On May 2, 7:04 am, "Jimmie D" wrote:
"K7ITM" wrote in message

ups.com...



On Apr 1, 1:16 am, "Beech Creek" wrote:
I have an old Murch Ultimate Transmatch that I would like to put a switch
in
so that I can switch the transmatch in or out of the circuit. What kind
of
switch should I use that will handle the 2KW power rating of the
transmatch?

Thanks in advance for any advice.

Cal Barton
WB5CYS

The switch has to handle not only the 50 ohm input, but the output,
whatever that may be. Note that 2kW at 10 ohms is 14 amps RMS, and at
RF where the current flows on the surface of the conductors and
resistance is quite a bit greater than at DC, you can get a lot of
heating with 14 amps. And note that 2kW at, say, 600 ohms is over
1000 volts RMS. At that sort of power level, where you don't really
know what impedance you're driving, I'd suggest you should be using a
pretty heavy-duty ceramic switch, heavy contacts and wide spacing. I
wouldn't even think of using a "miniature" switch in an application
like that.

If things stayed at 50 ohms, you'd only need to handle a relatively
puny 6.3A and 316V RMS, but if things stayed at 50 ohms, you wouldn't
need an antenna tuner, would you?

Cheers,
Tom

Of course one would assume the only time he would use the switch is when
working into 50 ohms, either the input to the tuner or the input of a 50 ohm
more or less antenna.

Jimmie

;-) Well, when I read "switch the transmatch in or out of the
circuit," I envision switching both ends of it. If indeed it's a
switch to select between antennas that don't need a tuner and one that
does, where that antenna is always connected to the tuner output, then
a more modest range of impedances could be assumed. I'd assume at
least 2:1 SWR relative to 50 ohms, though; a typical 2kW amplifier
would have a tuned output that likely could handle that range easily.
So even then you're getting into possible currents and voltages I
wouldn't trust to just any old switch I found kicking around. I do
understand that a common ham design philosophy is, "build it with
whatever's around and if it fails, rebuild it with something a bit
beefier." I'd prefer to put a bit of thought into the anticipated
worst-case stress levels before I build it, but that's just my
personal feeling.

Cheers,
Tom