whip or a Super Loop/Wellbrook FLG-100 combination in,
either - simply too much input from strong locals. Obviously, based on Guy
Atkins' 18" ferrite experiment, at least that amount of signal is acceptable -
maybe there is something about ferrites (reduced stray capacitance, etc.)
that also is a factor? At any rate, those wanting to plug an air-core loop
straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


Rik
 

It's only 11 turns spread over about six inches wide. Matching the inductance sounds like a very good idea, but I am not sure I'd want any more gain! All previous attempts were bothered by that white noise blocking everything to my west, which IS almost everything domestic from Connecticut. If that does not return, then I will be even more interested in winding both MW and LW air core loops.

Interesting Gary's ferrite never heard the white noise interference.

My four foot loop overloaded the PL-360,and received images on LW and the inductance is way too high as well.

My 2 foot loop is a sorry looking thing. I wound it with wire reclaimed from a motor, which is not straight and parallel. It was just a quick prototype. It is a much handier size than the 4 footer!

I did not notice overloading on strong stations, but I was looking for new catches, so I only tuned in weak signals.

-FARMERIK

--- In ultralightdx@..., "dhsatyadhana" <satya@...> wrote:

Hey Farmerik:

Good to hear that your PL-360 is not overloading. I have several 50 kw stations within 10-15 miles, plus two 5-10 kw stations only 3 miles away, so my location is probably the extreme. If you have a little more breathing room, perhaps you can continue to rake 'em with your 2 foot loop!

I would be curious to know if you've determined the inductance of your 2-foot loop - is it around 350 uH? Per:
http://www.ourhousenews.com/loopAnt.php4
a 24-inch loop with a 5-inch coil depth should have about 18 turns. If your 2-footer is wound so that it resonates over the whole band with a tuning capacitor, you're probably looking at only about 250 uH, or about 15 turns. So, you may be able to throw some more wire on to get the inductance up to where the PL-360 likes it, and even more gain!

Kevin S
Bainbridge Island, WA


--- In ultralightdx@..., "farmerik" <farmerik@> wrote:


I tried my 2 foot air core loop again tonight, and the white noise blocking my reception to the west is gone at the moment. I found a good signal on AM 620, and it turned out to be WVMT from Burlington VT. That is a 5kW I don't recall hearing here before, but I get it when I am in Northern VT. So a 2 foot loop may not be too large for the PL-360. I do have to shut off my CRT computer monitors, and I don't have any local stations to worry about. I tried AM 650 WSM, but just got noise on that frequency. Also a pretty steady signal on AM 540, but it's all in Spanish, so I wouldn't be able to ID it easily. - FARMERIK


--- In ultralightdx@..., D1028Gary@ wrote:

Hi Kevin (and Stephen),

Thanks for sharing the results of your PL-360 air core loop
experimentation, and thanks again for your positive comments on the 7.5" plug-in
loopstick.

Regarding the overloading issue with air core loops directly connected to
the PL-360 (or other Ultralights), my own experience has been very similar
to yours. Inductive coupling to the large PVC loops had always provided
satisfactory results with the new DSP radios, and avoided any RF overloads from
local pests. As you have also documented in your excellent tuned passive
loop articles, the extra tuned circuit in the Crate Loop helps for filter
out RF overloads from local pests, and provides additional selectivity to
enhance DX reception. With the Crate Loop or PVC Loop tuning capacitor
optimized on a DX station's frequency, the splatter and spurious signals from
local pest stations is significantly reduced. From actual DXing experience I'm
sure that we would both agree that this extra tuned circuit is not a
hindrance-- it's actually a great advantage.

Probably the easiest way to understand the performance challenges in
creating Ultralight radio antennas is to remember that all designs have a
tradeoff in portability, gain, and size. The very best designs have good
performance in all three aspects, although this a subjective judgement. The 7.5"
plug-in loopstick was designed to offer good portability and gain with
reasonable size (although some would dispute that last point :-) Your Crate Loop
might provide more gain with slightly less portability, and a 9' PVC loop
would provide massive gain with almost no portability. Each DXer needs to
decide what the objectives are, and one design certainly won't work for
everybody.

73, Gary DeBock (in Puyallup, WA)








In a message dated 5/12/2010 10:06:21 P.M. Pacific Daylight Time,
satya@ writes:




Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has
produced, I decided to see what the performance of the PL-360 would be using a
large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19
turns of wire, spaced over about a 10-inch coil depth. It measures out to
just over 350 uH, matching the inductance of Gary's ferrite rig. I simply
connected both of the coils leads to a 1/8" plug, and connected it to the
jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The
7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in
at 56 db signal and 24 db SNR, and it really sounded like a huge boost as
well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the
air-core started having problems, apparently with overloading, as I got into the
MW band. Tuning up and down the dial, I often had the dreaded "50-00"
reading on the PL-360, which I interpret to mean that it has so much signal
that it is desensitized/swamped. DX targets could often be heard, but at much
reduced volume. The ferrite, on the other hand, was still behaving well and
receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop,
even on fairly vacant channels, except again at the far end of the band,
away from my various strong locals. It wasn't quite as bad as when my locals
were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton
e100 - it was simply too much, and near locals the desensitizing was
significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a
broadband source, at least in an urban area. I know that I cannot plug and
untuned active whip or a Super Loop/Wellbrook FLG-100 combination in,
either - simply too much input from strong locals. Obviously, based on Guy
Atkins' 18" ferrite experiment, at least that amount of signal is acceptable -
maybe there is something about ferrites (reduced stray capacitance, etc.)
that also is a factor? At any rate, those wanting to plug an air-core loop
straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


sdwillingham
 

Hi Kevin,

Thanks for sharing the results of your air-loop experiments. I always enjoy reading about such experiments and considering how those results square with theory. I think there are a couple of issues that could be affecting your experience with the air loops.

