Mixed bag with the Air Core Antenna add-on to Tecsun PL310


Gary DeBock
 

Hello Gregory (and Chris),
 
Thanks for reporting the results of the 7.5" loopstick transplant in your PL-310 model, and congratulations on the new stations you have logged with it.
 
Since this 7.5" loopstick transplant procedure was developed for the PL-380 model (not the PL-310), the 530-1700 kHz performance testing was only done here on the PL-380 model. Using the exact same material and procedure as described in the transplant article, the PL-380 model here had a very great AM sensitivity boost on all frequencies from 530-1700 kHz (averaging 20 dBu), with no issues on the high band like you describe.
 
Concerning whether this PL-380 transplant procedure developed for the PL-380 will work with equal effectiveness in the PL-310, as far as I know, you are the first hobbyist to try it in the PL-310 model. I have a PL-310 model here hooked up to the same Amidon loopstick with an 81-turn Slider coil, and no matter how I tune the Slider coil, the PL-310's AM sensitivity performance does not equal that of the finished PL-380 7.5" transplant model. It comes fairly close on most frequencies when the Slider coil is peaked for optimum inductance match with the DSP chip, but it is still about 5 dBu less than that of the PL-380 transplant model on most AM frequencies. The reason for this is a mystery-- it may possibly be due to improved circuitry in the PL-380 model installed by Tecsun to compensate for the smaller stock loopstick. In any case, the testing here indicates that the PL-310's performance with the 7.5" Amidon loopstick transplant seems slightly inferior to that of the equivalent PL-380 model, although there were no high-band issues on the transplant-model PL-310 here, like you described.
 
The symptoms you describe DID occur here on the PL-310 experimental loopstick composed of 113 turns of 40/44 Litz wire on the stock ferrite bar, which was tested here as a possible replacement for the stock PL-310 loopstick. This experimental loopstick (documented in a photo album on Ultralightdx) had a coil inductance of 554 uh, and did boost up AM sensitivity significantly on all AM frequencies up to about 1300 kHz. Above 1300 kHz, however, it was inferior to the stock loopstick in performance, with symptoms exactly like you describe. Presumably the coil "Q" of this experimental loopstick (practically covered with Litz wire) was unacceptable at the high AM frequencies.
 
Recent experimentation here (and by Nick Hall-Patch, and others) indicates that any change in an experimental loopstick parameter (ferrite bar size or length, ferrite mix, Litz wire, coil "Q" or Si4734 Radio model) can change the optimum loopstick coil inductance required to match the DSP chip varactor for peak AM sensitivity. The loopstick parameters in the PL-380 7.5" loopstick transplant article were developed and tested on the PL-380, but may not be directly applicable to the PL-310 model, Gregory. In any case, if you wish to optimize the AM sensitivity in your PL-310 transplant model on all frequencies, you may need to ensure that the 554 uh coil inductance is optimal for your particular experimental loopstick. The easy way to try this would be to add or subtract a few turns of Litz wire from the coil, to see if the high band issue would be corrected or not. You are also welcome to borrow the 81-turn Slider coil here, if you wish to perfectly fine-tune your new loopstick for optimal AM sensitivity.
 
73, Gary DeBock      
 
 
 
In a message dated 1/1/2010 9:25:01 A.M. Pacific Standard Time, gmosherat@... writes:
 

A few days ago I completed a Air Core Add-on to my Tecsun PL310, essentially I performed the 7.5 Inch Loopstick Transplant as described by Gary DeBock (BTW Gary - thants for all your work on this stuff - much appreciated!).

I can report mixed results, read on.

Antenna replacement consists of a Amidon 7.5" Type 61 ferrite bar wound with 81 turns of 40/44 Litz wire starting at 43mm from one end of the bar. Wire is wrapped around the Ferrite Rod first covered by J&J waterproof tape (Sticky side out). The one difference between my antenna and the one described in Gary's article is that I used 1 inch width J&J tape rather than the 2-inch width variety (I can't find the 2-inch stuff locally). Also, the original antenna was left in the radio, albiet, disconnected.

