Great questions that anyone venturing into SO2R should be asking. One of
the things that you need to ask is whether you are simply trying to protect
the 2nd radio from damage? Or actually use the 2nd radio in the vicinity of
the interference. They are massively different answers.
I personally use S9 +60db as my max limit for receiver protection decisions.
This is clearly shy of the actual number but who wants to experiment with
the actual number. The best thing to do is try your own tests at much lower
power and then calculate the difference when power is raised. Aim antennas
in all normal configurations plus at each other (always the worst case).
I run yagis on the same tower that are 10 ft separated (15 and 20M or 40 and
80M). To do anything meaningful on SO2R, you are going to have to get
bandpass filters of some kind. With those installed at the output of the
transceiver, my worst case harmonic is S9+20 at 100W. Even with the amp on,
its no more than S9+40. The issue is that for a few Khz either side you
can't function there on SSB and about 1khz wide on CW. But I accept that.
It should be noted that with 1.5kW - more than 10khz off of any harmonic -
there is virtually no evidence of the second radio interaction. And that is
really the most important criteria.
The bigger issue is same band SO2R - which will definitely blow up the radio
if not managed. I use a receiver protector through the RX antenna port when
doing same band SO2R.
Ed N1UR
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