First, your loop is very big. I realize many people on this forum have success with big loops, but I'm not convinced they're necessary or beneficial. They certainly create more overload potential. According to a National Radio Club paper by Dallas Lankford, an air loop 6" in diameter is approximately equivalent to a 7.5" ferrite rod. Furthermore, in all but rare circumstances, a 1 to 2 foot air loop has a noise floor lower than atmospheric noise and interference. Bigger loops yield bigger signals, but only with correspondingly bigger noise. Once the noise floor exceeds the radio's electronics, there is just no further performance gain. So, while I haven't personally carried out extensive tests, I'm pretty confident that the best air loops for the PL-360 will not be larger than 1 to 2 feet.

The second problem with air loops plugged into the PL-360 is the unbalanced nature of the PL-360 input circuit. An unbalanced loop antenna acts like a superposition of two antennas: the desired loop plus a 'parasitic' whip-antenna. The parasitic whip depends on the physical size of the wire coil.

A ferrite-core loop has the advantage of a compact wire coil, so it is affected the same way, but to a much smaller extent. A large (unbalanced) air loop will have a substantial effect. This parasitic antenna picks up a lot of signal. That's not bad in itself, but it ruins the loop nulling ability, and the whip-effect reception has generally lower signal-to-noise ratio. It picks up a lot of local man-made noise. On my PL-380 I've done this experiment: disconnect the ferrite, replacing it with a simple high-quality inductor. Then I add a 12" piece of wire to the "hot" side of the circuit. Just this tiny tuned antenna brings in signals with huge RSSI. On local stations, it can be 25 dB stronger than the internal ferrite. But the reception quality is much worse. The unbalanced circuit does the same thing -- lots of RSSI but poor still poor reception. Combine this with strong pest stations, and the results are even worse.

The solution for air loops involves a balun transformer. Jim K's hoop-loop design works very well, combining a balun transformer with impedance transformation. I have a similar setup in my modified PL-380, connected to a small rectangular loop (60 x 133 mm, 7 turns, 10 uH). With a compact, shielded transformer and input circuit, I get fantastic balance and signal quality. Even with the tiny loop, I generally have a noise floor well above the radio's floor. In the day time, I could use a 1 to 2 foot loop for more gain. At night, I'm not too sure. I can null 70 dB local stations to the point of hearing co-channel stations at night. Now that I'm satisfied with my transformer design, I'll probably make a 12-14" loop.

-Scott-

--- In ultralightdx@..., "dhsatyadhana" <satya@...> wrote:

Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has produced, I decided to see what the performance of the PL-360 would be using a large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19 turns of wire, spaced over about a 10-inch coil depth. It measures out to just over 350 uH, matching the inductance of Gary's ferrite rig. I simply connected both of the coils leads to a 1/8" plug, and connected it to the jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The 7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in at 56 db signal and 24 db SNR, and it really sounded like a huge boost as well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the air-core started having problems, apparently with overloading, as I got into the MW band. Tuning up and down the dial, I often had the dreaded "50-00" reading on the PL-360, which I interpret to mean that it has so much signal that it is desensitized/swamped. DX targets could often be heard, but at much reduced volume. The ferrite, on the other hand, was still behaving well and receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop, even on fairly vacant channels, except again at the far end of the band, away from my various strong locals. It wasn't quite as bad as when my locals were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton e100 - it was simply too much, and near locals the desensitizing was significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a broadband source, at least in an urban area. I know that I cannot plug and untuned active whip or a Super Loop/Wellbrook FLG-100 combination in, either - simply too much input from strong locals. Obviously, based on Guy Atkins' 18" ferrite experiment, at least that amount of signal is acceptable - maybe there is something about ferrites (reduced stray capacitance, etc.) that also is a factor? At any rate, those wanting to plug an air-core loop straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


Rik
 

Scott- If you have time, go back to my recent posts about using an untuned 2 foot loop with a short lead on my PL-360. It did work and get me several new logs in a short time the other night, but might be at the limit in size for the PL-360. I am not trying to suggest it is a good set up, but just reporting what happened. I'd like to build something better.

I had been trying to avoid using a matching transformer, thinking I would not need one if I kept the radio close to the loop. Actually, I was more concerned I would make an error with the loop or the balun, and not know which was not working correctly. So my question is it a good idea to wind a loop in the inductance range the PL-360 should tune, or better to use a matching transformer?

-FARMERIK

--- In ultralightdx@..., "sdwillingham" <sdwillingham@...> wrote:









Hi Kevin,

Thanks for sharing the results of your air-loop experiments. I always enjoy reading about such experiments and considering how those results square with theory. I think there are a couple of issues that could be affecting your experience with the air loops.

