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Cecil Moore[_2_] · March 23rd 07, 05:38 PM posted to rec.radio.amateur.antenna

wrote:
I have followed this thread with interest There are some who seem to
dispute the existance of reflected power and its ability to do damage.
My answear is that reflected power can certainly do so any one want to
run a completly detuned antenna to prove me wrong may do so at their
expence and risk.

Reflected power doing damage is an overly simplistic
way of viewing things. The reflected wave is capable
of causing damage without giving up any of its energy.
It is not the energy in the reflected wave, per se,
that causes the damage. It is the interference pattern
established by superposition of the forward wave and
the reflected wave that causes the damage.

If the reflected voltage arrives back at the source
in phase with the forward voltage, that constructive
voltage interference can cause an over-voltage condition
that blows the finals. This over-voltage condition actually
reduces the dissipation in the finals because it is
accompanied by a destructive interference, reduced-current
condition. The finals are actually cooler than normal
when the over-voltage begins to fry them.

Of course, a Thevenin equivalent source doesn't suffer
from an over-voltage problem. Power consumption within
a Thevenin equivalent source actually decreases during
constructive interference between the forward and
reflected voltages.

If the reflected current arrives back at the source
in phase with the forward current, the result can
be an over-current condition which can indeed cause
over-heating and failure.

What happens in the finals depends upon the phase
of the reflected wave. Solid-state finals are
usually protected from both over-voltage and
over-current by detecting the SWR. A fold-back SWR
limit of 3:1 limits both the ratios of Vmax/Vmin
and Imax/Imin to 3:1.
--
73, Cecil http://www.w5dxp.com

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