> But, you say you use 1.5 KW to a KW Matchbox? Didn't think they could handle
> it.
>
Hi Greg,
When the Johnson Kilowatt Matchbox was manufactured, the FCC rules for
Amateur Radio transmitter power stated the maximum DC input power to the
final power amplifier stage. The limit was 1000 watts input. (If you
used more than 900 watts input you had to have separate metering of the
anode voltage and current. At under 900 watts you could use a selectable
meter, or perhaps no meter at all.) A class C push-pull final amplifier
stage could be close to 90% efficient. So that could be 900 watts of
unmodulated carrier power output. Then when you modulate it with a class
B push-pull modulator, to 100% modulation, the RF voltage output on the
modulation peaks could be as much as 2X the RF voltage of the
unmodulated carrier. Two times the voltage is 4X the power. So the peak
power output could be 3600 watts. (Don't try this with a tuner built in
Starkville, Mississippi, at least not with any I have seen.)
Additionally in the old days tuners were made to be used when you really
needed a tuner, that is when the SWR might be really high. So the peak
voltages and/or currents on the feedline or in the tuner, could be many
times what they are with the moderate SWRs that most present day tuners
are able to safely match. Take a look at the insides of a Johnson 250
watt Matchbox, and you'll find that the capacitor plate spacings and
inductor wire cross sections are larger than most modern day "1500 watt"
tuners.
Few tuners built today can really handle the RF power they claim to be
good for, when the SWR is high. Most will handle their supposed rated
power only when the SWR is so low that you hardly even needed a tuner at
all. (with the old tube rigs with wide range Pi-Net outputs, you didn't
really need a tuner for about 3:1 SWR or lower)
Tuners I know of that are really built to handle a kW are the Drake
MN-2000 and Millen 92201. They are for coax, not balanced feeders. There
are probably others. Even the Ten-Tec 238 is not nearly as beefy,
however since it is an L network there are never any voltages or
circulating currents inside the tuner which are greater than either the
input or output voltage or current. I am quite happy with mine, though I
do not run a full kW of 100% modulated AM.
A watt ain't watt it used to be!
DE N6KB
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