First, your loop is very big. I realize many people on this forum have success with big loops, but I'm not convinced they're necessary or beneficial. They certainly create more overload potential. According to a National Radio Club paper by Dallas Lankford, an air loop 6" in diameter is approximately equivalent to a 7.5" ferrite rod. Furthermore, in all but rare circumstances, a 1 to 2 foot air loop has a noise floor lower than atmospheric noise and interference. Bigger loops yield bigger signals, but only with correspondingly bigger noise. Once the noise floor exceeds the radio's electronics, there is just no further performance gain. So, while I haven't personally carried out extensive tests, I'm pretty confident that the best air loops for the PL-360 will not be larger than 1 to 2 feet.

The second problem with air loops plugged into the PL-360 is the unbalanced nature of the PL-360 input circuit. An unbalanced loop antenna acts like a superposition of two antennas: the desired loop plus a 'parasitic' whip-antenna. The parasitic whip depends on the physical size of the wire coil.

A ferrite-core loop has the advantage of a compact wire coil, so it is affected the same way, but to a much smaller extent. A large (unbalanced) air loop will have a substantial effect. This parasitic antenna picks up a lot of signal. That's not bad in itself, but it ruins the loop nulling ability, and the whip-effect reception has generally lower signal-to-noise ratio. It picks up a lot of local man-made noise. On my PL-380 I've done this experiment: disconnect the ferrite, replacing it with a simple high-quality inductor. Then I add a 12" piece of wire to the "hot" side of the circuit. Just this tiny tuned antenna brings in signals with huge RSSI. On local stations, it can be 25 dB stronger than the internal ferrite. But the reception quality is much worse. The unbalanced circuit does the same thing -- lots of RSSI but poor still poor reception. Combine this with strong pest stations, and the results are even worse.

The solution for air loops involves a balun transformer. Jim K's hoop-loop design works very well, combining a balun transformer with impedance transformation. I have a similar setup in my modified PL-380, connected to a small rectangular loop (60 x 133 mm, 7 turns, 10 uH). With a compact, shielded transformer and input circuit, I get fantastic balance and signal quality. Even with the tiny loop, I generally have a noise floor well above the radio's floor. In the day time, I could use a 1 to 2 foot loop for more gain. At night, I'm not too sure. I can null 70 dB local stations to the point of hearing co-channel stations at night. Now that I'm satisfied with my transformer design, I'll probably make a 12-14" loop.

-Scott-

--- In ultralightdx@..., "dhsatyadhana" <satya@> wrote:

Hi all:

Inspired by the success of the 7.5" plug-in ferrite that Gary has produced, I decided to see what the performance of the PL-360 would be using a large air-core loop jacked directly in. I have an 18x32 inch PVC frame with 19 turns of wire, spaced over about a 10-inch coil depth. It measures out to just over 350 uH, matching the inductance of Gary's ferrite rig. I simply connected both of the coils leads to a 1/8" plug, and connected it to the jack on the PL-360.

Using stable daytime targets, the first was a TIS-type station on 540. The 7.5" ferrite had it at 34 db signal and 6 db SNR. The air-core clocked in at 56 db signal and 24 db SNR, and it really sounded like a huge boost as well! I felt my pulse quicken just a bit...

Alas, being in an urban area near several powerful broadcasters, the air-core started having problems, apparently with overloading, as I got into the MW band. Tuning up and down the dial, I often had the dreaded "50-00" reading on the PL-360, which I interpret to mean that it has so much signal that it is desensitized/swamped. DX targets could often be heard, but at much reduced volume. The ferrite, on the other hand, was still behaving well and receiving stations as it should.

At night, everything was still at or above 50 SNR with the air-core loop, even on fairly vacant channels, except again at the far end of the band, away from my various strong locals. It wasn't quite as bad as when my locals were on day patterns, but still quite swamped.

I had similar results when I jacked in a large air-core loop to the Eton e100 - it was simply too much, and near locals the desensitizing was significant, although I dare say the PL-360 is having an even tougher time.

So, perhaps there is a limit to what the PL-360 can accept directly from a broadband source, at least in an urban area. I know that I cannot plug and untuned active whip or a Super Loop/Wellbrook FLG-100 combination in, either - simply too much input from strong locals. Obviously, based on Guy Atkins' 18" ferrite experiment, at least that amount of signal is acceptable - maybe there is something about ferrites (reduced stray capacitance, etc.) that also is a factor? At any rate, those wanting to plug an air-core loop straight into a PL-360 had better live outside an urban area!

Kevin S
Bainbridge Island, WA


sdwillingham
 

Farmerik,

I have read the posts about your experiments, also with interest. I'm not saying that an unbalanced air-loop won't bring in strong signals. More precisely, I think it will be inconsistent, difficult to null, and prone to noise pickup.

The "whip effect" is also not limited to the lead-in wires. The loop itself, when unbalanced, acts like a fat whip. So in your experiments, it does help to eliminate the the lead-in wires, but since air loops are big, there is still considerable effect. In my experiments, even a few inches of wire on the "hot" side of the unbalanced, tuned circuit picks up a lot of RF.

