Re: Connecting the Tecsun PL-360 to a large air-core loop
Hi Kevin,toggle quoted message Show quoted text
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: