TopBand: loops & preamps

[Peter Chadwick]

Tom asks

>How do you avoid unwanted common mode excitation of the push-pull 
>follower? High input Z amps, especially those with ground referenced 
>supply and signal leads (generally the case with tubes), require 
>special precautions.


Careful balanced construction, with a dual gang capacitor motor driven
to tune it. Vertical effects (common mode) are very low, as evidenced by
the symmetry of the pattern when I rotate the loop. That's a good test
for a loop - vertical pick up leads to either asymmetry (the nulls
aren't 180 degrees apart) or to the nulls being of different depths, or
both. The maximum effect of vertical pick up, of course, is having
enough  to give a cardioid pattern, as is used for sensing in D/F. Small
amounts of asymmetry can show up as semi-circular, quadrantal or
octantal error.

Having said all that, although it works, I'm not convinced that it is as
good a receive antenna as could be arranged. Any comments on what the
polarisation of top band DX signals is at the rx end? Is it possible
that a horizontallly polarised antenna would be better? Or do I just
have too low a ratio of signal field strength to noise field strength?

73

Peter G3RZP
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