For air loops with high inductance, directly matching the Si4734's tuning range, it is difficult to make a 1:1 balun transformer. It takes a lot of windings on both the primary and secondary, leading to more parasitic capacitance and series resistance. Also, a larger core must be used. The high-impedance loop will also be more susceptible to stray capacitance, so limiting your lead-in wire length. So my answer to your question is that a low-inductance loop (10-20 uH) with matching transformer is a better setup.

Here's the recipe for the balun transformer I'm using, which should work for air and ferrite loops in the range of 10-20 uH:

1) FT50-61 ferrite core
2) Secondary winding is 80 turns of 32ga wire. If you can measure it, the open-circuit inductance of this winding should be 440 uH. I needed 84 turns to get there, but it's not vital. Arrange the windings neatly with several millimeters gap between the ends of the coil. Secure the ends lightly with super-glue.
3) Primary winding is 28ga wire (easier to work with), two strands twisted loosely together. Wind 8 turns of this bifilar wire over the primary windings. Secure with super-glue.
4) Primary connections: Separate the twisted wires and identify the two wires A and B and two ends 1 and 2. Twist wires A1 and B2 together and connect to the radio's RF ground. Twist wires B1 and A2; each wire connects to one side of your loop lead-in.
5) Secondary wires. One attaches to the radio's RF ground, the other to the AM input. It is vital to keep these leads short. It helps further to keep the "hot" wire (AM input) and toroid shielded by RF ground.

This setup is designed so that a 16 uH loop will be transformed to an effective 220 uH as seen by the radio chip. I'm using it with a 10 uH loop and the chip sees 180 uH, just within the range tunable by the chip over the AM broadcast band. With a 60x133 mm, 7-turn, 10 uH rectangular loop, I get RSSI readings on local stations roughly 6 to 10 dB higher than with the stock antenna on my PL-380.

-Scott-

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:


Scott- If you have time, go back to my recent posts about using an untuned 2 foot loop with a short lead on my PL-360. It did work and get me several new logs in a short time the other night, but might be at the limit in size for the PL-360. I am not trying to suggest it is a good set up, but just reporting what happened. I'd like to build something better.

I had been trying to avoid using a matching transformer, thinking I would not need one if I kept the radio close to the loop. Actually, I was more concerned I would make an error with the loop or the balun, and not know which was not working correctly. So my question is it a good idea to wind a loop in the inductance range the PL-360 should tune, or better to use a matching transformer?

-FARMERIK


jim_kr1s <jkearman@...>
 


--- In ultralightdx@..., "sdwillingham" wrote:
>

> For air loops with high inductance, directly matching the Si4734's tuning range, it is difficult to make a 1:1 balun transformer. It takes a lot of windings on both the primary and secondary, leading to more parasitic capacitance and series resistance. Also, a larger core must be used.

To put this in perspective: For an untuned balun, we like the reactance of a winding to be 4 times the source or load reactance at the lowest frequency of interest. Reactance is proportional to frequency, so we'll calculate at 530 kHz. Let's go with an inductance toward the low end of the range to minimize coil size. We'll use 240 uH.

At 530 kHz a 240-uH inductor has a reactance of 800 ohms. So the winding reactance should be 3200 ohms at 530 kHz. As reactance is directly proportional to inductance as well as frequency, the inductance should be 960 uH. Both windings will have the same inductance. It takes about 113 turns on a 1.14-inch toroid to get 960 uH, and it would take mighty thin wire. As Scott mentions, that raises the resistance and losses.

As Scott mentioned last night, a 6-inch air-core loop has about the same signal-gathering ability as a 7.5-inch ferrite rod antenna. Your relative RSSI readings on your 24-inch air-core loop vs the 7.5-inch ferrite rod sound about right. A 6-inch, low-inductance loop is easy to tote around. As you can locate it a few feet from the radio, so you can turn the antenna and not have to turn the radio. I find that to be a great convenience, even with the larger, 23-inch Hoop Loop. Cores and wire are available from http://kitsandparts.com/ (With a quick transformer change the loop will work on LW, though I'd use something larger than 6 inches there. I don't think you could squeeze a LW transformer onto a half-inch toroid, as you need >2000 uH.) Quilting and embroidery hoops are available in a variety of shapes and sizes. Use your favorite search engine or check out a local Michael's, Jo-Ann Fabrics, or similar store.

73,
 
Jim, KR1S
http://kr1s.kearman.com/ 
http://qrp.kearman.com/ 


Rik
 

Thanks for the great explanation, Scot.

Now does anyone know of a commercial source for a balun like that for AM BCB and LW? If not any volunteers to wind a few? Maybe a cottage industry? - FARMERIK

--- In ultralightdx@..., "sdwillingham" <sdwillingham@...> wrote:



Farmerik,

I have read the posts about your experiments, also with interest. I'm not saying that an unbalanced air-loop won't bring in strong signals. More precisely, I think it will be inconsistent, difficult to null, and prone to noise pickup.

The "whip effect" is also not limited to the lead-in wires. The loop itself, when unbalanced, acts like a fat whip. So in your experiments, it does help to eliminate the the lead-in wires, but since air loops are big, there is still considerable effect. In my experiments, even a few inches of wire on the "hot" side of the unbalanced, tuned circuit picks up a lot of RF.

