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



For nulling, I recommend starting with Jim K's hoop-loop design. I think that further experimentation may improve upon his balun transformer design, but clearly, Jim's already getting great results. I'm no expert in AM antenna design, but I think a reasonable air loop will be mostly limited in nulling by the transformer design and by keeping the loop away from nearby metal.

According to results in this very interesting article, http://www.kongsfjord.no/dl/Antennas/Loop%20Antenna%20Sensitivity.pdf , I would stick with a maximum of a two-foot loop to start, maybe even one-foot. Bigger loops are unlikely to give better signal-to-noise, and will be harder to manipulate for nulling.

Again, I don't have a lot of experimental experience with AM antennas, so my opinions should be taken as informed, but not authoritative. It seems to me the big-loop with inductive coupling is just as viable for catching difficult signals. The excess size doesn't give more SNR, but adds some signal strength that compensates for weaker inductive coupling. Clearly, Gary gets strong results using his big loops. The reason I recommend Jim's loops is for operational convenience. They are easily manipulated and tuning is automatic.


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

Scott - I have the opposite conditions in my area [CT]. There are no nearby 50kW stations, but there are signals on almost every AM channel, and many with more than one on the same frequency. I am enjoying the radios with highly directional ferrite rod antennas, but I am also interested in making a large [perhaps 3-4 foot] air loop, and wondered what factors would make it more directional for DXing, and LESS directional for regular listening? I am asking for an opinion, and I am sure experts will disagree. - FARMERIK

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


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

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

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!


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