[Elecraft] RFI issues

[Jim Brown]

On 2/20/2020 10:22 AM, Victor Rosenthal 4X6GP wrote:
> Not so fast. The K3's T/R switch can be overwhelmed by very strong 
> signals. The result is intermod junk all over the bands. This happened 
> to me with a 50 kW AM BC signal at about 1000 kHz that is line-of-sight 
> to my antenna. I don't see why a super-strong VHF signal couldn't have 
> the same effect.

AM BC signals are far more likely to cause issues because 1) they are 
much closer in frequency (as a percentage) than VHF/UHF, so 2) our 
antennas tend to be far less efficient receiving them, and 3) feedline 
loss is high at VHF/UHF, due both to normal increase in loss with 
frequency and because the feedline is poorly matched to the RX at 
VHF/UHF. Also, AM BC transmitters often combine in non-linear junctions 
to produce IMD that ON_FREQUENCY in our bands. I first experienced this 
more than 60 years ago, when local 5kW stations on 930 kHz and 1470 kHz 
put a monster product on 3870 kHz, wiping out the WV AM net on 3890.

The IM often occurs in the TX output stage; because FM and TV TX 
antennas are often in close proximity to other BC and 2-way TX sites, it 
is common for these TX to include a device called a "circulator" that 
prevents signal from another TX to backfeed into their own output stage, 
where rectification can occur, but it is far less common for AM stations 
to include comparable networks. In addition, of course, rectification 
can occur almost anywhere.

73, Jim K9YC