[Elecraft] K3/100 RF Feedback/Spurious oscillations ?

[Stewart Baker]

Very strange effect noticed today when tuning up on 20m.

At power levels above 20 w on 14.175 my external Palstar SWR meter 
would suddenly leap from 1:1 to above 5:1. Also the K3 SWR display 
showed similar. High current was also reported. No problems with 
either a TS850S or K2, both OK up to 100W.

Switching the RX Ant switch to the main antenna or removing the RX 
Antenna  BNC lead cured the problem.

I use the RX antenna input for 2 x Pennant loops for the LF bands, 
but find that they sometimes give a better SNR than the main 
antenna on higher bands. The main antenna system is a doublet fed 
with open wire feeders into a fully balanced (no toroid on output) 
ATU.

My initial thoughts is that there might be some spurious output 
generated by an obscure feedback path from the RX antenna input. 
If this output were on a frequency well separated from the desired 
then it could show up as as high SWR.

What think Ye ?

73
Stewart G3RXQ