Guy said:
<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.>
Very well said and worth repeating. This is exactly the reason for my occasion
to measure the phase and amplitudes of received signals. While the radial
effects I measured were small on the top and heavily bottom loaded elements
ala W8JI the effects were sometimes large on both Hi-Z and transformer
coupled elements. The transformer fed elements were all over the place and I
deemed them not worth more time.
I did not get a chance to research in depth the top and bottom loaded
elements. I can certainly agree on having the radials around a receiving
antenna
very evenly spaced and electrically the same lengths. I was able to observe
several degree changes as the radials were moved around physically at the
base of an antenna.
What I found was that with Hi-Z antennas there was more stability with no
radials at all, just a ground rod. Using radials often caused several degrees
of
phase shift and several percent of voltage change. Completely unpredictable
as they were moved around. What I also found was that the ground
characteristic differences were observable as they could cause several
degrees of phase and several percent voltage change while using ground
rods or radials. After adding water around the base/radials of one antenna
and making measurements I concluded that the ground characteristics
indeed have a profound affect on the accuracy of an array.
The one really good thing I did uncover was that with Hi-Z elements and a
single ground rod in very fertile even farm ground amongst the sprouting
wheat I could repeatedly measure about +/-one degree phase and about
+/-1% voltage error between two elements. Accurate enough for the best of
arrays. Out of curiousity I may set up again in the field and make a few more
measurements. The first time was a big learning experience that took a lot of
time. Perhaps in time, knowing the range of some of these effects will help us
decide why arrays sometimes just refuse to work as good as we think they
should.
Lee K7TJR
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