Guys, I think it's time to get back to basics. The question posed is: "How much power should you run through a 350 w duplexer?"
The simple answer is: "Not more than 350 watts!" Many of the responses asked about how much power do you "need" to run, but that is not the issue. We don't need to digress into infinite stories about what you or a friend did, or what the coordinator recommends, or what your personal opinion is of alligator repeaters, or some discussion about "balance." When a duplexer has an insertion loss of, let's say, 2.2 dB, that represents power that is dissipated in the duplexer. If one has a repeater transmitter that puts out a true 125 watts into a dummy load, that same transmitter will (assuming matched impedances) lose about 40% of its output, or about 50 watts, in the duplexer. In a typical 4-cavity BpBr duplexer, that's about 25 watts per can. Not too much. Now, let's run the numbers on a 350 watt transmitter. That same duplexer will now be dissipating about 70 watts per can, and that is no longer a trivial amount. Any cavity that is heated with 70 watts will quickly become hot to the touch. Ignoring for the moment whether or not the duplexer is capable of properly isolating the receiver from the transmitter, the duplexer is going to endure "pumping" due to temperature cycling. Even if the center tuning rod is made of a material that has a coefficient of expansion close to that of the cavity cylinder, I guarantee that there will be some slight movement of the tuning fingers as the cavity heats and cools during successive transmissions. Remember the caution about not making tuning adjustments while transmitter power is applied? Guess what- that's exactly what is happening when a duplexer is run at high power! Sooner or later, the silver plating on the inside of the cavity (some cheaper cavities aren't plated) will get worn through to the base metal in the finger contact area, and now you have a duplexer that "mysteriously" became noisy without anyone touching it. It should not be a surprise that duplexers run at high power levels have this problem much more often than similar duplexers run at lower power levels. Choose wisely... 73, Eric Lemmon WB6FLY
