Dan McGillis wrote:
Can that do a decent job of nulling a 40m sky-wave signal?
Not sure I understand. I don't operate 40 very much, so don't keep track of its propagation. I can definitely null by rotating the loop.
A ground-wave signal comes in at, theoretically, zero degrees of elevation, and would be broadside to the loop plane. A sky-wave signal will arrive at angles somewhere above about 14 degrees. I'm sure you could null sky-wave signals; I just wonder how useful that capability is on HF.
that let the radio tune the antenna, or at least put the antenna tuning at the radio.
Yes, now that I see how sharply the antenna cap tunes, I would like remote tuning. How about a bias-Tee and a varactor diode(s) at the antenna? should be easy to do. I probably want to keep DC current out of the input side of my input RF transformer ( a re-worked 10.7 MHz IF can). That should be easy to do with blocking caps. Maybe difficult to cover two bands that way, however. I have bandswitching for 40 and 31 meters. Not sure I want one big varicap to try to cover all of that?
The bias-tee would work, and a 0.1-uF would block dc from the rf input transformer. The advantage of converting the impedance down, then up again at the receiving end, where the tuning cap is connected, is that you don't have to bother with all that.
Instead of a coupling loop, the higher-inductance loop is transformed down in impedance, say with a 2:1 turns-ratio (not critical if both transformers have the same ratio) transformer, T1. T2 is identical except
it has an additional secondary to present low Z to the receiver,
is connected in reverse, so the low primary impedance is transformed to present the same impedance to the tuning cap as does the part of the loop you're currently tuning,
and has a grounded, center-tapped primary. That aids balance but it may not be necessary with your loop setup. If you aren't hearing stations with the feedline shorted at the antenna end, you don't need the grounded centertap.
While the turns ratio isn't critical, the primary of T1 and the secondary of T2 should have the same inductance as the loop. I'm thinking about 30 turns on a T50-2 core. Wind the smaller windings over the center of the larger ones. Whatever you're doing to change bands at the loop end now, you can do at the high-Z secondary of T2. Now you can stick the loop wherever you want it and tune from the radio end.