Paul,
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
relay.
I did it this way during many years with a TX antenna 5 meters away from my
K9AY loops.
Kees, PE5T
--------------------------------------------------
From: "Paul Ferguson" <p...@paulferguson.us>
Sent: Friday, September 30, 2016 9:10 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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