On Mon, 07 Jun 2010 13:36:41 -0700, Roy Lewallen
wrote:

I haven't followed this thread since it's been beaten to death so many
times here before. But there's an interesting fact that might have been
overlooked, and might (or might not) be relevant:

If you put a directional coupler such as a Bruene circuit at the output
of a transmitter, and use its "forward" output to control the
transmitter power to keep the "forward" directional coupler output
constant, you'll find that the power output vs load resistance
characteristic is exactly the same as if the transmitter had 50 ohm
output impedance. This is assuming that the directional coupler is
designed for a 50 ohm system, and that the load is purely resistive. It
also assumes that any load is left in place long enough for the feedback
circuit to stabilize. The effective source impedance to a very rapidly
varying load (that is, one changing so fast that the ALC feedback system
doesn't have time to respond) would be the open-loop output impedance
which could be quite different. I haven't taken the time to analyze how
it behaves with a complex load.

I stumbled across this some time ago when designing a rig using this ALC
method and found it interesting. I believe many if not most modern
solid-state transmitters use this ALC method.

Roy Lewallen, W7EL

Hi Roy,

This application that you describe was written up in exactly the same
terms within the recent HP Journals I have posted extracts here. HP
used Directional Couplers (the Bruene circuit, also called a bridge,
qualifies too but uses a non-wave design) to separate out the forward
from the reverse power reflected from the mismatch to create a
reference power. Later, HP and others strapped the signals back into
the source in much the manner you describe.

The rudimentary version can be found in HP Journal v.6 n.1-2. HP
Journal v.12 n.4 strengthens the concept with hard copy sweeps of the
reflection coefficient of a load. By HP Journal v.16 n.6, we have the
description of automatic level control. For the 45 years beyond that
last article, more refinements.

73's
Richard Clark, KB7QHC