First, the rest of the story on my RX antenna...

Last winter I used a 9m tall "low noise vertical" for receiving:

http://blog.n1bug.com/2017/11/21/

It was only about 12m away from the TX antenna. It seemed to work OK
for a omnidirectional antenna.

This Spring, when I moved it away from the TX antenna, it stopped
working. The signal level from the antenna was too low to amplify. I
ended up with the noise floor being largely noise of the preamp itself.

Today I put the RX antenna back in the original location. Output
level from the antenna increased more than 20 dB to the same level
it had been last winter. My LF transmitter is not ready so I made
some isolation tests at low power with a signal generator.

First I connected the signal generator directly to the receiver and
verified calibration is linear, eg. for every 10 dB change in
generator output level I see exactly 10 dB change at the receiver. I
calibrated the receiver so that it would read the same dBm level as
the signal generator.

Next I connected the signal generator to the TX antenna and measured
signal level from the RX antenna into the receiver. Isolation is
only 21 dB!

This means that during last winter when I was transmitting with 200
watts I probably had about 1.5 watts coming into the receiver.

Paul N1BUG