KCBQ overloads PL-380 (Re: Tecsun PL-360 3" Loopstick Testing)


pianoplayer88key
 

Ah... but one thing is I notice that my noise floor when I'm farther from the stations is 15dBu (the minimum it will display - I've seen stations at that level with a 15dB SNR sometimes). Am I asking too much to be able to have that 10+ kHz away from a local 50kW pest a few miles away?

And yes, an external directional antenna would not help for the walking around operation. I wouldn't mind better Q though so I can pull in weak stations under the elevated noise floor in my area.

My sat seems to do ok with coupling with the PL-380, but I still can't seem to get much of anything on 1150 KTLK (sandwiched between 1130 KSDO and 1170 KCBQ) or 1180 KERI (next to KCBQ), for example. Am I asking too much to be able to hear a signal that is maybe 0.5┬ÁV/m (or whatever level would be 6 dB below the level of atmospheric noise, and I do believe it is possible to hear stations a little below the noise floor) 10kHz removed from one that is, say, 1000mV/m?

BTW did you hear the recordings I posted in the previous message? What would be causing the PL-380 to behave the way it was when faced with a signal like that?


What originally got me to thinking my PL-380 wasn't quite as selective or overload resistant as I had thought, was a trip I took to my grandparents house in San Gabriel, CA, about a month or two ago. I'll post a partial bandscan, but I should mention I didn't log every single frequency. You will see some with an elevated noise floor where I could ID a station (that in many cases themselves would have been a local city-grade signal), but the SNR was 0dB. These were done in 1 kHz bandwidth, barefoot.

530 - 30,14 - WNHV296
540 - 39,14 - XESURF
570 - 63,25 - KLAC
590 - 40,25 - KTIE
600 - 42,25 - KOGO
620 - 30,00 - XESS (faint)
640 - 63,25 - KFI
670 - 37,22 - KIRN
690 - 48,25 - XEWW
710 - 63,25 - KSPN
740 - 58,25 - KBRT
760 - 39,00 - KFMB (faint)
790 - 58,25 - KABC
800 - 43,00 - XESPN (weak)
820 - 35,00 - TIS in Monrovia
830 - 61,25 - KLAA
860 - 39,16 - XEMO
870 - 63,25 - KRLA
890 - 41,00 - unID'd weak signal
900 - 60,25 - KALI
920 - 41,00 - weak image of 1430
930 - 47,25 - KHJ
950 - 43,25 - XEKAM
960 - 45,00 - KIXW? (weak)
980 - 63,25 - KFWB
1020 - 63,25 - KTNQ
1030 - 47,00 - XESDD (better when tuned to 1031 or 1032 (in 2 kHz mode)
1070 - 63,25 - KNX
1090 - 49,18 - XEPRS
1110 - 63,25 - KDIS
1150 - 60,25 - KTLK
1170 - 50,00 - KCBQ (weak)
1180 - 50,00 - KERI (weak)
1190 - 53,25 - KXMX
1230 - 50,00 - KYPA
1260 - 50,00 - KGIL
1280 - 50,00 - KFRN
1300 - 63,25 - KAZN
1330 - 50,00 - KWKW
also possible stations on 1350 and 1370 but my list doesn't comment much. I think I may have heard a carrier or something but I can't tell.
1390 - 50,15 - KLTX
1430 - 63,25 - KMRB
1460 - 50,00 - KTYM
1520 - 50,00 - image of 1430
1540 - 53,25 - KMPC
1560 - 49,00 - image of 1430 (although on that frequency I'd rather hear KNZR, even though radio-locator says that location is outside their coverage by about 2-3% or so)
1580 - 49,25 - KBLA
1600 - 50,00 - image of 1300
1650 - 45,00 - KFOX
1670 - 49,00 - KHPY

I should mention they live a few blocks away (0.345mi) from a 50kW (daytime) transmitter on 1430, KMRB. Here's the readings for some of the harmonics:
2nd - 2860 - 59,25
3rd - 4290 - 48,16
4th - 5720 - 37,01
5th - 7150 - 28,01
6th - 8580 - 27,00
7th - 10010 - 25,00

So what can I do to be able to get better barefoot reception of stations close to 1430 when I visit them? I won't quite go so far as to expect to get the groundwave from TISs across the pond on 1431, though. ;)

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




Stephen,

Thanks for posting your thorough band-scan data. It is very interesting
and helpful.