For air loops with high inductance, directly matching the Si4734's tuning range, it is difficult to make a 1:1 balun transformer. It takes a lot of windings on both the primary and secondary, leading to more parasitic capacitance and series resistance. Also, a larger core must be used. The high-impedance loop will also be more susceptible to stray capacitance, so limiting your lead-in wire length. So my answer to your question is that a low-inductance loop (10-20 uH) with matching transformer is a better setup.

Here's the recipe for the balun transformer I'm using, which should work for air and ferrite loops in the range of 10-20 uH:

1) FT50-61 ferrite core
2) Secondary winding is 80 turns of 32ga wire. If you can measure it, the open-circuit inductance of this winding should be 440 uH. I needed 84 turns to get there, but it's not vital. Arrange the windings neatly with several millimeters gap between the ends of the coil. Secure the ends lightly with super-glue.
3) Primary winding is 28ga wire (easier to work with), two strands twisted loosely together. Wind 8 turns of this bifilar wire over the primary windings. Secure with super-glue.
4) Primary connections: Separate the twisted wires and identify the two wires A and B and two ends 1 and 2. Twist wires A1 and B2 together and connect to the radio's RF ground. Twist wires B1 and A2; each wire connects to one side of your loop lead-in.
5) Secondary wires. One attaches to the radio's RF ground, the other to the AM input. It is vital to keep these leads short. It helps further to keep the "hot" wire (AM input) and toroid shielded by RF ground.

This setup is designed so that a 16 uH loop will be transformed to an effective 220 uH as seen by the radio chip. I'm using it with a 10 uH loop and the chip sees 180 uH, just within the range tunable by the chip over the AM broadcast band. With a 60x133 mm, 7-turn, 10 uH rectangular loop, I get RSSI readings on local stations roughly 6 to 10 dB higher than with the stock antenna on my PL-380.

-Scott-

--- In ultralightdx@..., "farmerik" <farmerik@> wrote:


Scott- If you have time, go back to my recent posts about using an untuned 2 foot loop with a short lead on my PL-360. It did work and get me several new logs in a short time the other night, but might be at the limit in size for the PL-360. I am not trying to suggest it is a good set up, but just reporting what happened. I'd like to build something better.

I had been trying to avoid using a matching transformer, thinking I would not need one if I kept the radio close to the loop. Actually, I was more concerned I would make an error with the loop or the balun, and not know which was not working correctly. So my question is it a good idea to wind a loop in the inductance range the PL-360 should tune, or better to use a matching transformer?

-FARMERIK


jim_kr1s <jkearman@...>
 


--- In ultralightdx@..., "farmerik" wrote:
> Now does anyone know of a commercial source for a balun like that for AM BCB and LW? If not any volunteers to wind a few? Maybe a cottage industry? - FARMERIK

They must be made somewhere, as low-inductance AM antennas are standard on new consumer AM/FM tuners and receivers. In The Age of the Landfill, getting replacement parts is tough, especially a part that generally wouldn't become defective. So I don't know where you could get one. And I'm sure no one makes a transformer that would work at LW.

As far as "cottage industry," there isn't enough demand to warrant putting my other projects on hold. There's the need to fabricate an enclosure for the transformer, ordering plugs and jacks...  I'm glad to help anyone who cares to build  one; it really isn't hard.  You cut the wire to length and thread it through the core. Tying fishing flies is harder! The number of turns is not critical. If you are off a few turns it will still work fine, but all you have to do is count them. I think some people are intimidated by the bifilar primary winding. I just added a sketch to my Hoop Loop page  that shows how that winding is done. (I also added information on using the Hoop Loop with other receivers.)  This page

http://toroids.info/FT50-61.php 

will help you calculate the lengths of wire for various cores. For LW and MW I've been using -61 cores, available from http://kitsandparts.com/  He also sells short lengths of magnet wire, and will probably sell you a single core if that's all you want.

Making the antenna -- sanding, staining and applying polyurethane to the wood, then fabricating the tilt axles -- took 10 times longer than winding the transformers. Again, contact me if you have questions.

73,
 
Jim, KR1S
http://kr1s.kearman.com/ 
http://qrp.kearman.com/ 


Rik
 

Is the core you used the best one or something out of your parts box?

If you will recommend a core for AM and LW broadcast bands, I'll use the calculator and give it a try. I just don't want to start off with the wrong core for those frequencies. A larger core might be better for my hands to work with, but if it is too small, I might be able to get someone here to help.

I can rig a shield and solder on the 1/8th inch phone plug, no problem. I would think the shield should be aluminum, like an IF can, not steel. Is that correct? - FARMERIK

--- In ultralightdx@..., "jim_kr1s" <jkearman@...> wrote:


--- In ultralightdx@..., "farmerik" <farmerik@> wrote:
Now does anyone know of a commercial source for a balun like that for
AM BCB and LW? If not any volunteers to wind a few? Maybe a cottage
industry? - FARMERIK

They must be made somewhere, as low-inductance AM antennas are standard
on new consumer AM/FM tuners and receivers. In The Age of the Landfill,
getting replacement parts is tough, especially a part that generally
wouldn't become defective. So I don't know where you could get one. And
I'm sure no one makes a transformer that would work at LW.

