Richard Fry wrote:
. . .
The source impedance of most transmitters is not published even today. If
it was, probably we wouldn't be having all of this confusion about it, and
its effects.


Who's confused? It has no effect.

Roy Lewallen, W7EL

"Bob Nielsen" wrote in message
...
(Jumping in after this argument has gone on for a long time)

This has evolved somewhat into an "apples and oranges" discussion. What
Richard Fry appears to be concerned with is the reflection of a video
pulse by the load (antenna) and its re-reflection (at a later time,
dependent on the length and Vp of the transmission line) by the source
(transmitter) and subsequent radiation (as a ghost image--the effect
repeats, ad infinitum).

While this is certainly a real phenomenon, its effect is a function of
both the mismatch at the load (S11) and the mismatch looking from the
transmission line back toward the source (S22). It does NOT influence
the VSWR and could, obviously, be significantly reduced by the use of a
circulator at either end of the transmission line (which may not be
practical at the power levels of televison transmitters, to use his
example.)

73,
Bob Nielsen, N7XY

To shed more light on this, 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. One was the case of multiple
transmitting antennas on one tower; a particular antenna could pick up
another transmitter, and then re radiate it with some delay. The second was
the case of a wide band spread spectrum transmitter, where it is not
possible to have a good antenna match on all frequencies.

Tam/WB2TT

On Sun, 5 Sep 2004 12:47:45 -0500, (Richard Harrison)
wrote:

Richard Fry wrote:
"At least there appears to be an acknowledgement that some RF amplifiers
do not have a source impedance that is the conjugate of their load
impedance."

Those may be anomalous. I recommend King, Mimno, and Wing to anyone
desiring the complete story on the conjugate matches.

To the extent that the amplifier is designed for a performance on demand
that stresses it to its maximum safe dissipation, an amplifier of the
Class C variety is designed for a perfectly matched load. It`s the
economical thing to do.

You supply the tube with about all the volts it can safely take. Then
you supply it with just enough load impedance to limit its current to
all it can take under the heaviest loading it well encounter. That would
be when it is conjugately matched to a 50-ohm load, the usual cable
impedance specification.

The tank circuit is mostly a harmonic filter providing a very high
impedance to the fundamental frequency and shorting out the harmonics.

Best regards, Richard Harrison, KB5WZI

Gotta' disagree here, Richard, conjugate matching a Class C tube amp doesn't
require the heaviest loading it will encounter. A conjugate match can be
obtained with any loading you choose. For example, you can set the drive that
will permit loading to any particular level of maximum output power into any
load within the tank circuit's available matching range. So let's assume you set
the drive to provide a max output of 100 w into a 50-ohm load by adjusting the
tank controls for maximum output. The source resistance is now 50-ohms, there is
a conjugate match, and the amplifier has not been loaded to the heaviest
loading it will encounter. In addition, if the load was 70 ohms and the tank
adjusted for maximum output, the source resistance would now be 70 ohms, and
there'd also be a conjugate match.

If you question my statements above see the data from my measurements using
professional grade instruments in either QEX, May/Jun 2001, Chapter 19 in
Reflections II, or from my web page at http://home.iag.net/~w2du.

Walt

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).

As for the specification for output impedance gone missing :-)
MW-10B SPECIFICATIONS
RF OUTPUT IMPEDANCE: 50 ohms, unbalanced.
Other output impedances available on special order.

Harris Platinum Z FM transmitter
100 ohm output impedance (unbalanced)

HARRIS SW-50 A
RF Output Impedance 300 ohms balanced, 2.0 to 1 maximum VSWR

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

On Sun, 5 Sep 2004 10:44:04 -0500, "Richard Fry" wrote:

snip
When this test shows a 5% pulse return 2 µs after the incident pulse time
(for example), then the same pulse passed through the tx also shows nearly
exactly the same reflection % and time separation -- assuming there is
enough RF delay in the system for the reflection to be resolved in the
demodulated waveform.

