Ian White, G3SEK wrote:
W5DXP wrote:
If reflected energy could not cause over-current/over-voltage
problems in the source there would be no need for protection
circuitry. I once burned up a pair of ARC-5 1625's because my
feedline came unsoldered at the antenna. If there had not been
any reflected power, it would not have happened.

Transmitters are damaged *only* by excessive heat dissipation, voltage
or current, all caused by the wrong value of load impedance. Reflections
don't come into it.

Reflections are the *CAUSE* of the wrong value of load impedance. The load
impedance on a transmitter when reflections are allowed to reach the
transmitter is (Vfor+Vref)/(Ifor+Iref). Please note that half the terms
in the impedance equation are reflected terms. Please note that without
the reflected terms, the transmitter would probably be perfectly happy
with the "right" value of impedance at (Vfor/Ifor).

Think what would have happened if you had measured the impedance at the
TX end of your o/c transmission line (very high or very low, depending
on the length) and replaced it with a resistor and inductor/capacitor
giving the same value of R +/- jX.

There's no transmission line, so no traveling waves of anything, and no
reflections - just a transmitter with a very wrong value of load
impedance. The 1625s would have burned up just the same.

Yes they would, but in that case reflections are not the cause of the
impedance. In the first case, reflections are the *CAUSE* of the
impedance that burned up the transmitter. Without the reflections, the
transmitter would see (Vfor/Ifor) as the "right" impedance. With the
reflections, the transmitter sees (Vfor+Vref)/(Ifor+Iref) as the "wrong"
impedance.
--
73, Cecil http://www.qsl.net/w5dxp



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