As far as "cottage industry," there isn't enough demand to warrant
putting my other projects on hold. There's the need to fabricate an
enclosure for the transformer, ordering plugs and jacks... I'm glad to
help anyone who cares to build one; it really isn't hard. You cut the
wire to length and thread it through the core. Tying fishing flies is
harder! The number of turns is not critical. If you are off a few turns
it will still work fine, but all you have to do is count them. I think
some people are intimidated by the bifilar primary winding. I just added
a sketch to my Hoop Loop page
<http://kr1s.kearman.com/html/hooploop.html> that shows how that
winding is done. (I also added information on using the Hoop Loop with
other receivers.) This page

http://toroids.info/FT50-61.php <http://toroids.info/FT50-61.php>

will help you calculate the lengths of wire for various cores. For LW
and MW I've been using -61 cores, available from
http://kitsandparts.com/ <http://kitsandparts.com/> He also sells
short lengths of magnet wire, and will probably sell you a single core
if that's all you want.

Making the antenna -- sanding, staining and applying polyurethane to the
wood, then fabricating the tilt axles -- took 10 times longer than
winding the transformers. Again, contact me if you have questions.

73,

Jim, KR1S
http://kr1s.kearman.com/ <http://kr1s.kearman.com/>
http://qrp.kearman.com/ <http://qrp.kearman.com/>


Rik
 

I should have read more carefully the links you gave, but still I'd like your recommendation for what cores to order for each broadcast band. I am not picturing it in my mind yet, but it is getting clearer, and I have a local Elmer who mentioned he winds baluns, so he can fill in. When you said so many turns per inch, I thought you meant the spacing between turns on the core, but now I see that is for twisting the primary winding BEFORE it is wrapped. - FARMERIK

--- In ultralightdx@..., "farmerik" <farmerik@...> wrote:

Is the core you used the best one or something out of your parts box?

If you will recommend a core for AM and LW broadcast bands, I'll use the calculator and give it a try. I just don't want to start off with the wrong core for those frequencies. A larger core might be better for my hands to work with, but if it is too small, I might be able to get someone here to help.

I can rig a shield and solder on the 1/8th inch phone plug, no problem. I would think the shield should be aluminum, like an IF can, not steel. Is that correct? - FARMERIK

--- In ultralightdx@..., "jim_kr1s" <jkearman@> wrote:


--- In ultralightdx@..., "farmerik" <farmerik@> wrote:
Now does anyone know of a commercial source for a balun like that for
AM BCB and LW? If not any volunteers to wind a few? Maybe a cottage
industry? - FARMERIK

They must be made somewhere, as low-inductance AM antennas are standard
on new consumer AM/FM tuners and receivers. In The Age of the Landfill,
getting replacement parts is tough, especially a part that generally
wouldn't become defective. So I don't know where you could get one. And
I'm sure no one makes a transformer that would work at LW.

As far as "cottage industry," there isn't enough demand to warrant
putting my other projects on hold. There's the need to fabricate an
enclosure for the transformer, ordering plugs and jacks... I'm glad to
help anyone who cares to build one; it really isn't hard. You cut the
wire to length and thread it through the core. Tying fishing flies is
harder! The number of turns is not critical. If you are off a few turns
it will still work fine, but all you have to do is count them. I think
some people are intimidated by the bifilar primary winding. I just added
a sketch to my Hoop Loop page
<http://kr1s.kearman.com/html/hooploop.html> that shows how that
winding is done. (I also added information on using the Hoop Loop with
other receivers.) This page

http://toroids.info/FT50-61.php <http://toroids.info/FT50-61.php>

will help you calculate the lengths of wire for various cores. For LW
and MW I've been using -61 cores, available from
http://kitsandparts.com/ <http://kitsandparts.com/> He also sells
short lengths of magnet wire, and will probably sell you a single core
if that's all you want.

Making the antenna -- sanding, staining and applying polyurethane to the
wood, then fabricating the tilt axles -- took 10 times longer than
winding the transformers. Again, contact me if you have questions.

73,

Jim, KR1S
http://kr1s.kearman.com/ <http://kr1s.kearman.com/>
http://qrp.kearman.com/ <http://qrp.kearman.com/>


jim_kr1s <jkearman@...>
 


--- In ultralightdx@..., "farmerik" wrote:
>
> Is the core you used the best one or something out of your parts box?

Yes. :) The -61 core is the best for MW, and I had one in my parts box. Scott suggested I try a -43 core on LW but I haven't done it yet. There are slightly better mixes for LW (I don't think -43 is one of them, though). But as Scott has said, if you can hear the noise floor you have all the gain you can use. I can hear the noise floor on LW with transformers wound on -61 cores.

>
> If you will recommend a core for AM and LW broadcast bands, I'll use the calculator and give it a try. I just don't want to start off with the wrong core for those frequencies. A larger core might be better for my hands to work with, but if it is too small, I might be able to get someone here to help.


