[Lowfer] Accidental Receive Antenna - What's Happening

[N1BUG]

Hello Chris, Andy, Lower

>   I believe you made the TX antenna resonant on or around a frequency
>   where there was significant RX noise and it "sucked out" some of this
>   by coupling to the RX antenna. Paul, N1BUG posted on the LF reflector
>   about a similar phenomenon where his TX antenna de-tuned or weakened
>   his RX ability on a different vertical RX aerial unless the TX one
>   was disconnected and / or grounded.


Actually with the TX antenna resonated, it increases noise in the RX 
antenna.

My TX antenna is a 90 foot vertical with capacity hat for top loading, 
resonated by a large coil at the base. The RX antenna, about 50 feet 
away, is a 30 foot un-tuned vertical or so-called LNV.

What I found is that I get better RX S/N when the TX antenna is 
de-resonated, that is to say not resonated on or near the RX frequency. 
When the TX antenna is resonated on the RX frequency, it is possible 
that noise and interference conducted on the coax going to the TX 
antenna are strongly radiated by that antenna and then picked up by the 
RX antenna. It is also possible the TX antenna picks up more noise and 
interference from the surrounding area due to its larger size and 
height, then re-radiates this into the RX antenna. Either way, it is 
undesirable.

When my TX antenna is resonated at 2200m and I am receiving on 2200m, I 
have a lot of discreet "carriers", squiggles and other interfering 
signals on the receiver. When I de-resonate the TX antenna the overall 
noise floor decreases nearly 20 dB, many of these local interfering 
signals go away, and signal to noise ratio of WSPR signals increases 
several dB.

I have now added a relay at the base of the TX antenna so that I can 
de-resonate it from the operating position. The relay, when energized, 
disconnects the bottom of the TX antenna loading coil from the secondary 
of the impedance matching transformer. This disconnects the TX antenna 
ground connection, which is made through the impedance matching 
transformer secondary winding. I have not measured the self-resonant 
frequency of the antenna in this configuration but I believe it to be 
well removed from 137 kHz (higher).

73,
Paul N1BUG