I only performed one test between the old and new antenna, at 1180 Khz (local 50Kw station). The new antenna gave me a 40 dBu increase over the stock antenna.

Pervious to this modification I had logged 115 stations barefoot. In the past two days (using the new antenna) I have added 15 new stations, not bad! I suspect not all of these can be attributed to the new antenna, but on the whole the radio is more sensitive.

Why do I say it's a mixed bag? It seems something is wrong at approx above 1370-1400khz. dBu seems ok, even on frequencies where there I hear nothing it hovers around 49-50. The problem is dB S/N... It stays at 00 for anything other that local stations (1370 and 1460). For example, On 1520 khz is a 50kw station 65 miles away. I logged this station prior to the antenna mod, It came in loud and clear. I can hear it on the car radio, and other radios in the house just fine. Now after the antenna mod when I tune in 1520kHz on the PL310 I have to raise volume to the highest level and use headphones to hear the station. Reading are: dBu: 49, dB S/N is 00.

So.. It seems that above approx 1370-to 1400 Khz dB S/N ratio stays at 00. Even with this phenomenon five of my new loggings are from w/in this frequency range.

Has anyone else performed this mod on their PL310 or PL380 and if so are you seeing the same results?

Any of you out there have an explination for what I am seeing?

Regards,

Gregory Mosher

.


gmosherat
 

A few days ago I completed a Air Core Add-on to my Tecsun PL310, essentially I performed the 7.5 Inch Loopstick Transplant as described by Gary DeBock (BTW Gary - thants for all your work on this stuff - much appreciated!).

I can report mixed results, read on.

Antenna replacement consists of a Amidon 7.5" Type 61 ferrite bar wound with 81 turns of 40/44 Litz wire starting at 43mm from one end of the bar. Wire is wrapped around the Ferrite Rod first covered by J&J waterproof tape (Sticky side out). The one difference between my antenna and the one described in Gary's article is that I used 1 inch width J&J tape rather than the 2-inch width variety (I can't find the 2-inch stuff locally). Also, the original antenna was left in the radio, albiet, disconnected.

I only performed one test between the old and new antenna, at 1180 Khz (local 50Kw station). The new antenna gave me a 40 dBu increase over the stock antenna.

Pervious to this modification I had logged 115 stations barefoot. In the past two days (using the new antenna) I have added 15 new stations, not bad! I suspect not all of these can be attributed to the new antenna, but on the whole the radio is more sensitive.

Why do I say it's a mixed bag? It seems something is wrong at approx above 1370-1400khz. dBu seems ok, even on frequencies where there I hear nothing it hovers around 49-50. The problem is dB S/N... It stays at 00 for anything other that local stations (1370 and 1460). For example, On 1520 khz is a 50kw station 65 miles away. I logged this station prior to the antenna mod, It came in loud and clear. I can hear it on the car radio, and other radios in the house just fine. Now after the antenna mod when I tune in 1520kHz on the PL310 I have to raise volume to the highest level and use headphones to hear the station. Reading are: dBu: 49, dB S/N is 00.

So.. It seems that above approx 1370-to 1400 Khz dB S/N ratio stays at 00. Even with this phenomenon five of my new loggings are from w/in this frequency range.

Has anyone else performed this mod on their PL310 or PL380 and if so are you seeing the same results?

Any of you out there have an explination for what I am seeing?

Regards,

Gregory Mosher


Chris Knight <chris@...>
 

Gregory,

I have the same problem above 1370 kHz when either a G8 or PL-380 is coupled
to a Conti Loop facing south toward Denver. The problem is overloading from
a strong local station, KCKK, on 1510 about 20 miles SSW of me. One of their
lobes is right over my QTH. If I move the Conti Loop to face away from KCKK
or use the Quantum Phaser to phase-null 1510 the problem goes away. When
overloading occurs, the radio's display reads "4900" or "5000" (like you
noted) and it sounds like there's an open carrier. If you couple your PL-310
to a tuned loop my guess is the problem you're experiencing will go away.
Or, you can try pointing the radio in a direction perpendicular to your
strong stations and see if that has any effect.

