Dear J.B., LF group,
>I think the problem here is the
size of the coupling loop, which is only
>slightly smaller diameter than
the main loop.
My antenna is based on the
successful design by Steve, VE7SL, with
only two differences, 1) my pickup
loop is shielded, and 2) I used #12
wire rather than #20. Could
those two differences make that much of
a change in the loop
operation? I know that all other things being the
same, using the larger gauge wire
would have raised the Q, but would
it change things so drastically that
I'd have to use a smaller diameter
Steve suggested that I use a simple
non-shielded pickup as he used
on his antenna. I tried that,
but I found little or no difference between
it and my shielded pickup.
Both where spaced about 6 inches inside
the main loop, just as on his
antenna, which works fine. That's what
is puzzling me, why does
it work OK for him but not for me??!!
I don’t think the alterations in
loop construction would make a major difference – the loading of the loop
is not very critical provided it is in the right area, changes in Q will make
some difference, but not a huge difference. There are other things that can
make a big difference. One is receiver sensitivity – loops produce
relatively small output so require an RX with good sensitivity (or a preamp).
Amateur-type RXs vary enormously in sensitivity at LF, a few are quite good,
most are lousy, 20dB or more down on their HF performance. So this is a
possibility, unless you have measured the sensitivity and know otherwise.
Another possibility is that the input impedance of yours and Steve’s RXs
are greatly different, producing different loading impedance on the loops.
The tuning adjustment should give you a
very sharp peak – if not, either the impedance matching is drastically wrong,
or something else in it’s construction is reducing the Q of the loop.
If I go the transformer
route as you and several others have suggested,
do I need to physically remove the
pickup loop or simply disconnect it?
The coupling loop should make little
difference provided it is left open circuit.
Cheers, Jim Moritz
73 de M0BMU
