Nulls are created by cancellation of signals that very nearly
equal and very nearly 180 degrees out of phase at the point of
observation. For the null to be 20 dB deep, the field strength of
the two signals must be within 10% of each other, which is only
0.9 dB. Thus, very small changes in one element of a directional
array can make large changes in the field strength in null
directions.
Adding the radials to only one antenna could easily increase the
field strength of that tower by 0.5 dB. That would significantly
reduce the depth of one null and increase the depth of the other
one, and/or shift their azimuth and/or elevation.
Another way of thinking about this is to realize that two signals
of equal strength can add by only 6 dB (precisely in phase), but
they could, in very well controlled systems, cancel to -infinity
(precisely equal and precisely 180 degrees out of phase).
73,
Jim Brown K9YC
--Original Message Text---
From: W8AV@aol.com
Date: Fri, 13 Apr 2007 13:57:46 EDT
Jim:
I really do not thing that adding addition radials on the null area of the
pattern should not make that
much of a difference in the null area either. I agree that nulls are
important, that's why I have an 80
meter array. :-)However, if you take a field strength meter out in the field
and measure the nulls of the
pattern before lengthening the radials and then again after you lengthen them,
I doubt that you will see a
lot of change in the radiated field.
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