On Mon, 06 Sep 2004 01:23:04 GMT, Richard Clark wrote:

On Sun, 5 Sep 2004 20:25:31 -0400, "Tam/WB2TT"
wrote:

when I tried to find information in the
literature about transmitter output impedance, I ran across two cases where
reflections from the transmitter are bad.

Hi All,

It doesn't take much research into the matter of matching and Harris
Transmitters to find from their Digital AM stations:

The newest Harris AM Transmitters of 10 KW and higher (up to 1
megawatt and more) are totally solid state and use a completely
different modulation system called Digital AM Modulation. This
scheme is another Harris Patented system, using a large quantity
of plug-in modules, each one generates RF at a different
modulation level; in order for this to function, any incoming
audio (if analog) is broken down into digitized data, then used to
drive the various modules at the varying levels needed to have an
amplitude modulated output. It sounds pretty simple, in fact,
each module has a torroid (coil) which is the load for that
module; all those torroids are lined up and an iron pipe is run
through these torroids so that the combined output of all the
modules is coupled into this pipe. One end of the pipe is at
ground, the other end is the RF output of the transmitter. There
is an output network to match impedance of the pipe, which is
probably only a few Ohms, to the required output impedance which
is usually 50 Ohms, although sometimes high power transmitters are
set up to provide 75 Ohms, once in a while even 300 Ohms, to drive
an open wire type transmission line system.

Hardly a wit difference between Geoff's first FM amplifier design that
was documented for FCC type acceptance and now when it comes to
matching - all very commonplace and not even a hair out of place in
the conventional wisdom found in Terman (one of his references).
snip
Well, instead of reciting their complete catalogue, it is easier to
simply say I could not find any product that did not specify an output
impedance, much less that others were available on special order.

One wonders if it doesn't matter, do they just change the spec page
and the sticker above the connector? ;-)

73's
Richard Clark, KB7QHC

To Rich Fry,

Rich, I just now saw Richard Clark's post above. Until reading it I was totally
unaware that high-power BC transmitters used solid-state final amps.

Consequently, my comments were all directed toward tx's using tubes and LC
networks to isolate the non-linear input from the linear output. I'm not at all
familiar with the circuitryused in solid-state amps, please interpret my
comments accordingly.

Walt