As the directional coupler driving the normal demodulator at the TV station
is looking at forward power only, it is clear that the reflection from the
far end of the antenna system has been re-reflected from the TV tx output
stage, and NOT absorbed by it in its "conjugate impedance."

RF


Rich, I've got to tell you that you've just proved that the output circuitry of
the tx doesn't absorb the reflection from the antenna, but instead, re-reflects
it, because the output source resistance is not dissipative, only the cathode to
plate resistance is dissipative.

And the output source resistance is not dissipative or absorptive if the
impedance match is 'conjugate'. Contrary to what Mendenhall said that the tx
would absorb half the power if the source resistance is 50 ohms, there is
nothing special about the conjugate relationship that would produce this effect.
The tx will re-reflect whatever the match relationship is between the tx and its
load.

Walt

On Sun, 5 Sep 2004 17:01:14 -0500, "Richard Fry" wrote:

"Walter Maxwell"
Thank you Rick, I've received the Mendenhall paper, which
I'll review and comment later. (etc)
______________

Thanks for your very civil response. I was on Geoff Mendenhall's staff for
some years before I retired, and will contact him to ask for his response to
whatever of your points seem appropriate for that. He's a very busy person
these days and may not have the time to get involved, but there's no harm in
trying.

RF

Hi Rich, it's interesting that you were on Geoff's staff. On the paper you sent
me, the location of the company appears as Quincy, IL. When I bought the tx for
new BC stn WCEN in 1948 it was from Gates Radio in Quincy. Is it possible that
Geoff's business is a spinoff from Gates?

It's also interesting that you were also with RCA BC Div. I was with the RCA
Labs in Princeton, and working with me in the same lab room in 1960 was Don
Peterson, who was running an experiment suggested by Dr. George Brown. The
experiment was in developing the use of TDR for finding discontinuities in RF
feed lines for TV. With the success of his experiments we could pinpoint the
location of a discontinuity that produced a reflection causing no greater than
1.03 SWR. He developed a kit for use in the field for locating ghost problems in
the lines connecting the tx to the antenna. Were you aware of Don's work in this
area?

Walt

On Sun, 5 Sep 2004 18:30:53 +0000 (UTC), "Reg Edwards"
wrote:

|
|"Wes Stewart" wrote in message
.. .
| On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards"
| wrote:
|
| |What's a directional coupler?
| |What do they look like?
| |Don't bother answering those questions.
| |
| |Why do the arguers, when caught in a tight corner, always escape to UHF
|for
| |help from directional couplers?
| |
| |There are NO directional couplers at HF. They are as scarce as real swr
| |meters. So they cannot be used in futile attempts to explain what really
| |happens at HF.
| |
| |You're next move will be to drag in scattering-matrices.
|
| Why not. I have used an HP3577 network analyzer with an S-parameter
| test set that was specified to work over the frequency range of 100 Hz
| to 200 Mhz.
|
| I guess the guys at HP didn't realize that you can't do this.
|
|=================================
|
|Wes,
|
|Why do USA citizens invariably introduce the type numbers of their
|favourite, indeed worshipped articles when they have not the slightest
|bearing on an argument. To base one's position on a lifeless piece of
|hardware rather than logic is surely unsafe.

Well, I cannot speak for all US citizens and *invariably* is a little
too all encompassing for me. But I suppose it's similar to declaring
so unequivocally and "logically" that there are NO directional
couplers at HF. When presented with a real-life instrument that
includes these non-existant couplers that you refer to, you wish to
attack and belittle the presenter.

If you don't know what an HP 3577 is, you might ask in a civil manner,
in which case I would say that it is a vector network analyzer,
covering the frequency range of 5 Hz to 200 MHz that was widely used
in this country as well as around the world.

An accessory to the instrument is an S-parameter test set that offered
the ability to measure full two-port parameters over the frequency
range of 100 Hz to 200 MHz, which last time I checked, includes the hf
spectrum.


|What on Earth is an HP3577?

