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


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.


--- 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

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