This topic deserves more attention than it will get from its previous
placement deep in some recent threads.

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
transmitter product line: AM, FM & TV.

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.

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 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 2dB or less is
rather common. At 2dB the reflection coefficient is over 79%.

PAs comprised of multiple devices combined by balanced methods (e.g. 3dB
hybrids, Wilkinsons) can provide a source impedance closer to 50 ohms
(higher return loss). In these cases, power that is reflected off the load
and NOT re-reflected by the tx mostly is dissipated in resistive networks in
the PA combiner. However these networks do not provide a load for the
forward power from the tx, only for reflected power from the output
termination.

--RF

Visit http://rfry.org for FM broadcast RF system papers.