73,

Chris Knight (N0IJK)

-----Original Message-----
From: ultralightdx@... [mailto:ultralightdx@...] On
Behalf Of gmosherat
Sent: Friday, January 01, 2010 10:18 AM
To: ultralightdx@...
Subject: [ultralightdx] Mixed bag with the Air Core Antenna add-on to Tecsun
PL310

A few days ago I completed a Air Core Add-on to my Tecsun PL310, essentially
I performed the 7.5 Inch Loopstick Transplant as described by Gary DeBock
(BTW Gary - thants for all your work on this stuff - much appreciated!).

I can report mixed results, read on.

Antenna replacement consists of a Amidon 7.5" Type 61 ferrite bar wound with
81 turns of 40/44 Litz wire starting at 43mm from one end of the bar. Wire
is wrapped around the Ferrite Rod first covered by J&J waterproof tape
(Sticky side out). The one difference between my antenna and the one
described in Gary's article is that I used 1 inch width J&J tape rather than
the 2-inch width variety (I can't find the 2-inch stuff locally). Also, the
original antenna was left in the radio, albiet, disconnected.

I only performed one test between the old and new antenna, at 1180 Khz
(local 50Kw station). The new antenna gave me a 40 dBu increase over the
stock antenna.

Pervious to this modification I had logged 115 stations barefoot. In the
past two days (using the new antenna) I have added 15 new stations, not
bad! I suspect not all of these can be attributed to the new antenna, but on
the whole the radio is more sensitive.

Why do I say it's a mixed bag? It seems something is wrong at approx above
1370-1400khz. dBu seems ok, even on frequencies where there I hear nothing
it hovers around 49-50. The problem is dB S/N... It stays at 00 for
anything other that local stations (1370 and 1460). For example, On 1520
khz is a 50kw station 65 miles away. I logged this station prior to the
antenna mod, It came in loud and clear. I can hear it on the car radio, and
other radios in the house just fine. Now after the antenna mod when I
tune in 1520kHz on the PL310 I have to raise volume to the highest level and
use headphones to hear the station. Reading are: dBu: 49, dB S/N is 00.

So.. It seems that above approx 1370-to 1400 Khz dB S/N ratio stays at 00.
Even with this phenomenon five of my new loggings are from w/in this
frequency range.

Has anyone else performed this mod on their PL310 or PL380 and if so are you
seeing the same results?

Any of you out there have an explination for what I am seeing?

Regards,

Gregory Mosher


Chris Knight <chris@...>
 

What Gregory Mosher, Mark Connelly, and I are experiencing is an interesting
problem. In the past, an outboard Slider antenna could work to get rid of
overloading/desensitizing, because sliding the coil along the ferrite rod is
effectively tuning the resonant frequency. One would not have that luxury
using a fixed coil. The Si4734 has a tuner circuit, an on-chip varactor,
which tunes the coil to resonance at the particular frequency of interest.
I'm interested in knowing how well that function works in the presence of
strong RF signals.

Chris Knight

-----Original Message-----
From: ultralightdx@... [mailto:ultralightdx@...] On
Behalf Of Chris Knight
Sent: Friday, January 01, 2010 11:51 AM
To: ultralightdx@...
Subject: RE: [ultralightdx] Mixed bag with the Air Core Antenna add-on to
Tecsun PL310

Gregory,

I have the same problem above 1370 kHz when either a G8 or PL-380 is coupled
to a Conti Loop facing south toward Denver. The problem is overloading from
a strong local station, KCKK, on 1510 about 20 miles SSW of me. One of their
lobes is right over my QTH. If I move the Conti Loop to face away from KCKK
or use the Quantum Phaser to phase-null 1510 the problem goes away. When
overloading occurs, the radio's display reads "4900" or "5000" (like you
noted) and it sounds like there's an open carrier. If you couple your PL-310
to a tuned loop my guess is the problem you're experiencing will go away.
Or, you can try pointing the radio in a direction perpendicular to your
strong stations and see if that has any effect.

