Richard Harrison wrote:
Cecil, W5DXP wrote:
"The superposed forward voltage and reflected voltage can damage an
unprotected transmitter."

To do so, they would be in-phase and not out-of-phase.

Nope, they don't have to be in-phase. The reflected voltage can
arrive with any phase with respect to the forward voltage. You
trimmed out the example I gave. I will repeat it so you can
study it closer.

Just a for instance - assume the transmitter is putting out 70.7v
in phase with 1.4a at zero deg. The arriving reflected wave is 50v
at 90 deg and 1.0a at -90 deg. The load seen by the transmitter is
86.6v at 35 deg and 1.72a at -35 deg. Over voltage and over current
exist at the transmitter output. The forward power is 100w and the
reflected power is 50w. The net power being delivered to the reactive
"load" seen by the transmitter is 86.6*1.72*cos(70.4) = 50w.

In the above example, the designed for output voltage is 70.7v and
the designed-for output current is 1.4a. The superposed voltage is
86.6v, higher than the designed-for voltage. The superposed current
is 1.72a, higher than the designed-for current. In this example,
we have both over-voltage and over-current occurring *at the same
time* even when the forward voltage and reflected voltage are 90
degrees out of phase. The superposed voltage is not smaller than
the forward voltage until the phase angle between them is in the
neighborhood of 120 degrees. Two superposed voltages with a 90
degree phase angle are *always* larger than either voltage component,
i.e. SQRT(V1^2+V2^2) is always larger than either V1 or V2.

The superposed voltage will be higher than the forward voltage for
any phase angle from zero to 90 degrees. At some phase angle higher
than 90 degrees, in the neighborhood of 120 degrees, the superposed
voltage starts to decrease.

If you sit down and draw these phasors on a piece of paper, you
will discover that you are mistaken. It appears that you are
thinking one-dimensionally instead of two-dimensional phasors.
--
73, Cecil http://www.qsl.net/w5dxp


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