Typically a transmitter will fold-back delivered power when its output Z
is fixed (e.g., 50 or 70-ohm) and SWR exceeds some predetermined amount
set by the manufacturer. This is typical of broadbanded solid-state
amplifiers with a fixed output Z that use no output matching network.
These transmitters are designed to work into just one line Z and hence a
strict maximum SWR at the transmitter's output terminals.
From my perspective/research/experience from nearly 40 years in the
broadcast industry before I retired in 1999 -- including 15 yrs at RCA
Broadcast, then 19 years at Harris Broadcast Division ...
Normally the impedance at the active output elements (plates/collectors etc)
of a transmitter is much lower than 50 or 75 ohms, so that it can drive a 50
or 75 ohm load connected to the transmitter output connector with good
efficiency. If the net impedance at the output elements of the transmitter
exactly matched the load impedance of the antenna system, only 50% of the
available r-f output power would be dissipated by that antenna system.
Broadcast/other transmitters using vacuum tubes often included adjustable
matching components in the cabinet so that the transmitter safely could
produce 100% of its maximum rated output power into a load SWR of 1.7:1, or
so. But many of them also include(d) power foldback to protect the
transmitter if the load SWR exceeded that 1.7:1 nominal value.
Some solid-state broadcast transmitters do not include such matching
components, and consequently their maximum load SWR for full rated ouput
power is about 1:3:1 or so, and they fold back if load SWR exceeds that
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