73,

Chris Knight (N0IJK)



-----Original Message-----
From: ultralightdx@... [mailto:ultralightdx@...] On
Behalf Of gmosherat
Sent: Friday, January 01, 2010 10:18 AM
To: ultralightdx@...
Subject: [ultralightdx] Mixed bag with the Air Core Antenna add-on to Tecsun
PL310

A few days ago I completed a Air Core Add-on to my Tecsun PL310, essentially
I performed the 7.5 Inch Loopstick Transplant as described by Gary DeBock
(BTW Gary - thants for all your work on this stuff - much appreciated!).

I can report mixed results, read on.

Antenna replacement consists of a Amidon 7.5" Type 61 ferrite bar wound with
81 turns of 40/44 Litz wire starting at 43mm from one end of the bar. Wire
is wrapped around the Ferrite Rod first covered by J&J waterproof tape
(Sticky side out). The one difference between my antenna and the one
described in Gary's article is that I used 1 inch width J&J tape rather than
the 2-inch width variety (I can't find the 2-inch stuff locally). Also, the
original antenna was left in the radio, albiet, disconnected.

I only performed one test between the old and new antenna, at 1180 Khz
(local 50Kw station). The new antenna gave me a 40 dBu increase over the
stock antenna.

Pervious to this modification I had logged 115 stations barefoot. In the
past two days (using the new antenna) I have added 15 new stations, not
bad! I suspect not all of these can be attributed to the new antenna, but on
the whole the radio is more sensitive.

Why do I say it's a mixed bag? It seems something is wrong at approx above
1370-1400khz. dBu seems ok, even on frequencies where there I hear nothing
it hovers around 49-50. The problem is dB S/N... It stays at 00 for
anything other that local stations (1370 and 1460). For example, On 1520
khz is a 50kw station 65 miles away. I logged this station prior to the
antenna mod, It came in loud and clear. I can hear it on the car radio, and
other radios in the house just fine. Now after the antenna mod when I
tune in 1520kHz on the PL310 I have to raise volume to the highest level and
use headphones to hear the station. Reading are: dBu: 49, dB S/N is 00.

So.. It seems that above approx 1370-to 1400 Khz dB S/N ratio stays at 00.
Even with this phenomenon five of my new loggings are from w/in this
frequency range.

Has anyone else performed this mod on their PL310 or PL380 and if so are you
seeing the same results?

Any of you out there have an explination for what I am seeing?

Regards,

Gregory Mosher


Chris Knight <chris@...>
 

Hi Gary,

 

I found a website which has simplified resonant frequency formulas manipulated to easily figure out capacitor and inductor values: http://www.electronics-tutorials.com/basics/resonance.htm . In an earlier email response to John B. (dated 12/20) you mentioned a break-even point (1340 kHz.) where below that there was noticeable improvement over the stock coil. Since the coil you used has an inductance of 554uH, I placed 1.34M and 554uH into the formula to solve for capacitance and came up with 25pF (at 1340 kHz). Since we now know the si4734 cannot provide less than 7pF of capacitance, I subtracted 25pF-7pF = 18pF. This is the stray capacitance present in your PL-310. In the si4734 datasheet there’s a note saying “Stray capacitance on antenna and board must be < 10 pF to achieve full tuning range at higher inductance levels.” That would imply these ULR circuit boards are being manufactured with higher-than-optimum stray capacitance. Next, I used 1.7MHz which is where you should need 25pF so the si4734 can tune through the high end of the band. The result was 350uH inductance. Plugging 350uH at .54 MHz into the formula, I got 248pF. Assuming the si4734 can provide 248pF on its internal varactor, you should be able to tune the entire AM BCB with a fixed coil inductance of 350uH and maybe have enough left over to cover a nice chunk of the longwave band.