Scott has managed to make his MW coil on an FT50-61 core. "FT" means "ferrite," as opposed to the T prefix for iron-powder materials. The 50 is the OD of the core in hundredths of an inch. Thus a 50 is 1/2 inch; a 114 is 1.14 inches, and so on. The number after the dash is the core mixture. My Hoop Loop page has inductances for both bands, and the number of turns I used on one of my MW transformers. http://kr1s.kearman.com/html/hooploop.html A half-inch core just fits inside a PL-380 or similar radio. There are 8/10-inch cores but Kits and Parts doesn't sell them in -61 material. A 1.14-inch core (smallest size after 1/2-inch sold by Kits and Parts) would be easier to manage, especially as you could use a larger wire gauge. Scott used 32-ga, which is pretty fine. The smallest size I have is 28-ga. I managed to wind a MW coil with it on a 1/2-inch core.

Going to a larger core means a larger shield box. For LW, even if you used a different core mixture, you can't get enough inductance on a half-inch core. Then you need at least 1.14-inch. I got some 1.4-inch cores and one 2.4-inch just to try. Here's how you can determine if a core is large enough. That calculator page I mentioned, http://toroids.info/ , gives the ID of each core. Multiply that times 3.14 to get the inside circumference (I use that figure for pi to give myself some slack). Here are common wire gauges and their thickness in inches.

24

0.0201

25

0.0179

26

0.0159

27

0.0142

28

0.0126

29

0.0113

30

0.01

31

0.0089

32

0.008


Magnet wire is a little bit thicker, due to the insulation coating. If you divide the inner circumference by the wire thickness you'll have a good measure of how many turns you can fit on a core. Coils wound on toroids work better if they only cover about 2/3  of the core (better Q) but this isn't a big issue in this application. I think the package of magnet wire sold by Radio Shack includes 30-ga, but I'm not sure. I used 28-ga for the large secondary windings as it is the thinnest I have. For the primary windings I've used 26- and 24-ga only because they're easier to work with.

> I can rig a shield and solder on the 1/8th inch phone plug, no problem. I would think the shield should be aluminum, like an IF can, not steel. Is that correct? - FARMERIK

If you can get a plug with a metal shield you can use the shield as a nut, to fix the plug to the box (a plastic "shield" might work loose). Aluminum or scrap PC board soldered together will work. There's no advantage to steel. The box should be large enough that the walls are about half the core diameter away from the core. Toroids are largely self-shielding. I haven't yet shielded my LW transformers and I'm not having any problems. Even on MW, the shield is not as important as that bifilar winding. Before making a shield, try the transformer loose and with the antenna disconnected, while tuning the band. If you don't have strong locals you may be able to use a plastic box.

73,
 
Jim, KR1S
http://kr1s.kearman.com/ 
http://qrp.kearman.com/ 



jim_kr1s <jkearman@...>
 


--- In ultralightdx@..., "farmerik" wrote:
>
> I should have read more carefully the links you gave, but still I'd like your recommendation for what cores to order for each broadcast band. I am not picturing it in my mind yet, but it is getting clearer, and I have a local Elmer who mentioned he winds baluns, so he can fill in. When you said so many turns per inch, I thought you meant the spacing between turns on the core, but now I see that is for twisting the primary winding BEFORE it is wrapped. - FARMERIK

I think all my explaining is making this sound harder than it is! The number of turns needed on the secondary, the larger winding that goes to the radio, is chosen such that the inductance is 240-350 uH. It helps to play around with that calculator to see how many turns are needed on a given -61 core, as they aren't the same for 1.14-inch cores as 0.5-inch cores. You can build a MW transformer on an FT50-size core, but you have to use thinner wire to get enough turns. Pick a core size and determine the number of turns. Then you can figure out the fattest wire that will let you wind that number of turns over about 2/3 of the core, give or take -- not critical.

The primary is wound over the middle of the secondary. I use a single layer of the cheapest, thinnest electrical tape sold at Lowe's (in colors!) over the secondary, but you don't have to. The primary should be wound in the same 'sense' as the secondary; in other words the wire goes on in the same direction. Probably not highly critical. You twist the two primary wires a little, which reduces capacitance between them and helps keep them together while winding. I spread the primary out a little. Placing the turns closer together increases the inductance slightly; spreading them out may help the primary couple better into the core. I don't spend a lot of time worrying about that.

Note: That calculator assumes 1-inch pigtails. You may need longer leads, cutting them before installation. Longer wires make it easier to pull them snug, and you need a little extra length on the primary winding to bring the two ends together. So add a few inches to the calculated lengths. It's easier to cut wire than stretch it!

When I wind a large number of turns, I slide the core into the middle of the wire, and wind in both directions in turn. On the LW coil, which had a LOT of turns, I made a shuttle from a piece of plastic, so I could slip the wire through the core more easily. I managed to make my MW windings without using a shuttle, which is hard to manage on a half-inch core in any event. By starting with the core centered on the wire, you only have to haul half the length through as you start. Starting at one end on a large winding will drive you crazy.

If you have a magnifier or better yet, a lighted magnifier, it helps to check the large winding every few turns, to make sure you didn't overlap one turn over the previous one. This is easy to do. It won't kill the coil, but it will reduce Q. Unwinding is a pain, so check frequently.

73,
 
Jim, KR1S
http://kr1s.kearman.com/
 
http://qrp.kearman.com/ 


Rik
 

The 1.4 and the 2.4 inch sound best to me. Are you planning on trying those out soon?

