Received several emails about tuning the rearwad null on K9AY loops,
and several folks mentioned they had seen the same thing using NEC 4 models.
A couple asked for plots and the model, those are on my website.
K5QY added the series C and lowered value of R last night and said that his
initial impression is that it helped the F/B a lot on a lower-angle signal.
There is a value of R that needs no compensation, lower than that value of R
lowers the angle above the horizon of the notch, but requires capacitance to
compensate. Higher than that value of R raies the angle of the notch, and
requires
inductance to compensate. As R gets lower and lower, the capacitive reactance
needed gets more - lower value of C means more reactance.
I've posted the NEC2 model:
http://www.tapr.org/~n5eg/index_files/antenna/k9ayLoop-original.nec
And 4 plots (jpg). Note that 4NEC2 plots show the angle from the zenith, the
angle from the horizon is (90 degrees - angle).
Notch at 15 degrees above the horizon:
http://www.tapr.org/~n5eg/index_files/antenna/K9ay_orig_300R_750PF.jpg
Notch at 35 degrees above the horizon:
http://www.tapr.org/~n5eg/index_files/antenna/K9ay_orig_350R_1000PF.jpg
Notch at 50 degrees above the horizon:
http://www.tapr.org/~n5eg/index_files/antenna/K9ay_orig_420R_No_X.jpg
Notch at 60 degrees above the horizon:
http://www.tapr.org/~n5eg/index_files/antenna/K9ay_orig_500R_9UH.jpg
-- Tom, N5EG
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