 

I wonder what would happen if you went from 554uH down to 350uH in this case (PL-310)? If you have already tried it, please let me know. I’m also wondering what variations of stray PCB capacitance we can expect from these Chinese manufacturers.

 

73,

 

Chris Knight

 


From: ultralightdx@... [mailto:ultralightdx@...] On Behalf Of D1028Gary@...
Sent: Friday, January 01, 2010 12:56 PM
To: ultralightdx@...
Subject: Re: [ultralightdx] Mixed bag with the Air Core Antenna add-on to Tecsun PL310

 

 

>>The symptoms you describe DID occur here on the PL-310 experimental loopstick composed of 113 turns of 40/44 Litz wire on the stock ferrite bar, which was tested here as a possible replacement for the stock PL-310 loopstick. This experimental loopstick (documented in a photo album on Ultralightdx) had a coil inductance of 554 uh, and did boost up AM sensitivity significantly on all AM frequencies up to about 1300 kHz. Above 1300 kHz, however, it was inferior to the stock loopstick in performance, with symptoms exactly like you describe. Presumably the coil "Q" of this experimental loopstick (practically covered with Litz wire) was unacceptable at the high AM frequencies.<<

 


jim_kr1s <jkearman@...>
 

--- In ultralightdx@..., "Chris Knight" <chris@...> wrote:


I wonder what would happen if you went from 554uH down to 350uH in this case (PL-310)? If you have already tried it, please let me know. I'm also wondering what variations of stray PCB capacitance we can expect from these Chinese manufacturers.
Good analysis, Chris. I think the length of the leads from antenna to board probably contribute 3-5 pF by themselves. It appears the maximum capacitance the chip's varactor can achieve is between 325 and 375 pF. Let's say 350 pF. Scott was saying 270 uH was about the max for the G8. For the sake of discussion and uniformity across models, let's say 270 uH and 325 pF maximum C. That combination will resonate at 537 kHz. If you minimized stray capacitance then, a 330-uH coil as supplied by Tecsun could resonate across the US MW band. The problem with Tecsun antennas at MW is probably losses in the core material vs -61, rather than inductance.

At the low end of the band where it can be resonated, there is a clear technical advantage to using a larger inductance. The reactance of the coil increases with more turns, more rapidly than does the ohmic resistance. The "Q" is calculated by dividing reactance by resistance. If reactance rises faster than resistance as inductance is increased, then a larger coil will have higher Q. But to obtain the benefit of this Q, the coil must be resonated by the tuning capacitance. (Reactance and resistance are both measured in ohms; check Wikipedia or a copy of the ARRL Handbook for Radio Amateurs at your local library for more info on the differences.)

It doesn't appear that the range of capacitance available will allow a single antenna coil to resonate on both MW and LW. A 554-uH coil could resonate down to 375 kHz or so; as you note, however, it won't resonate at the high end of the MW band. A second coil and a tiny slide switch, to select one or both coils, could do the trick.

73,

Jim, KR1S


Chris Knight <chris@...>
 

Jim,

I thought about the slide switch and believe this could be the right answer
for the reasons you mentioned. The MW band is a pretty large swath of real
estate for a single fixed coil to cover. Another good method of implementing
a change of inductance for lower band (and LW) would be the ability to
easily swap coils. I was thinking of using speaker terminals to make it
"plug-and-play"
http://www.radioshack.com/product/index.jsp?productId=2103795 . A person
could wind coils for different portions of the MW and LW bands and swap them
out easily. I'm using this technique with a 0-365pF variable cap in a
plastic project enclosure that can be swapped among loop antennas.

73,

Chris (N0IJK)

-----Original Message-----
From: ultralightdx@... [mailto:ultralightdx@...] On
Behalf Of jim_kr1s
Sent: Friday, January 01, 2010 5:47 PM
To: ultralightdx@...
Subject: [ultralightdx] Re: Mixed bag with the Air Core Antenna add-on to
Tecsun PL310



--- In ultralightdx@..., "Chris Knight" <chris@...> wrote:


I wonder what would happen if you went from 554uH down to 350uH in this
case (PL-310)? If you have already tried it, please let me know. I'm also
wondering what variations of stray PCB capacitance we can expect from these
Chinese manufacturers.