-FARMERIK

--- In ultralightdx@..., "jim_kr1s" <jkearman@...> wrote:


--- In ultralightdx@..., "farmerik" <farmerik@> wrote:

Is the core you used the best one or something out of your parts box?
Yes. :) The -61 core is the best for MW, and I had one in my parts box.
Scott suggested I try a -43 core on LW but I haven't done it yet. There
are slightly better mixes for LW (I don't think -43 is one of them,
though). But as Scott has said, if you can hear the noise floor you have
all the gain you can use. I can hear the noise floor on LW with
transformers wound on -61 cores.


If you will recommend a core for AM and LW broadcast bands, I'll use
the calculator and give it a try. I just don't want to start off with
the wrong core for those frequencies. A larger core might be better for
my hands to work with, but if it is too small, I might be able to get
someone here to help.


Scott has managed to make his MW coil on an FT50-61 core. "FT" means
"ferrite," as opposed to the T prefix for iron-powder materials. The 50
is the OD of the core in hundredths of an inch. Thus a 50 is 1/2 inch; a
114 is 1.14 inches, and so on. The number after the dash is the core
mixture. My Hoop Loop page has inductances for both bands, and the
number of turns I used on one of my MW transformers.
http://kr1s.kearman.com/html/hooploop.html
<http://kr1s.kearman.com/html/hooploop.html> A half-inch core just fits
inside a PL-380 or similar radio. There are 8/10-inch cores but Kits and
Parts doesn't sell them in -61 material. A 1.14-inch core (smallest size
after 1/2-inch sold by Kits and Parts) would be easier to manage,
especially as you could use a larger wire gauge. Scott used 32-ga, which
is pretty fine. The smallest size I have is 28-ga. I managed to wind a
MW coil with it on a 1/2-inch core.

Going to a larger core means a larger shield box. For LW, even if you
used a different core mixture, you can't get enough inductance on a
half-inch core. Then you need at least 1.14-inch. I got some 1.4-inch
cores and one 2.4-inch just to try. Here's how you can determine if a
core is large enough. That calculator page I mentioned,
http://toroids.info/ <http://toroids.info/> , gives the ID of each
core. Multiply that times 3.14 to get the inside circumference (I use
that figure for pi to give myself some slack). Here are common wire
gauges and their thickness in inches.


24

0.0201

25

0.0179

26

0.0159

27

0.0142

28

0.0126

29

0.0113

30

0.01

31

0.0089

32

0.008

Magnet wire is a little bit thicker, due to the insulation coating. If
you divide the inner circumference by the wire thickness you'll have a
good measure of how many turns you can fit on a core. Coils wound on
toroids work better if they only cover about 2/3 of the core (better Q)
but this isn't a big issue in this application. I think the package of
magnet wire sold by Radio Shack includes 30-ga, but I'm not sure. I used
28-ga for the large secondary windings as it is the thinnest I have. For
the primary windings I've used 26- and 24-ga only because they're easier
to work with.

I can rig a shield and solder on the 1/8th inch phone plug, no
problem. I would think the shield should be aluminum, like an IF can,
not steel. Is that correct? - FARMERIK

If you can get a plug with a metal shield you can use the shield as a
nut, to fix the plug to the box (a plastic "shield" might work loose).
Aluminum or scrap PC board soldered together will work. There's no
advantage to steel. The box should be large enough that the walls are
about half the core diameter away from the core. Toroids are largely
self-shielding. I haven't yet shielded my LW transformers and I'm not
having any problems. Even on MW, the shield is not as important as that
bifilar winding. Before making a shield, try the transformer loose and
with the antenna disconnected, while tuning the band. If you don't have
strong locals you may be able to use a plastic box.

73,

Jim, KR1S
http://kr1s.kearman.com/ <http://kr1s.kearman.com/>
http://qrp.kearman.com/ <http://qrp.kearman.com/>


jim_kr1s <jkearman@...>
 


--- In ultralightdx@..., "farmerik" wrote:
>
> The 1.4 and the 2.4 inch sound best to me. Are you planning on trying those out soon?

I have. If you look at the toroids.info page you'll see a brief summary of the intended use of various materials. For -61 it says "High-Q Inductor 0.2 - 10 MHz." FT50-61, FT-114-61, FT-240-61 -- it doesn't matter, as long as the inductance is in the ranges previously mentioned. I tried larger-diameter wire on the larger cores, with no discernible improvement over the FT50-61 on MW. For LW I had to use a larger core diameter because of the number of turns. Use the smallest diameter core you can work with. I think you can get enough turns for LW on a 1.14-inch core, if you use smaller wire; I know it can be done on a 1.4-inch core. The 2.4-inch core weighs almost as much as a hocky puck and is nearly as large! Larger-diameter wire or Litz wire might raise the measurable Q of the coil, but in practice there is no improvement in performance. The performance of the balanced, low-inductance antenna far outweighs any deficiencies in a smaller core.

If you recall Roy Dyball's early experiments, he got better performance with plain wire-wrap wire wound directly on the original ferrite rod, than from the rod with the original Litz, by spacing out the turns. I tried Litz wire on the antenna, with no improvement. So pick a core, buy some wire, and wind one. You can get two FT140-61 cores for $5.00 + shipping. Get a pack of wire at Radio Shack and have fun!

73,
 
Jim, KR1S
http://kr1s.kearman.com/ 
http://qrp.kearman.com/