[Elecraft] Inter-K3 interference

[Howard Hoyt]

Hi folks,

For Field Day we have a K3 with the upgraded synth and a K3S, each has a 
3rd order Cauer elliptical bandpass filter but they are operating with 
antennas in close proximity.  The only interference we can see is when 
either one is transmitting on 40m and the other receiving on 20m.  
Obviously we are listening far away from 2nd harmonic, which comes in at 
S9+20 dB, but we get S7 interference everywhere else in the 20m band 
with what sounds like composite or phase noise.

It would seem that, if the transmit composite noise was being bandpass 
filtered along with the desired transmit signal, then 20 m should be ~60 
dB down off the passband.  We have tried adding a second bandpass to 
each with zero change to received noise, and if it was indeed received 
noise you would expect it to be reduced even further.

It is not common-mode currents, several large Mix31 cores with multiple 
turns on feedlines did not attenuate it a bit.  The rigs are on separate 
power supplies and have no galvanic connection to each other, even with 
the ground lifted there is no change.

Any ideas?

Thanks in advance,
Howie / WA4PSC