Good analysis, Chris. I think the length of the leads from antenna to board
probably contribute 3-5 pF by themselves. It appears the maximum capacitance
the chip's varactor can achieve is between 325 and 375 pF. Let's say 350 pF.
Scott was saying 270 uH was about the max for the G8. For the sake of
discussion and uniformity across models, let's say 270 uH and 325 pF maximum
C. That combination will resonate at 537 kHz. If you minimized stray
capacitance then, a 330-uH coil as supplied by Tecsun could resonate across
the US MW band. The problem with Tecsun antennas at MW is probably losses in
the core material vs -61, rather than inductance.

At the low end of the band where it can be resonated, there is a clear
technical advantage to using a larger inductance. The reactance of the coil
increases with more turns, more rapidly than does the ohmic resistance. The
"Q" is calculated by dividing reactance by resistance. If reactance rises
faster than resistance as inductance is increased, then a larger coil will
have higher Q. But to obtain the benefit of this Q, the coil must be
resonated by the tuning capacitance. (Reactance and resistance are both
measured in ohms; check Wikipedia or a copy of the ARRL Handbook for Radio
Amateurs at your local library for more info on the differences.)

It doesn't appear that the range of capacitance available will allow a
single antenna coil to resonate on both MW and LW. A 554-uH coil could
resonate down to 375 kHz or so; as you note, however, it won't resonate at
the high end of the MW band. A second coil and a tiny slide switch, to
select one or both coils, could do the trick.

73,

Jim, KR1S


jim_kr1s <jkearman@...>
 

--- In ultralightdx@..., "Chris Knight" <chris@...> wrote:

The MW band is a pretty large swath of real estate for a single fixed coil to cover.
Everything was swell until they added X band! But I think SiLabs guarantees it will tune the US AM BC band with one coil. If the range of inductances is correct, that varactor has a wider range than any commercial discrete varactor I know of.

By virtue of the higher reactance at 1700 kHz, you're going to get more Q up there, so if the varactor just wheezes its way to the top, it should work pretty well. A given capacitance range will always tune the same number of octaves no matter the inductance. Assuming the chip can tune 530-1710, a 3.23 ratio, it should almost cover the 153-513 kHz LW band. You'd have to sacrifice a few kHz, but chances are the varactor can do a little better. But -- the inductance to resonate with 350 pF at 153 kHz is a whopping 3092 uH! No wonder the little guys don't hear well on LW. Even with a larger winding you'd have to add some loading inductance to make it resonate. You'd probably need about 1500-2000 uH, and a high-Q coil would be rather large. I'm thinking of how large those old 2.5-mH (2500-uH) National rf chokes were, and they had lousy Q by design. (By the way, those old chokes are a good source of fine Litz wire, if you unwind them carefully.)

Here's where your plug-in coils would solve the problem! I think you could get 3000 uH on a 7.5-inch rod.

I also think Tecsun should fly us all over for some Szechuan chicken, a walk on the Great Wall, and a consult on their next model!

73,

Jim, KR1S


Nick Hall-Patch
 

--- In ultralightdx@..., D1028Gary@... wrote:
Recent experimentation here (and by Nick Hall-Patch, and others) indicates
that any change in an experimental loopstick parameter (ferrite bar size
or length, ferrite mix, Litz wire, coil "Q" or Si4734 Radio model) can
change the optimum loopstick coil inductance required to match the DSP chip
varactor for peak AM sensitivity.
********************
As Gary mentioned, I tried a 3" long, 0.375" diameter Amidon type 61 rod with the PL-380, so here are some preliminary results.

