Richard Clark wrote in
:

On Mon, 26 Feb 2007 23:24:13 GMT, Owen Duffy wrote:

Could that be exlained without introducing transmission lines and the
reflected power issue by examining how the PA works on a load different
to its rated load?

Hi Owen,

Let's treat this like the Chinese Box problem.

If you didn't know what the load was, could you explain it any
differently? No. Apriori knowledge is not a proof.

Richard, I content that:
- the power output of the PA; and
- the efficiency of the PA may be (and usually are) sensitive to the load
impedance.

A steady state analysis is usually adequate for most ham radio
applications, though there may be cases where establishment of steady
state brings its own issues. This discussion is about the steady state.

Though it is often asserted that the PA will get hotter as a result of
"reflected power" being dissipated in the dynamic output impedance of the
PA, and that this may / will damage the PA, the power explanation doesn't
work numerically in the general case.

The constrained ratio of V/I is an aspect of the field setup in a
transmission line, and the launching of a reflected wave to reconcile the
load V/I with the constrained (Vf+Vr)/If-Ir) at the load end of the line
is a solution for what happens on the transmission line. At the source
end of the line, (Vf+Vr)/If-Ir) at that point (a function of (Vf+Vr)/If-
Ir at the load end, and the complex propagation coefficient and Zo of the
line) give us the equivalent (complex) impedance seen by the PA, and we
can predict / explain the behaviour of the PA (maximum power out,
efficiency) on that equivalent load. This method of analysis does work
numerically.

Owen