The use of a phased array for receive for *receiving*, where the main
purpose is rejection of an unwanted signal vs. a wanted signal, changes the
purpose of the array from the max gain of a TX antenna to the *max
rejection* of a RX antenna.
On transmit a small change in phase may only change the max gain by a tenth
of a dB, while a similar change in a rejection direction may lose dB's of
rejection of unwanted signals.
One source of change in "radial induced phase change" is the ground-induced
variation in velocity factor of signals riding the radials. We have directly
measured velocity factor of wires laying on the ground (dipole on ground)
down to and occasionally below point 5 in the general Raleigh-Durham area.
Another source of change is the degree of "shielding" provided between the
vertical portion and the ground immediately underneath by the radials,
This suggests a need to "equalize" radials in an RX array. One would need to
measure resonance of each radial IN PLACE (not make them resonant at the
operating frequency), and adjust lengths to bring them to the same
resonance, and provide an equal radial count at each base.
We have instances of a "dipole on ground" measuring .75 vf at one point and
.55 at another on the same residential property, separated by less than the
typical vertical spacing used in these RX arrays.
With that kind of variation, it would seem that the ground itself would be
capable of inducing enough variation to undo useful nulls that would
otherwise contribute to rejection of unwanted signal voltage.
73, Guy.
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