See above.

|In the whole of my career I have never heard of
|the number 3577.

Clearly, you have lived a sheltered professional life.

|Is it a prime? Traditionally, in the UK, the letters HP
|on the side of a savoury sauce bottle stand for "Houses of Parliament"

Perhaps, but me thinks that the sauce that you are more familiar with
says something different.
|
|Unless a 3577 can unambiguously measure the swr on a non-existent
|transmission line I'm afraid its presence will serve only to further
|agravate the argument.

Surprising to you perhaps, yes it can.

On Sun, 5 Sep 2004 19:24:02 +0000 (UTC), "Reg Edwards"
wrote:
snip

In brief, how can you have a conjugate match with the source impedance
hopping about trying to follow the load? ;o)
----
Reg, G4FGQ

Reg, I have a myriad of measurements which prove that when the pi-network is
adjusted to deliver max power for any reasonable drive level into any reasonable
value of load, the source impedance is the conjugate of the load impedance. When
adjusting the load control of the network for max output the output resistance
equals the resistance of the load. And when the plate tuning is adjusted for max
out the reactance injected into the network equals the reactance opposite to
that in the load. To be practical, be assurred that the tuning and loading
controls are alternately adjusted continuously until they converge and the max
power is delivered.

Having performed these measurements many, many times, I assure you that the
resulting data isn't a series of coincidences. So instead of the source
impedance 'hopping about trying to follow the load', when the network is
correctly adjusted, the source impedance 'finds' the load and stays put until
the load is changed and the network readjusted to accomodate the new load.

Walt

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

On Sun, 05 Sep 2004 19:06:03 -0700, Wes Stewart wrote:


| On Sun, 5 Sep 2004 12:18:09 +0000 (UTC), "Reg Edwards"
| wrote:
|snip
| |There are NO directional couplers at HF. They are as scarce as real swr
| |meters. So they cannot be used in futile attempts to explain what really
| |happens at HF.
| |
| |You're next move will be to drag in scattering-matrices.
|
| Why not. I have used an HP3577 network analyzer with an S-parameter
| test set that was specified to work over the frequency range of 100 Hz
| to 200 Mhz.
|
| I guess the guys at HP didn't realize that you can't do this.
|
|=================================
|
|Wes,
|
|Why do USA citizens invariably introduce the type numbers of their
|favourite, indeed worshipped articles when they have not the slightest
|bearing on an argument. To base one's position on a lifeless piece of
|hardware rather than logic is surely unsafe.

Well, I cannot speak for all US citizens and *invariably* is a little
too all encompassing for me. But I suppose it's similar to declaring
so unequivocally and "logically" that there are NO directional
couplers at HF. When presented with a real-life instrument that
includes these non-existant couplers that you refer to, you wish to
attack and belittle the presenter.

If you don't know what an HP 3577 is, you might ask in a civil manner,
in which case I would say that it is a vector network analyzer,
covering the frequency range of 5 Hz to 200 MHz that was widely used
in this country as well as around the world.

An accessory to the instrument is an S-parameter test set that offered
the ability to measure full two-port parameters over the frequency
range of 100 Hz to 200 MHz, which last time I checked, includes the hf
spectrum.


|What on Earth is an HP3577?

See above.

|In the whole of my career I have never heard of
|the number 3577.

Clearly, you have lived a sheltered professional life.

|Is it a prime? Traditionally, in the UK, the letters HP
|on the side of a savoury sauce bottle stand for "Houses of Parliament"

Perhaps, but me thinks that the sauce that you are more familiar with
says something different.
|
|Unless a 3577 can unambiguously measure the swr on a non-existent
|transmission line I'm afraid its presence will serve only to further
|agravate the argument.

Surprising to you perhaps, yes it can.

And Reg, in conjunction with the HP-8405 Vector Voltmeter, the HP-778D dual
directional coupler can measure SWR from 1 MHz to 1 GHz with no transmission
line whatever. This combination is the workhorse of my RF Lab.

Walt