On 4/17/2013 5:03 PM, Richards wrote:
The input impedance changes with drive, so I've been told. Therefore,
I allow the auto tuner to tune the antennas in bypass mode then ture
off the auto tuner when driving the Amp.
That's a good move.
The SWR indicated by the rig when you drive the amp shows whatever
mismatch there might be between the rig nd the amp. It's quite common
for there to be a moderate mismatch, but because most amps require
one-half or less of the rated power of the exciter (the rig), that SWR
doesn't matter. That SWR would matter ONLY if it was so high that the
exciter "folded back" (reduced power) to protect itself. Unlikely at
half power or less.
The mismatch is the result of several factors, depending on the design
of the amp, including the dynamic characteristics of the amplifier's
input circuit, and bandpass filters or matching networks in the input
circuit, if there are any.
Most tube power amps have variably tuned output networks that can match
a moderate range of load impedances without the need for an external
antenna tuner while still keeping the output stage happy (that is,
making the tube see a load that it is happy with). "Moderate" in this
context suggests less than 2 or 3 to 1.
One good reason to use an external tuner with a tube amp is so that you
can switch between different antennas on the same band that look
different from each other. That's what I did when I had several Ten Tec
238s, each dedicated to an antenna. These are legal limit tuners, on the
output of the amp. I could instantaneously switch between my NE/SW
dipole and my SE/NW dipole without having to retune the amp. I'm doing
the same thing now, but with the new Elecraft KAT500 tuner, which has
three switched outputs and remembers settings for each output for each
band. This is the sort of thing you care about when you're a serious
contester -- here, I might be running JA on 80 or 40 and a VK calls me,
but he's too weak to copy. :)
73, Jim K9YC
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