It has been theorized that a circuit consisting of a Class C vacuum-
tube r-f amplifier using a tuned tank circuit in its output network
provides an operational “non-dissipative source resistance” of 50 ohms
for energy present at the output connector of the transmitter.

However the information and measured data provided in the text
excerpts below is not very supportive of that theory.

These excerpts discuss and show the effects of the energy entering a
transmitter at its output connector by frequencies offset from the
transmitter frequency.

There is direct applicability of the conclusions of the paper showing
that the operational source impedance of the transmitter near/at the
carrier frequency is much different than 50 ohms.

If it WAS a functional 50 ohms, then the termination provided to the
transmission line for signals entering the transmitter by its output
connector (whether on or off frequency) would not be present at the
plate of the PA tube to react with the power being generated by the PA
tube.

Rather the data leads to a logical conclusion that the operational
source impedance of this configuration at the carrier frequency will
be very low (approaching zero), when it is optimally tuned/adjusted to
produce its rated output power.

Further discussion or comment is invited.

RF

From:

A STUDY OF RF INTERMODULATION BETWEEN FM BROADCAST TRANSMITTERS
SHARING FILTERPLEXED OR CO-LOCATED ANTENNA SYSTEMS, by Geoffrey N.
Mendenhall, P.E.*

II. INTERMODULATION AS A FUNCTION OF "TURN-AROUND-LOSS".
"Turn-Around-Loss" or "Mixing Loss" describes the phenomenon whereby
the interfering signal mixes with the fundamental and its harmonics
within the non-linear output device. This mixing occurs with a net
conversion loss, hence the term "Turn-Around-Loss" has become widely
used to quantify the ratio of the interfering level to the resulting
IM level. A "Turn-Around-Loss" of 10dB means that the IM product fed
back to the antenna system will be 10dB below the interfering signal
fed into the transmitter's output stage.

"Turn-Around-Loss" will increase if the interfering signal falls
outside the passband of the transmitter's output circuit, varying with
the frequency separation of the desired signal and the interfering
signal. This is because the interfering signal is first attenuated by
the selectivity going into the non-linear device and then the IM
product is further attenuated as it comes back out through the
frequency selective circuit.

"Turn-Around-Loss" can actually be broken down into the sum of three
individual parts:
(1) The basic in-band conversion loss of the non-linear device.
(2) The attenuation of the out-of-band interfering signal due to the
selectivity of the output stage.
(3) The attenuation of the resulting out-of-band IM products due to
the selectivity of the output stage.

Of course, as the "Turn-Around-Loss" increases, the level of
undesirable intermodulation products is reduced and the amount of
isolation required between transmitters is also reduced.

The small portion of the interfering signal that is not reflected is
what causes intermodulation products to be generated. Obviously the
lower the output source impedance, the more complete the reflection
(lower return loss), with the result being less production of
intermodulation products.

III. EQUIPMENT PARAMETERS THAT AFFECT INTERMODULATION LEVELS.
The interfering signal must be coupled into the transmitter's output
stage before the IM products are produced and the output level of the
intermodulation products will be related to the interfering signal
level.

The two parameters (outside of the filterplexing equipment) that most
affect the interfering signal level into the transmitter's output
circuit are the output loading and the circuit's frequency selectivity
(loaded "Q"). These two parameters are interrelated because the degree
of output loading will change the loaded "Q" of the output circuit
while also affecting the return loss of the interfering signal looking
into the output circuit.

"Output Return Loss" is a measure of the amount of 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 transmission 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 resistive 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.

VI. CONCLUSIONS
1. "Turn-Around-Loss" is a function of the particular non-linear
device and the amount of loading on its output circuit.
2. "Turn-Around-Loss" increases as the interfering signal and the
resulting IM products are moved away from the carrier and out of the
output circuit passband.
3. "Turn-Around-Loss" will be least when the interfering signal is
within the transmitter's passband.

The figure posted at the link below shows the measured data supplied
with this paper.

http://i62.photobucket.com/albums/h8.../TAL_Chart.gif


* Geoffrey Mendenhall presently is Vice President, RF Engineering at
Harris Corporation Broadcast Division, and a recognized authority on
transmitter system design. Harris Broadcast is one of the largest
manufacturers in the world of AM/FM/TV broadcast transmitters, rated
for power outputs up to 2,000 kW.