Posted by
Ron D'Eau Claire-2 on
Mar 01, 2006; 9:31pm
URL: http://elecraft.85.s1.nabble.com/Coax-Switch-between-two-rigs-tp387104p387122.html
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
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