Rolf,
The likelihood of this happening depends on the width of the
null, which is a function of the spacing between the two
antennas (main antenna and noise antenna). If I remember my
antenna theory correctly, wider spacings will generally
produce sharper nulls. The wider the null, the larger the
blind spot that you will have. For a given angular
separation between between the offending noise source and a
desired signal, the degree of Signal to Noise (SNR)
improvement will depend on the amplitude taper of the null.
For example if the taper of the null is 5db/degree and the
two sources are 3 degrees apart in azimuth, you should be
able to improve the SNR by about 15dB. In some cases you
will actually be putting the desired signal into the null,
but not as far as the interfering signal. Its kinda like
tuning for maximum amplitude slope. This might expain why I
sometimes get the best SNR on a signal if I don't null the
noise completely. This is all theory and conjecture of
course. Good luck with your ANC-4. I would be dead in the
water on topband without mine.
73 de Mike, W4EF
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