Ralph, it is very easy to see if the mixing is happening in a RX or spectrum analyzer is to just dial in some attenuation. If the problem is within the device and you put in 10 dB of attenuation you will typically see a much greater decrease in the inter mod, probably 20 dB or much more. If the mixing happens outside the RX, a 10 dB attenuation should result in a 10 dB reduction of the inter mod. On the lower HF bands, with reasonable antennas, preamps are not usually required or even recommended. Sometimes even folks use some attenuation on say 160 meters. There are many possibilities for external mixing, not just the ones you mentioned. For example in other devices in house, such as a transmitter, TV preamp, SWR meter on another antenna, etc. Just one example I recently encountered, My 160 meter preamp I used on my RX array mixed two AM radio stations. It had back to back “protective” diodes. After I abandoned this published design, I built a very robust preamp, no protective diodes were necessary, and I was able to use the array from below 500 KHz to well over 7 MHz without a BPF or HPF. I would guess that the mixing is occurring in your immediate neighborhood, not many hundreds of yards away. I would walk around the area with a portable SW RX, even a less than $100 one or even a thrift store battery powered one will probably be plenty sensitive, at least mine picks up SW stations many thousands of miles away with it’s 3 foot whip antenna. Rick KL7CW no problems with my K3S.
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