There could be many reasons for this, Joe. But one possible cause is
direct-line switching supplies near your antennas. By this I mean those
cheap, light wall-warts that power everything in your house - cell phone
chargers, telephone power supplies, computer supplies, etc. These
typically have diodes hung on the AC line. RF gets into that diode, and
the AC line creates a nice mixer which can remodulate clean transmitted
RF into another band. Once the RF is in-band, no amount of receive
filtering can remove it.
A tipoff is that the receive junk has a 60-Hz component to it. Another
tipoff is that it goes away when you unplug the offending device. Maybe
turn off house circuit breakers one at a time to see if it goes away.
Yes, it should be possible to make SO2R cleanly. You may need stubs on
the transmitters to clean up their output (another whole topic in
itself), and bandpass filters on RX to keep the receivers from
overloading with out-of-band stuff.
GL,
-Tony, K1KP
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