Well said, George.

Roy Lewallen, W7EL

I agree with every word, George.

Why do folks make this so complicated, Ian?

Mostly by insisting on asking questions that may not even *have* an
answer!

The US Constitution guarantees every citizen's right to ask whatever
questions they wish - but the Universe does not guarantee there'll be
any answers.

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

Ian White, G3SEK wrote:



I agree with every word, George.

Why do folks make this so complicated, Ian?


Mostly by insisting on asking questions that may not even *have* an answer!


The problem is that finding the output impedance
analytically is usually very difficult.

Measurement methods (usually questionable) have
been proposed that "estimate" the output
impedance. These tests can often be manipulated to
get some desired result (for example 50 ohms).

The value of output impedance depends especially
on signal level and also several other parameters,
such as negative feedback.

Is the value of output impedance important?
Sometimes in critical situations it can be. For
example, a lowpass filter connected to the output
of the PA may not be exactly correctly terminated
at the input end. The error slightly affects the
filter response, especially at the filter cutoff
frequency. The passband ripple can also be
affected. Most lowpass filter types can be
designed for unequal values of generator and load
impedances.

If the PA is broadband solid-state a sweep method
can be used to optimize the filter design.

Usually these errors are unimportant, especially
in typical Ham Radio.

Bill W0IYH

"George, W5YR" wrote in message ...
....
I think that a great deal of confusion over this whole issue comes from two
sources:

1. vague efforts to apply the infamous "Maximum Power Transfer Theorem" from
the early days in undergrad EE school; and

2. confusing an r-f transmitter output stage with the classical "signal
generator" with a dissipative 50-ohm internal resistance.

Forget both of those irritants and concentrate on the required load for the
transmitter, which the designer will provide and insist upon, and then
adjust the antenna system to provide that load and all will be well.

Those of us who _do_ have to worry, in intimate detail, about
generator source impedances, are most thankful that we do NOT when we
put loads on our ham rigs. Thanks for a great posting that nicely
summarizes what a lot of us have been saying for a long time. Perhaps
Reg is right. Perhaps we SHOULD quit calling it an SWR meter and
instead call it a "Transmitter Load Indicator" (or perhaps transmitter
load error indicator).

When you plug an appliance into the mains, do you worry about what the
mains source impedance is, so long as it's low enough to maintain the
proper voltage? When you connect speakers to an amplifier, do you
worry about what the source impedance is, so long as it's low enough
to not materially affect damping? If not, why would you worry about
transmitter source impedance? Why would you not worry instead about
proving the proper load so the amplifier can do it's job right?

Cheers,
Tom

proper voltage? When you connect speakers to an amplifier, do you
worry about what the source impedance is, so long as it's low enough
to not materially affect damping? If not, why would you worry about

I would not go there on the audio . The speakers do need to match the design
of the amp just as the load on a transmitter needs to match the design
impedance. Most power output devices are designed to produce maximum power
and /or minimum distortion into a specific load.

"Ralph Mowery" wrote:
proper voltage? When you connect speakers to an amplifier, do you
worry about what the source impedance is, so long as it's low enough
to not materially affect damping? If not, why would you worry about

I would not go there on the audio . The speakers do need to match the design
of the amp just as the load on a transmitter needs to match the design
impedance. Most power output devices are designed to produce maximum power
and /or minimum distortion into a specific load.

Virtually all "HiFi" audio amplifiers are designed to have
"damping" factor of something significantly more than 10.
Damping factor is the ratio of the load impedance to the
amplifier's output impedance. You might even find a few (more
expensive units) that have ratios greater than 1000, which is to
say that the output impedance of the amp (designed to drive
speakers in the range of 4 to 16 Ohms), has an output impedance
of 4/1000 of an Ohm).

Just as with RF, the output impedance has little to do with the
power delivered to the load impedance. The amplifier can
generate a maximum voltage by design, and how much power is
actually delivered depends solely on the impedance of the load.
Hence the same amplifier can deliver twice the power to a 4 Ohm
speaker as it can to an 8 Ohm speaker, and that is twice what it
will deliver to a 16 Ohm speaker. And it also works just fine to
drive a 600 Ohm headset, with significantly less power.

--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

"Ralph Mowery" wrote in message ...
proper voltage? When you connect speakers to an amplifier, do you
worry about what the source impedance is, so long as it's low enough
to not materially affect damping? If not, why would you worry about

I would not go there on the audio . The speakers do need to match the design
of the amp just as the load on a transmitter needs to match the design
impedance. Most power output devices are designed to produce maximum power
and /or minimum distortion into a specific load.

Just my point, Ralph... you should worry about your speakers being
within the range of LOAD impedances that the amplifier is designed
for, but you seldom would worry about the SOURCE impedance. You
should worry about the antenna-load you present to your transmitter or
amplifier being within the range of impedances for which the
transmitter or amplifier is designed, but why worry about the
transmitter or amplifier source impedance?

(Some folk worry about audio amp damping factor, but it tends to be
grossly overemphasized...see postings over the years by Dick Pierce in
the audio groups for the simple explanation why it doesn't matter all
that much.)

Cheers,
Tom

"Tom Bruhns" wrote:

...... Perhaps
Reg is right. Perhaps we SHOULD quit calling
it an SWR meter and instead call it a "Transmitter
Load Indicator" (or perhaps transmitter load error
indicator).

I propose that we call it a "Reflected Energy
Guessimator", or REG for short.

Jim, K7JEB k7jeb(at)arrl(dot)net
Glendale, AZ

Richard Clark wrote:
"I keyed down and made sure the excitation was disconnected---."

Supposing you were measuring the impedance the source shunts its output
with, and supposing your amplifier is linear so it can be used for AM
and SSB, and supposing that it is Class AB or Class B for more
efficiency than Class A, and supposing that it has a small amount of
forward bias to reduce crossover distortion, you should measure a much
higher impedance while idling than when the transmitter puts full power
into a load.

The output impedance has a meaning at maximum power output. This can be
determined by trying different loads to find the load that gets the most
power from the transmitter. The source impedance is its conjugate.

Best regards, Richard Harrison, KB5WZI

On Fri, 22 Aug 2003 09:48:56 -0500 (CDT),
(Richard Harrison) wrote:

Richard Clark wrote:
"I keyed down and made sure the excitation was disconnected---."

Supposing you were measuring the impedance the source shunts its output
with, and supposing your amplifier is linear so it can be used for AM
and SSB, and supposing that it is Class AB or Class B for more
efficiency than Class A, and supposing that it has a small amount of
forward bias to reduce crossover distortion, you should measure a much
higher impedance while idling than when the transmitter puts full power
into a load.

This would be true of commercial equipment (that is for commercial
stations, not equipment sold commercially in the retail trade).


The output impedance has a meaning at maximum power output. This can be
determined by trying different loads to find the load that gets the most
power from the transmitter. The source impedance is its conjugate.

Best regards, Richard Harrison, KB5WZI

Hi Richard,

And that too was performed, as I described, consistent with others'
reports to their methods (much as you describe).

73's
Richard Clark, KB7QHC