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.

"Richard Fry" wrote in message
...
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.


Richard,
Is there a description somewhere that details which losses in an amplifier
have an effect on the output impedance? For example, the fairly high
parallel equivalent collector resistance would manifest itself as a fairly
low value series output resistance. There are other collector and emitter
losses that would have an effect on output impedance. What happens to the
~30% loss inherent in a class B amplifier? Is that a DC loss?

Tam/WB2TT

On Tue, 25 May 2004 11:23:03 -0500, "Richard Fry"
wrote:
This topic deserves more attention than it will get from its previous
placement deep in some recent threads.

Below is a quote
Which is a creationist-model of negative definition.

This is poor substitute for expressing a simple complex value derived
from actual measurement. The methods for accomplishing this are rote
material practiced by designers every day. The quality of their
product is in proportion to the accuracy of their determination.

73's
Richard Clark, KB7QHC

TWIMC:

OK, argue amongst yourselves. I have tried to post some real world data and
experience here, using a respectful tone. What I read back is mostly filled
with haughtiness, invective and intimidation.

Back to lurk mode (if that).

Richard Fry

Richard Fry wrote,

TWIMC:

OK, argue amongst yourselves. I have tried to post some real world data and
experience here, using a respectful tone. What I read back is mostly filled
with haughtiness, invective and intimidation.

Back to lurk mode (if that).

Richard Fry



That's a valid criticism, Richard. Some of these posts - including my own -
are more noteworthy for their sciolism than for their content and
clarity.
73,
Tom Donaly, KA6RUH

"Richard Clark" wrote in message
...
On Tue, 25 May 2004 11:23:03 -0500, "Richard Fry"
wrote:
This topic deserves more attention than it will get from its previous
placement deep in some recent threads.
Below is a quote
Which is a creationist-model of negative definition.

This is poor substitute for expressing a simple complex value derived
from actual measurement. The methods for accomplishing this are rote
material practiced by designers every day. The quality of their
product is in proportion to the accuracy of their determination.
73's
Richard Clark, KB7QHC

Side Comment:

Unfortunately, Richard Clark, the essence of your comment is completely
lost on me. I haven't the faintest idea what you mean. Are you criticizing
the quote or supporting it? Whatever...



Real content:

The quote posted by Richard Fry certainly explains well what we call
"Transmitter IM'" (the PA is a mixer and Tx frequencies mix producing other
signals). In the cellular telephone base station transmitter we have up to
20 transmitters on one antenna. In this case the IM is much more serious
because the power of the undesirable transmitter is not from a near antenna,
but a transmitter directly connected to the same antenna. Circulators in
each transmitter output are required to reduce the IM to acceptable levels
by diverting the unwanted TX signals to a load instead of the PA device..

I believe his intent (I'm guessing) is to show an authoritative report
that the output Z of a power amplifier is not equal to the load or
transmission line impedance, right?



As I stated before (with spelling corrected):

Get this MPT theorem blockage out of your minds... It is a synthetic
restriction.
The "maximum power theorem" (ZL=Zs) ONLY applies to ONE special case, NOT
all cases. That case is where the source's output power (or if you like
current) capability is limited ONLY by the two resistances (source and
load). That is, the
case is when the source can put out all the power needed by these resistors
and no other internal limit dominates. A common circuit can be shown to
give maximum power at other than Zs=ZL (apparently violating the above
referred-to theorem). There are things other than these resistances that
limit the output power of a practical source.



I add... In fact, in that quote, he comments that a low source impedance is
what makes for higher (50%) efficiency.


--
Steve N, K,9;d, c. i My email has no u's.

"Richard Fry" wrote in message
...
TWIMC:

OK, argue amongst yourselves. I have tried to post some real world data
and
experience here, using a respectful tone. What I read back is mostly
filled
with haughtiness, invective and intimidation.

Back to lurk mode (if that).

Richard Fry


Hi, Richard -

It's refreshing to read something that "feels" familiar. I must say it is
easy to understand since that has been my thinking for many years. Thanks
for the quotation.

John

On Wed, 26 May 2004 09:51:03 -0500, "Steve Nosko"
wrote:

Side Comment:

Unfortunately, Richard Clark, the essence of your comment is completely
lost on me. I haven't the faintest idea what you mean. Are you criticizing
the quote or supporting it? Whatever...

I am criticizing it AND the lack of response to the explicit question.
Cut-and-paste journalism is as cheap as a dime a page at the Xerox
machine.

If I asked you to give me a definition of Ohm's law expressed with
three variables (one dependant and two independent); then wouldn't it
seem obtuse to get a philosophy of laminar flow in pipelines?

Real content:

well, it is immaterial to the question at hand as it neither attends
Source Z nor Load Reflections. Except to allow:
by diverting the unwanted TX signals to a load instead of the PA device..

I believe his intent (I'm guessing) is to show an authoritative report
that the output Z of a power amplifier is not equal to the load or
transmission line impedance, right?

Where does it say that!? You had to guess, so that makes it less than
explicit, and now we have two questions to answer instead of one. Is
this accumulation of questions a linear or power progression? Web
debate is notorious for arguing the color of the table, then its
shape, then the number of chairs... before you get to the debate at
all.

If this posting had been headed with the answer "I don't know -
but...."

As I stated before (with spelling corrected):

Get this MPT theorem blockage out of your minds... It is a synthetic
restriction.

I didn't infer it, much less explicitly address it. I made a
statement of fact, and it is quantifiable and measurable. Whatever
occurs as a consequence of it is entirely another matter.

73's
Richard Clark, KB7QHC

Richard Fry wrote:
"This topic deserves more attention---."

Then Richard Fry attached a paper which includes:
"If the source impedance were equal to the fifty 0hm line impedance,
half the transmitter`s output power would be dissipated in its internal
output source impedance."

For a specific transmitter, that is certainly possible, but the
statement is wrong as a general statement.

Many transmitters, such as the familiar type that uses a Class-C final
amplifier, are 60 to 80% efficient..This does not mean it is not
adjusted for maximum power transfer. It may well be conjugately matched
and still exhibit high efficiency, the reason for using the Class-C
configuration.

The internal resistace of a Class-C amplifier consists of two parts. One
part gets hot and the other does not.

The part that gets hot is ortdinary loss. The part that does not get hot
is the switched-off time of the final amplifier. It is just as adept at
opposing transmitter output as is the common resistor, but it makes no
heat.

Best regards, Richard Harrison, KB5WZI