What is the IP3 of your amplifier?
How strong are the strongest local AM and FM broadcast stations? (These might need
additional notching if they are too strong.)
Do you have any local SW broadcast stations? A couple switchable 10/20dB pads in the
input path might not be a bad idea, just in case.
I remember a story in Technical Topics about one of the tube rigs that had a strong
LO output at the antenna port. (Possibly a HRO?) You might want check your rigs -
probably not a problem with newer ones using balanced mixers, but a pentagrid converter
doesn't give a lot of isolation. I think that would be the limiting factor, otherwise 40 to
50dB should be adequate.
The IP3 is somewhere around 9-11 dB
Using the AOR 8600MKII in spectrum analyzer mode I show three significant signals in the 88-108 MHz band;
89.9 MHz at -40 dBm
91.5 MHz at -6 dBm (really strong)
105.1 MHz at -10 dBm
There are 21 of other stations ranging from -65 to -110 dBm. The noise floor in the FM broadcast band is down around -115 dBm. (measured using a discone at 40' with no preamplifier).
In the AM band I have signals at;
570 KHz at -72 dBm
860 KHz at -78 dBm
1154 KHz at -82 dBm
1730 KHz at -77 dBm
Generally the noise floor in the AM BCB is around -110 dBm
The AOR is not really all that good for HF/BCB as the noise floor is not that great. Across the 1.8 to 30 MHz band the internally generated noise (with a 50 ohm terminator in place of the antenna connection) is right at -110 dBm. With a random length of wire for a HF antenna signal levels are between -60 and -80 dBm (radio in CW mode). There are four or five SWL broadcast stations between 15300 and 15700 KHz that come in around -40 dBm. For a comparison, WWV at 15000 KHz is at around -60 dBm with the same chunk of wire.
I could do the same thing with the HP or Hameg spectrum analyzer but using an SDR radio is just too tempting for a quickie email as I just need to fire up Spectrum Commander on my computer.
Back of the napkin numbers show me that the received signal for the FM broadcast stations should be down around -70 dBm for the NPR affiliate (91.5 MHz and the strongest FM signal in the area). I could make a quarter wave coaxial stub filter to attenuate that down further as it would have a low enough Q that it could also attenuate down the 89.9 MHz signal as well.
Into any of the decent receivers I usually see -120 to -130 dBm noise floors across HF. I do not have neighbors (at least who use 'lectricity <j/k>) who are closer than a few hundred meters. My locally generated RF noise is caused by the porch light that I can turn off with a switch and my computers.
I would think that if I have a receiver that is generating spurious signals that are visible at the antenna it would be the Hammarlund SP-600. Later designs like the R-390A (if the navy mods were done) and the newer surveillance receivers were specifically designed to be stealthy and the RF usually goes right into a mixer stage. Someone must have learned that a torpedo, missile or bomb could follow your receiver IF signal right back to the bull's eye. I knew that was of concern for the navy as there were worries that the German U-Boats were DF'ing convoys through that technique.