"Reg Edwards" wrote in message ...

I didn't say they were unimportant. I said they served only to add to the
confusion when considering operation of the usual amateur installation when
the generator internal resistance is unknown.

Indeed, and not only that, the generator (ham transmitter) is commonly
neither a linear system nor time invariant. Also, maximum power
(conjugate-matched load) from a linear generator is generally not the
most efficient case. A great many generators and amplifiers are
distincly NOT designed to deliver power to a matched load, but rather
to deliver power efficiently to a specific load which is mismatched
with respect to the output impedance of the generator/amplifier.

There are times when knowing that a generator is a linear 50 ohm
source (within some small tolerance) is important--I deal with them
all the time in the work I do--but in a typical ham transmitter
application, that's very seldom if ever the case.

Cheers,
Tom

"Tom Bruhns" wrote
"Reg Edwards" wrote

I didn't say they were unimportant.
I said they served only to add to the
confusion when considering operation of the usual amateur installation
when
the generator internal resistance is unknown.

Indeed, and not only that, the generator (ham transmitter) is commonly
neither a linear system nor time invariant. Also, maximum power
(conjugate-matched load) from a linear generator is generally not the
most efficient case. A great many generators and amplifiers are
distincly NOT designed to deliver power to a matched load, but rather
to deliver power efficiently to a specific load which is mismatched
with respect to the output impedance of the generator/amplifier.

There are times when knowing that a generator is a linear 50 ohm
source (within some small tolerance) is important--I deal with them
all the time in the work I do--but in a typical ham transmitter
application, that's very seldom if ever the case.

====================================
Tom, To add a bit more -

50-ohm generators as used in laboratories (so that measured reflexion loss,
mismatch loss etc, mean something) are effectively constant voltage
generators in series with a 50-ohms resistor, or constant current generators
in shunt with a 50-ohm resistor. They may be followed by an ampifier whose
output impedance is held constant at 50-ohms by some automatic means. None
of these circuits bear much resemblance to a pair of 807's and a tuned tank.

The best that can be said about Rg of the usual HF radio transmitter is that
Rg is indeterminate. IT EVEN VARIES AS THE LOAD IMPEDANCE IS CHANGED which
most of the Guru's contributing to this newsgroup appear to be unaware of or
at least choose to disregard. So what does "adjusting RL to equal Rg" mean?
To use it in a description of feeder + antenna behaviour further propagates
myths, including those surrounding SWR, forward power, reflected power, SWR
meters, etc.

Does Terman ever bother to mention Rg of a Tx PA? If he doesn't it can't
matter very much to him. The ARRL handbook, when numerically designing a
transistor linear HF PA, makes no mention of Rg.
----
Reg, G4FGQ

I referred to Terman as "him".

It should, of course, have been "HIM". ;o)
---
Reg.

Reg Edwards wrote:
I referred to Terman as "him".

It should, of course, have been "HIM". ;o)

Not really - "Him" will do nicely. Just spell his surname in capitals
:-)

Seriously, people like Terman, Kraus and Jasik do deserve our respect,
for developing textbooks that have become 'standards'. Over several
editions they have been subject to searching examination from thousands
of teachers and students, so there aren't many errors left in there.

That's the valid reason for using those names as touchstones. To
contradict one of those standard texts, you'd better have some good
arguments prepared.

- BUT -

Never quote a textbook as a substitute for doing your own thinking. That
is the ultimate disrespect to the original authors.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

"Reg Edwards" wrote in message ...
....
====================================
Tom, To add a bit more -

50-ohm generators as used in laboratories (so that measured reflexion loss,
mismatch loss etc, mean something) are effectively constant voltage
generators in series with a 50-ohms resistor, or constant current generators
in shunt with a 50-ohm resistor. They may be followed by an ampifier whose
output impedance is held constant at 50-ohms by some automatic means. None
of these circuits bear much resemblance to a pair of 807's and a tuned tank.

The best that can be said about Rg of the usual HF radio transmitter is that
Rg is indeterminate. IT EVEN VARIES AS THE LOAD IMPEDANCE IS CHANGED which
most of the Guru's contributing to this newsgroup appear to be unaware of or
at least choose to disregard. So what does "adjusting RL to equal Rg" mean?
To use it in a description of feeder + antenna behaviour further propagates
myths, including those surrounding SWR, forward power, reflected power, SWR
meters, etc.

Does Terman ever bother to mention Rg of a Tx PA? If he doesn't it can't
matter very much to him. The ARRL handbook, when numerically designing a
transistor linear HF PA, makes no mention of Rg.

Amen, brother. I was thinking after making my last posting to this
thread that the one thing I DON'T bother thinking about when designing
a PA is what source impedance it will present. I worry about
currents, voltages, efficiency, distortion, a network to present the
proper load to the active device(s)... but not Rg.

In precision instrumentation systems, the output is commonly levelled
or monitored through a levelling splitter (not to be confused with a
power divider), so that a virtual zero-impedance point can be
established, with a 50-ohm (or other Zo) resistor from that point to
each output. And network analyzers are commonly calibrated with
precision loads so that the imperfections in their outputs and
reflectometers and cabling can be backed out by the calibration
software.

Cheers,
Tom

On 18 Jul 2003 04:50:07 -0700, (Tom Bruhns) wrote:


In precision instrumentation systems, the output is commonly levelled
or monitored through a levelling splitter (not to be confused with a
power divider), so that a virtual zero-impedance point can be
established, with a 50-ohm (or other Zo) resistor from that point to
each output. And network analyzers are commonly calibrated with
precision loads so that the imperfections in their outputs and
reflectometers and cabling can be backed out by the calibration
software.

Cheers,
Tom

Hi Tom,

H.W. Bode describes these characteristics through "Noise" Gain. It is
that portion of the "Open Loop" gain (or GBWP) that is returned to the
input.

The amount of "Noise" Gain is directly responsible for the significant
benefits (control over) of:
Input Impedance;
Output Impedance;
Noise (within the system);
Linearity (distortion);
Phase Response;
Gain variation;
Stability;
Frequency Response.

73's
Richard Clark, KB7QHC