Richard Harrison wrote:
Cecil, W5DXP wrote:
"Nope, they don`t have to be in-phase."

Cecil is right. The third side of a triangle can be longer or shorter
than one of the sides.

I should have said, it is entirely possible that the superposed forward
and reflected voltages can damage an unprotected transmitter, The
reflected voltage can add to the forward voltage applied to the
transmitter.

The phase of the reflected voltage should be out of phase with the
transmitter source voltage to maximize the volts across the internal
impedance of the transmitter if we want to damage an unprotected
transmitter.

Now assuming, as Richard H. logically did, that we don't want to damage
an unprotected transmitter, let's introduce the concept of an active
match Vs a passive match.

The match at the transmitter output is passive, not active. Consider
the following system:

XMTR---1/4WL 300 ohm feedline---1800 ohm load

What does it really mean when we say that the XMTR "sees" 50 ohms?

It means that an interference pattern is established at the XMTR
that re-reflects (re-directs) the reflected energy back toward the
load. The transmitter has absolutely nothing to do with that reflection.

We can reconfigure the system as follows:

XMTR---1WL 50 ohm feedline--+--1/4WL 300 ohm feedline---1800 ohm load

Now we can see that the match is passive and the XMTR has nothing to
do with the match.

Destructive interference on the XMTR side of '+' eliminates the reflections.
The resulting equal magnitude of constructive interference on the load side
of '+' re-reflects (re-directs) the reflected energy back toward the load.

Optics engineers understand interference. Seems RF engineers might not
fully understand interference. Interference is the cause of the
elimination of reflections in optic systems. Interference is the cause
of elimination of reflections in RF systems, including at antenna tuners.
--
73, Cecil http://www.qsl.net/w5dxp


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