A lot of responses have offered excellent information on this subject. There's one that hasn't been mentioned yet so I will: SWR. Coaxial switches are designed to operate at a low SWR.
Good switches not only show a very low SWR, they EXPECT a low SWR. They aren't designed for really significant RF voltages or really high RF currents that are common in "non-resonant" antenna feeder systems. Coaxial switch ratings are based on feeding a 50-ohm non-reactive load over the frequency range specified: typically 1.5 to 30 MHz. >From comments on the reflector it seems that a number of installations use a short length of coaxial line to run from the operating position to outside the wall where it connects directly or through a balun to an antenna system that shows a wide range of impedances depending upon the frequency in use. The operator recognizes that the losses on a coax line will be relatively high but, the reasoning goes, if the line is kept short it won't be a big issue. That's true as long as the coax line and any coaxial switches used in the line are kept within their normal ratings for current and voltage. But if you have a voltage loop (high impedance point) at the switch, it's quite easy to produce thousands of volts of RF there, especially with a K2/100, and arcing can occur between the hot lead in the switch and surrounding grounds. Sure, the other ports of the switch may be grounded through the switch mechanism, but how many people would willingly connect a jumper across the antenna jack of their rigs, then subject the jumper to an arc from a very high RF voltage source a few millimeters away? I wouldn't recommend it. Under the same conditions, coaxial line itself will easily fail. It's not always so obvious, at least not a first. What happens at excessive voltages is that the center insulation melts until the inner and outer conductors touch. Up to that point the coax is just a nice room-warmer, converting the RF into heat instead of radio waves <G>. At the other extreme, a current loop, conductors will also overheat with considerable losses. The most likely effect of excess current through a switch is damaged wiper contacts. Of course, up until the point of failure, they are simply eating up the RF power that you wanted to go to the antenna. Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
