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.
I'm sorry to say that , but the concept is wrong, the TX antenna resonance
as structure has nothing to do with the impedance. Floating the element
makes it 1/4 element, keeping it connect it works like a 1/2 wave resonant
antenna.
I presemted some solutions on my Webnair at wwwrof, still available and you
can download the slides with extra information. Like PY2XB inverted V when
disconnected from the feed line drops the noise 2 S units on the RX antenna.
Changing the impedance you can notice a change in the noise but only few
dbs.
JC
N4IS
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