Topband: Modeling Transmit Antenna Affect on Rx Antenna Performance?

[Lee STRAHAN]

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 at contesting.com] On Behalf Of Paul Ferguson
Sent: Friday, September 30, 2016 12:10 PM
To: topband at 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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