Paul, I suggest you talk to Gary KD9SV as he built a Hi-Z around his TX
antenna.
Incidentally the ground radials under the Hi-Z is not a good idea. I suspect
you would have to use NEC4 to see the real effect of the radials. I have not
made a test myself to see how close radials could be. Disconnecting the T
element on one band like 160 will work for that band but likely will not work
the same on 80 meters. Leaving it connected might be better. Same situation
with the feedline impedance looking back toward the radio. That may change with
band as well.
Lee K7TJR
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Paul Ferguson
Sent: Friday, September 30, 2016 12:10 PM
To: topband@contesting.com
Subject: Topband: Modeling Transmit Antenna Affect on Rx Antenna Performance?
For transmit I use a 160M T antenna with on ground radials. I am considering
installing a Circle-8 receiving array such as one made by Hi-Z. The receive
array would be very close to the T, maybe within 100 feet. I am using EZNEC to
see how the transmit antenna proximity affects the pattern of the Rx array.
I have a simple EZNEC model of a 4 square Rx array. The model uses a source on
each 20-foot Rx vertical, with the pattern determined by the phase of each
source.
I put models of the T and the Rx array in the same EZNEC model. With the source
removed from the T, the radials are connected to the T portion of the antenna.
This configuration shows a big degradation in Rx front to back ratio. If I open
the T at the feed-point by either putting in a source with zero current or
simply breaking the connection between the T and its radials, running the model
shows little to no affect on the Rx pattern.
I could use a relay at the antenna to disconnect the feed line near the
feed-point when not transmitting, but I would rather avoid the relay because of
running QSK and wanting to avoid the wiring run.
My modeling runs, using either an effectively open or shorted feed line at the
T, do not represent my real system because it is not open or shorted. The feed
is about 350 feet of coax to a K9YC-designed common-mode choke and a 1:1.56
unun. I am wondering if there might be value in disconnecting the feed line
from the antenna and measuring the impedance looking back into the
unun/choke/coax string. Take this impedance and add it as an RLC network
connected across the feed-point of the T antenna. My thought is this would be
close to what the Rx array is really seeing when I look for the affect of the
nearby T in the EZNEC run.
Am I on a decent path or lost in the woods?
73,
Paul, K5ESW
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