> 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.
Not quite. It only makes one trip Dick, and some of the concepts in the
text above are incorrect. This is not the proper reflector for a discussion
of PA's, or conjugate matching. I'll make one final (I hope) comment on
this topic.
Wave theory is just another way of explaining circuit action. Nothing
really bounces back and forth at all, it's just a concept. Power flows only
one direction, and that is from the source to the load.
Anything you can analyze and "prove" with wave theory, I can analyze and
"prove" with circuit theory and NO reflected waves. Both are models, and
both work properly within their boundaries.
> 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.
No, that isn't true either. Dissipation can be LESS with a mismatched load,
or more. It depends on the phase angle of the "reflection" or load
impedance. In general when dissipation is less, voltage is higher. So you
can destroy the PA devices from excess voltage as well as excess heat. When
you mismatch a tube PA, the same thing happens. You either wind up with a
lot of current and high dissipation, or a lot of voltage and low
dissipation. The voltage can be MANY times the supply voltage for the
output device, causing the output device or a circuit component (like a
bandswitch) to fail. Dissipation is generally confined to the output device
itself, and not to tank components.
> 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?)
Right, it has Nothing to do with the fact they are tubes, except tubes are
more forgiving for momentary overloads. A tube can handle several times its
rated voltage, current and dissipation for brief periods and survive. If
you could "tune" your solid state rig, high SWR would not matter a bit.
> 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?
No, bouncing power is a concept or "tool" we use to understand how a system
behaves. One reflection is all you get in this type of analysis. That is
what "creates" the standing wave.
73 Tom
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