Topband: Modeling Transmit Antenna Affect on Rx Antenna Performance?

Kees Nijdam clnijdam at
Fri Sep 30 16:18:23 EDT 2016

Your feedline is about 350 feet. That is (with RG213 -velocity 0,66-) close 
to 1 wavelength. With 1 wavelength feedline, the TX antenna is floating when 
the feedline is open in the shack.
Depending on your transceiver configuration (if the TX antenna is completely 
decoupled when using the RX antenna input) you may not even need a special 
I did it this way during many years with a TX antenna 5 meters away from my 
K9AY loops.

Kees, PE5T

From: "Paul Ferguson" <Paul at>
Sent: Friday, September 30, 2016 9:10 PM
To: <topband at>
Subject: Topband: Modeling Transmit Antenna Affect on Rx Antenna 

> 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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