[Elecraft] 12 meter K3 RFI from nearby FM station

[pa4la]

Hi Jon,

What you experience is mixing of the FM BC frequency  with the 3th, 5th, 7th etc. harmonic(s) of the K3's  VCO.
If the mix product result is 8,215 Mhz you can hear it.

I'm 3 km away from a BC tower hosting 10 station with >20kw each.
Used SDR-radio  on the K3-IF at 8,215 Mhz to identify the FM stations.
That was easy, stereo and even RDS could be decoded via the K3-IF.
the "mixing" frequencies could quickly be found.

You can calculate where 91,1 pops up

For the 3th VCO harmonic
((BCfreq-8,215)/3)-8,215 and
((BCfreq+8,215)/3)-8,215

That is 19,413 Mhz and 24,890 Mhz (you already knew :) )

For the 5th  8,362 Mhz and 11,648 Mhz
For the 7th  3,626 Mhz and 5,973 Mhz
And so on,
With a strong enough signal from a generator, mixing of the 11th harmonic could be detected.

I have only seen the BC mix phenomena when using the RX-in with MainRx and the SubRX on separate (wideband) antenna.
When using the RX-in, the K3 LPF  is bypassed and with the Subrx, there is no LPF in the KRX3 !
The LPF in the main antenna circuit has enough attenuation to stop BC FM leakage to the K3 mixer in my situation.

Since you have such a strong signal on 91.1 Mhz
Check if the products can be heard with the suggested external LPF connect to a dummy load on Ant1 (K3 LPF is inline too)
If that is still the case, the antenne input(s) is probably not the route the BC signal makes to the mixer.
Hope you find a solution.

73 Hugo pa4la