On Sat, 4 Sep 2004 17:57:26 -0500, "Richard Fry" wrote:

"Richard Clark" wrote
To date in this matter, I have yet to see any concrete value of source
Z offered from those of the NOT 50 Ohms camp. Further, I have yet to
see any of them offer any experimental confirmation of their assertion
________________

Please see the following. In the quote there, note the text starting "The
transmitter's output source impedance must be low...", and the following
sentences.

+ + +

Below is a quote from a paper titled "A Study of RF Intermodulation Between
FM Broadcast Transmitters Sharing Filterplexed or Co-located Antenna
Systems," by Geoffrey Mendenhall. Mendenhall is a registered professional
engineer, and now a VP for Harris Broadcast Division in Mason, OH. He is
responsible for the engineering research and design of the entire broadcast
transmitter product line for Harris: AM, FM & TV. Harris is the world's
largest supplier of broadcast transmitters.

This paper and quote has to be read here with some interpretation, because
it is an analysis of what happens when an in-band signal from one
transmitter is coupled into another transmitter when their antennas are
close together and/or when adequate filtering of the external signal is not
provided. But it is strictly applicable also for single tx and antenna
systems, where an antenna mismatch produces reflections back toward the
transmitter. In this case the "interfering signal" is not external, but a
reflection of the incident power of that tx.

A critical point made in the quote below is evidence of a serious
misunderstanding concerning the relationship between the source impedance of the
tx and the load impedance.

QUOTE: Output return loss is a measure of the interfering signal that is
coupled into the output circuit versus the amount that is reflected back
from the output circuit without interacting with the non-linear device. To
understand this concept more clearly, we must remember that although the
output circuit of the transmitter is designed to work into a fifty ohm load,
the output source impedance of the transmitter is not fifty ohms. If the
source impedance were equal to the fifty ohm line impedance, half of the
transmitter's output power would be dissipated in its internal output source
impedance.

The last sentence in the paragraph above is incorrect. This shows that the
writer of the quote is in the unbelievably large group that still believes
incorrectly that half of the tx power would be lost if if it were conjugately
matched. But we all know that efficiencies greater than 80% is achieved by Class
C amps, and greater than 60% is achieved by Class B amps when the source
impedance of the tx is 50 ohms resistive and the load impedance is also 50 ohms
resistive.

I have made appropriate measurements in a professional RF laboratory that prove
this point. The data from these measurements and the procedure used is available
for downloading from my web site at http://home.iag.net/~w2du under the title
"On the Nature of the Source of Power in Class B and C Amplifiers." This piece
is Chapter 19 in Reflections II, and also appears in QEX,, May/Jun 2001.

Unfortunately, like the statement made in the 'quote' above, there are all too
many RF engineers who fail to appreciate the true relationship between the two
separate resistances in the amp, the resistance resulting in dissipation and the
resistance responsible for delivering the power to the load. I guarantee the
reader of the piece referenced above will come away with something to think
about.

The transmitter's output source impedance must be low compared
to the load impedance in order to achieve good efficiency. The transmitter
therefore looks like a voltage source driving a fifty ohm load. While the
transmission line is correctly terminated looking toward the antenna (high
return loss), the transmission line is greatly mismatched looking toward the
output circuit of the transmitter (low return loss). This means that power
coming out of the transmitter is completely absorbed by the load while
interfering signals fed into the transmitter are almost completely reflected
by the output circuit. END QUOTE

The transmitter topology in this study was a single PA tube operating Class
C. For these designs, an on-carrier return loss value of 2 dB or less is
rather common. At 2 dB the reflection coefficient is over 79%.

snip
RF

73,

Walt, W2DU