Yikes! I didn't mean to start this particular thread. I recall one just like
it on the amps reflector. Eventually, we all got down to the same
terminology and conclusions that are emerging here. In English, I think it
goes like this:
1. A mismatch between the transmission line and the antenna will cause power
to be reflected back down the transmission line towards the transmitter. The
greater the mismatch, the lower the percentage of power radiated by the
antenna and the larger the percentage of power reflected back to the
transmitter.
2. Depending on the severity of the mismatch, a tuned output circuit (like
those found in amateur antenna tuners, tube-based power amplifiers and the
older tube-based transmitters) can be used to create a conjugate match that
presents a 50-ohm impedance when looking from the transmitter to the
transmission line while at the same time presenting a high impedance when
looking from the transmission line to the transmitter. The result is maximum
power transfer from the transmitter to the transmisstion line and
re-reflection of any reflected power back to the antenna. As in #1 above, a
percentage of the re-reflected power will be radiated by the antenna and a
percentage of it will be reflected back to the transmitter again.
Eventually, as the power "bounces" back and forth, all of the forward and
reflected power will be radiated by the antenna. This is why tube
transmitters and amplifiers can be successfully used with an antenna that is
mismatched to the transmission line.
3. If the transmitter contains a broadband 50-ohm output network, and no
tuner is used, there is no conjugate match and the reflected power will be
dissipated by the finals. In most solid-state designs, transistor finals
can't handle much additional heat dissipation without being destroyed, so
most modern amateur solid state transmitters automatically detect an
increase in reflected power and reduce forward power to protect the finals.
This is why solid state rigs can't be operated into high VSWR -- either the
finals will be destroyed or power will be reduced.
What I'm less sure about:
1. The conjugate match provided by the tuned output circuit is really what
makes it possible to use tubed rigs when the antenna and transmission line
are not matched. The somewhat greater ability of tubed finals to dissipate
heat has nothing to do with it. (Well, perhaps they have a greater ability
to withstand the tuning process?)
2. When a conjugate match is achieved, some power is lost as heat in the
tuned circuits and transmission line, but it's very small. Perhaps this
dissipation is what keeps the power from bouncing back and forth
indefinitely?
3. Poorly designed matching networks can and do dissipate power as heat. For
example, if you use a huge loading coil with a 10-foot pole to transmit on
160M, a lot of power will be lost in heating the coil.
4. Not all antenna tuners are created equal. I believe that some designs are
inefficient at some frequencies, resulting in power lost to heat.
5. Typical antenna tuners and tuned circuits in tubed PAs or transmitters
can handle only so much mismatch at a given frequency. Often the result
isn't more heat -- it's arcing.
Just my 2 cents...
73, Dick, WC1M
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