It is always possible to overload the front-end of any radio (except
possibly a crystal set). So the lesson from that is: avoid doing it --
excess gain is unneeded and unproductive. Most of the time, and
especially in urban situations, you cannot improve the noise floor
by elevating all signals.

Sometimes, you can get a marginal signal by more selective antenna gain.
But this involves raising the desired signal while reducing a pest. Two
methods are viable: a tuned, frequency-selective antenna or a directional
antenna.

The tuned approach is very helpful, and the Si4734's automatic tuning
takes full advantage. Like all methods, there are limits. Frequency
selectivity will not help by more than a couple dB when a pest is only
a channel or two separated from the desired. Technically, the Si4734
should support antenna resonances with a maximum Q of roughly 100. In
the stock PL-380, the Q is around 25. This means you will get only
about 3 dB reduction of a pest 40 kHz away. But pests further away
are attenuated more, and in an urban environment, chopping down several
pests will help a lot. The resonance will also help a lot at the
upper end of the band, reducing 2nd harmonic problems from pests low
in the band.

External antennas, like the SAT or Q-Stick+, can also help with frequency
selectivity. These can achieve higher Q resonance and that resonance is
added to the resonance already in you radio. My experience, however, is
that a Q-Stick+ does not couple well to the stock PL-380 antenna. I
don't have a SAT to compare to. I am currently experimenting with
transformer-coupled loops (like Jim K's hoop-loop, but smaller). My loop
couples great to the Q-Stick+ and I see excellent results, usually in the
form of greatly improving the SNR of a station. What is really interesting
is that the improved SNR happens simultaneously with _lowering_ the RSSI
strength! This is consonant with the idea that we don't always need
more signal, but we often need less of an undesired signal.

Directional antennas can help you sort desired stations from pests that
are close in frequency. So in an urban environment, your best friend is
an antenna with good nulling. So concentrate on nulling setups and stay
away from power poles! Unfortunately, directional antennas will not help
you in your desired "walking around" operation.

In examining your band scans, I don't really see any anomalies with your
radio's operation. It's unfortunate that the PL-380's display puts a
63 dB ceiling on RSSI measurements. I was hoping to get a better idea
of the magnitude of your locals. But in the situations where your radio
is not overloading, the RSSI at frequencies adjacent to strong stations
looks normal in my opinion. That is, seeing 30-35 dB RSSI in the slot
adjacent to a 63+ dB station is normal. I generally see 30-35 dB noise
floor during the day with 4-5 stations that are between 60 and 70 dBu.
At night, the noise floor can get higher. In more rural areas, the noise
floor will drop.

A good deal of the noise floor is just the RF environment. Some call it
"splatter", but that's a bit unfair (IMO). A 30 dB drop in signal strength
means that 0.1% of a transmitter's power is "escaping" out of band. At
40 dB, you're talking about 0.01%. Again, probably the most fruitful
method of working near a pest is a sophisticated nulling setup and skillful
operation.

Regarding your question about my location, I'm in Austin Texas, about
two miles from downtown. There are transmitter towers in all directions,
but power levels are not super high. I see about eight stations between
50 and 70 dBu during the daytime, and a "noise level" of 30 to 35 dBu.

I got my graduate degrees at UCLA, so I'm familiar with So Cal and have
been to San Diego many times. It's a wonderful place to live, except
for the politics, which I'll avoid commenting on further!

-Scott-

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