Topband: Modeling Transmit Antenna Affect on Rx Antenna Performance?

Paul Ferguson Paul at
Fri Sep 30 15:10:04 EDT 2016

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?

Paul, K5ESW

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