David, One need not have a "feeling" about the necessity of band pass filters. In addition to the concerns that Paul cited about isolation between ports in a triplexer or internal antenna port isolation in a transceiver, there are analytical methods to determine susceptibility to receiver damage and IMD.
A receive-only bandpass filter, whether external or internal, must suppress the fundamental frequency of the other TX in a M/2 station sufficiently to prevent overload and IMD generation. Do a calculation: TX power (dBW or dBm) minus band pass filter attenuation in its stop band at the other TX frequency equals worst case RX input power due to other TX (obviously RX antenna proximity, orientation, and sensitivity reduction out of band also reduce unwanted fundamental TX signal). The receive-only band pass filter cannot attenuate the in-band harmonic of the other TX. One can estimate the harmonic level, say in a 40 meter/20 meter M/2 setup, using the transceiver and amplifier manufacturers' specifications. If that level is at most equivalent to a strong distant station's received signal power, then it is likely of no greater concern than those loud stations. A 1500 watt amplifier that just meets the FCC's spurious emission limit at its 2nd harmonic (43 dB or greater attenuation relative to fundamental) yields approximately +19 dBm at the TX antenna input. In-band harmonic level at the other M/2 receiver again depends on antenna factors, but those factors combined would need to provide another ~70 dB of attenuation of the TX harmonic just to produce an S9+20 dB interferer from the other M/2 station. 73, Mike, K8CN ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
