Dr. Slick wrote:

W5DXP wrote:
It seems pretty
obvious that not all PA's are Z0-matched so they will always re-reflect
100% of the incident reflected power. But that is exactly what that
definition implies.

Your vocabulary is very confusing here, Cecil. "Incident"
usually refers to the forward power, so when you say "incident
reflected" it's extremely confusing.

The point was not to confuse. So allow me to re-word it.

Some say there is zero power reflected from a PA. For that to
be true, all PA's must exhibit an impedance of Z0, i.e. all PA's
must be Z0-matched. Doesn't that seem a little far-fetched?

Some say that 100% of the reflected power is re-reflected by
the PA. For that to be true, all PA's must exhibit an open,
short, or pure reactance to the reflected waves. Doesn't that
seem a little far-fetched?

The problem lies in the definition of "generated power" which
is forward power minus reflected power. A mental exercise will
illustrate.

XMTR---one second long feedline-----mismatched load

For the first two seconds, a directional wattmeter at the
XMTR reads 100W forward, zero watts reflected.

During steady-state, the directional wattmeter reads 100W
forward, 25 watts reflected.

If the XMTR is a 100W signal generator equipped with a circulator
and load resistor, we have no problem deciding that the signal
generator is generating a continuous 100W and dissipating whatever
reflected energy arrives.

If the XMTR is a ham transmitter, we say it generates 100W for
two seconds, and after that, it generates 75 watts, by definition.
This seems to me to be just a useful shortcut that avoids opening
Pandora's Box (in which the source is located). :-)

If a transmitter only generates (forward power minus reflected
power), it follows that the transmitter always re-reflects 100%
of the reflected power arriving at its terminals. Does that
sound reasonable?

Or, if reflected energy is never re-reflected from a PA, then
the PA must be dissipating the reflected power, which it previously
generated, just like a signal generator with circulator load does.
Does that sound reasonable?

Or, if there is a circulator load, the reflected waves contain
energy, but if there's not a circulator load, the reflected waves
don't contain any energy. Does that sound reasonable?

What sounds reasonable to me is that the reflected waves arriving
back at the source obey the rules of the wave reflection model
as described by Ramo and Whinnery. But since the impedance presented
to the reflected waves by the transmitter is usually unknown, we
are again up that proverbial creek without a paddle.

So, by all means, use the shortcuts, but recognize that they are
definitional shortcuts which may or may not represent reality.
--
73, Cecil, W5DXP



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