KCBQ overloads PL-380 (Re: Tecsun PL-360 3" Loopstick Testing)
--- In ultralightdx@..., "sdwillingham" wrote:
> 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.
I'm very interested in seeing what you come up with. If one could design the loop antenna mechanics to suit, rather than using a prefabricated frame, an inductance of 15 uH would be ideal. A 4:1 turns ratio would transform that to match a 240-uH tank coil for MW.
The Hoop Loop worked out to 16 uH, with either magnet wire or Litz wire. I suppose a 4:1 turns-ratio transformer with slightly higher-inductance (256 uH) secondary wouldn't hurt much. It would probably stop resonating at about 1660 kHz, and I've logged just about everything above there anyway. The problem is, I don't know how to confirm any improvement. Here's what I have now, for MW:
A 240-uH inductance wound on an FT114-61 toroid, turns spaced so the winding covers about 2/3 of the core. One layer of electrical tape over that winding. The primary winding is spaced to cover just about all of the secondary, the idea being to couple as much primary-secondary as possible. (You suggested placing the primary separately; I believe winding it nearer the ground side of the secondary is better as it reduces capacitance between the windings?) The primary winding was calculated, then tweaked for maximum signal strength, confirmed at three points across the dial at 530, 1000 and 1700 kHz. My assumption here was that maximum signal strength correlated with best impedance match between source (antenna) and load (receiver). The antenna is connected to the transformer through six feet of 22-gauge "Zip cord." A mono 1/8" plug/jack provide the connection. At the antenna, the feed line is soldered to the pads where the loop winding terminates. The transformer is currently positioned directly above the receiver
I'm able to position the Hoop Loop several feet from any metal objects to any side. This morning I ran a series of null tests on 530 and 1700 kHz. (I know that the anti-null lobes of a small loop are too wide for accurate determination of peak direction.) I ran the tests several times because even nearby stations fade. Positioning the loop for the deepest null and rotating it 180 degrees (positions marked on the turntable), SSI and S/N indications averaged within less than 2 dB, some of which I attribute to slight fading.
Then I selected some stations with SSI about 40 dBu. Comparing SSI at null with SSI when the loop was turned 90 degrees in either direction, null depths averaged 14 dB, with range 12-16 dB. This was worse than expected, but articles about small loops rarely mention null depth; they're all concerned with sensitivity, which isn't the proper function of a small loop.
When I had the LW transformer in place, I did similar tests on two local NDBs, with similar results. Just now I had the idea that disconnecting the antenna, and connecting a random-length wire to each primary terminal in turn should yield identical results. Aha! One side is slightly stronger than the other. So now I have an empirical method to compare transformers. My microwave oven converted itself into an IED (I love the smell of ozone in the morning) so I won't get to this right away. I don't have anything more sophisticated than a signal generator and scope, and can't think of a way to actually measure the balance with the transformer isolated on the bench, as both primary terminals have to be above ground. The only way I can think of is to fix the transformer above a ground plane and connect the generator to each terminal in turn, the generator common lead going to the ground plane, but that introduces capacitive coupling between ground plane and transformer. Further suggestions greatly appreciated!