I initially used Amidon's 32mH/1000t approximate AL value for this rod, to calculate a 125 turn coil of 10/44 Litz to create Gary's "secret sauce" 554uH winding. However, this number of turns formed about a 750uH coil instead , but I did some quick tests with it anyway, by disconnecting the leads of the stock ferrite antenna and soldering in the leads from the new coil instead. With the coil placed in approximately the same position as Gary's 7.5" rod relative to the PL-380, the maximum strengths of several semi-locals at midday were noted relative to the PL-380 stock core which had been observed a few minutes earlier prior to the changeover to the new coil. Some examples
stock 700uH coil
CFTE-1410 50dBu 57 dBu
KOMO-1000 41dBu 49 dBu
CKNW-980 50dBu 59 dBu
CHMJ-730 49dBu 60 dBu
KIRO-710 35dBu 56 dBu

all of these had a 25 dB S/N ratio using either coil....except KOMO, which dropped to 18dB using the 700uH coil (interesting; strength increase but S/N decrease...remember, big local CFAX-1070 is in the same direction). I tested other weaker signals, but these were showing fading, so I'm not sure how valid the comparisons were. KPOJ-620 and KXL-750 both showed a 5 or 6 dB improvement in signal strength with the 700uH coil, with S/N going from zero to a few dB, but they were fadey, so hard to claim accuracy without a proper A/B comparison. The semi-locals noted above had showed an increase in signal strength as much as 7dB at the top of the band to as much as 11 dB at the bottom of the band.

Now, I reduced the windings to approximately 100 turns, which gave about 550uH on an SRS LCR bridge, all frequencies tested also showed increased signal strength from the stock bar, and the increase was still more impressive at the bottom end of the MW band. Increased strength ranged from 2dB in the X-band through 4 or 5 dB from 1510 down to 1000kHz, and more like 9dB below 1000kHz (12 and 15dB increase was noted on the two stations from Portland, but propagation may have changed in the few minutes between changing one set of ferrite rod leads to the other). These are less than Gary's 7.5" rod improvements, hardly surprising, given the 2.5x difference in the length of the long rod, and its larger diameter. What is of interest, is that they are also somewhat less than the signal strength differences noted using the 700uH coil on the same rod.

What was quite important however, was position of the rod relative to the radio. If the rod was snugged up against the back of the radio (this is sort of approximate), the signal strength dropped, if it was placed above and away from the radio like the 7.5" loopstick was, signal strengths were at their best. I suspect this may have to do with effect of proximity of the radio's circuitry to the loop, which could drop the "Q", and therefore the voltage swing available from the loop. The RLC bridge I used to measure my new coil has a "Q" indication, and even though the bridge itself has a plastic case concealing its circuitry, if I left the coil on the case next to the test leads from the instrument, Q was about 70. If I propped the coil up a couple of inches above the case, Q went up to above 100 (Q is measured at the 100kHz clock frequency of the bridge, I believe, so not entirely relevant at MW, but the ratio of 100 to 70 is probably significant). As comparison, the PL380's stock rod in place measured 227uH with a Q of 23.5, so its location probably is not helping it any so far as Q is concerned.

So, Gary might want to move his rewound PL-310 core away from the radio (or even move the original one) and see if he gets improved signal strength!

I also had another 3" 61 type core left over from some other project with a 200uH coil scramble wound on it (all Litz used was 10/44), and tried it with the PL-380, and there were gain improvements over the stock rod even with that (but with the 3" rod away from the radio), and the improvements weren't wildly different from that observed with the 554uH coil. Makes it look as if the length and diameter of the rod had more effect than the inductance.

My results seem to imply that the Q of the coil is an important parameter, not necessarily just the inductance. (or at least the Q as influenced by the core's position relative to the radio; it doesn't seem the inductance changes with position), At the moment I don't understand that, as the Q is most important in a resonant circuit, and I don't see how a 750uH core can be resonant with this radio at the high end of the band. It makes sense that too high an inductance will not allow the varactor to peak the coil; but maybe the extra turns deliver enough extra voltage to overcome the lack of peaking???

I'm looking forward to Gary's slider experiments with a 3" rod.

best wishes,

Nick