To be able to work simultaneously on two adjacent bands like 40 and 20 m the isolation needs to be at least 90 dB. Of course this will depend on the following:
- antenna isolation – can be as low as 30-40 dB with close antennas or resonant antennas on harmonics.
- antenna port isolation on the transceiver- on the Flexradio series 6000 it is about 40-50 dB between the antenna ports.
So in total an isolation that can be as low as 70-90 dB. But is this a problem? Well with a 1.5 kW station you have a signal level of around +62 dBm which is way over S9+73 dB!!! And your station can not handle more than +10-20 dBm. To be on the safe side and also having the joy of working SO2R a good thumb of rule is at least 150 dB of isolation. This means that with a 1.5 kW station sending on say 40 m you will not have more than S 8 (+63 – 150 dB = -87 dB) . In this exercise I have more or less ignored effects of phase noise and second harmonics (which with an old TRX can be severe).
With my set-up I end up in the following isolation between the antennas and 4o3A High Power bandpass filters (they provide about -50 dB of isolation) :
As you see about 90 dB (green lines). To this we must add the transceiver isolation of 40-50 dB. So in total 130-140 dB of isolation or S7-S9. Not too bad. That is you may send with a kW on 40 m and do listening on 20 m with a 40 m signal at about S7-S9.