Guys, I'm considering the relative value of various coax protection schemes for a planned UHF ham repeater. I know PolyPhaser is the industry standard, but I also know they don't last forever, and can cause problems with spurious emissions in some failure modes.
A simple shorted quarter-wave stub on a tee will put the entire feedline at DC ground, but wouldn't be much of a match for even an indirect, nearby lightning strike. But...what if I could build a really stout quarter-wave stub out of non-ferrous metal, with enough diameter and wall thickness to put up a fight, or at least make the coax the "fuse"? Would I need to account for superheated air turning the cavity into a bomb when hit, or would the coax fail quickly enough? It would be fairly easy to construct out of copper, (and could be brazed rather than soldered,) but I'm likely to have to connect it to a large-gauge aluminum ground system on whatever rooftop I find for a site. Is there a good way to make this connection between dissimilar metals, or should I try to build it out of aluminum? Given the fact it will probably be connected in the feedline using a silver-plated tee anyway, am I overthinking the dissimilar metals issue? More important, is there something I'm missing in the comparison? A shorted cavity would seem to be a more elegant solution where DC does not need to be fed up the coax, and would have some mildly beneficial, if broad, bandpass characteristics. If you're a fan of the PolyPhaser aproach, which of their products would be preferred on a 440 MHz ham repeater? This system is being designed for limited HAAT and coverage, and we expect to be at a site in an industrial area with no other RF systems in close proximity. 73, Paul, AE4KR
