"Dr. Slick" wrote in message
om...

If you measure the SWR at point 1, you let Z1 (Zload) be 50 ohms,
and Zo is the output impedance of the PA. SWR meter1 should be
characterized to Zo.
snip
Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo)

It should be easy to see from this well known equation that a
perfect 50 Ohm Zl will definitely change SWR as you change Zo away
from 50 ohms.


Slick

Looking at a book. Has the same equation, only thing is that Zo is the
impedance of the *coax*.

Tam/WB2TT

"Dr. Slick" wrote in message
. com...

If you measure the SWR at point 1, you let Z1 (Zload) be 50 ohms,
and Zo is the output impedance of the PA. SWR meter1 should be
characterized to Zo.
snip
Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo)

It should be easy to see from this well known equation that a
perfect 50 Ohm Zl will definitely change SWR as you change Zo away
from 50 ohms.


Slick

Looking at a book. Has the same equation, only thing is that Zo is the
impedance of the *coax*.

Tam/WB2TT


Dang it, Tam, you just had to tell him! I was just having a good laugh when
you
had to go and spoil it.
73,
Tom Donaly, KA6RUH

(Tdonaly) wrote in message ...

Looking at a book. Has the same equation, only thing is that Zo is the
impedance of the *coax*.

Tam/WB2TT


Dang it, Tam, you just had to tell him! I was just having a good laugh when
you
had to go and spoil it.


Zo doesn't have to be the coax impedance. It can be any impedance
you want. What about interstage-matching networks between FETs? Do
you think there are no reflections there? Should you attempt to match
impedances there? I would think so.

let me rephrase:

PA--+SWR meter1+--50ohm line--+SWR meter2+--50ohm line--+50 ohm
(same Z as PA) (50 Ohms) dummy
load
1 2 3


If you measure the SWR at point 1, you let Z1 (Zload) be 50 ohms,
and Zo is the output impedance of the PA. SWR meter1 should be
characterized to Zo.

I've agreed that SWR meter2s SWR reading (at point 3) will not
change as you change PAs impedance, but the SWR at point 1 definitely
WILL.

So, Does the SWR change as you change the source impedance? IT
DEPENDS ON WHERE YOU MEASURE IT! Right at the output of the source,
it will!

From Pozar's Microwave Engineering (Pg. 606):

Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo)

It should be easy to see from this well known equation that a
perfect 50 Ohm Zl will definitely change SWR as you change Zo away
from 50 ohms.


Slick

"Dr. Slick" wrote in message
om...
(Tdonaly) wrote in message
...
Zo doesn't have to be the coax impedance. It can be any impedance
you want. What about interstage-matching networks between FETs? Do
you think there are no reflections there? Should you attempt to match
impedances there? I would think so.

You transform the input impedance of the driven stage to the optimum load of
the driving stage. Whether that results in a conjugate match is completely
immaterial. Trust me, I have done it .

As for the Zo thing, you are trying to misapply the formula.

BTW, the one place where you would think conjugate matching should always
apply, antenna to receiver input, does not (always). Lowest noise figure
often occurs at SWRs around 1.5

Tam/WB2TT

let me rephrase:

PA--+SWR meter1+--50ohm line--+SWR meter2+--50ohm line--+50 ohm
(same Z as PA) (50 Ohms) dummy
load
1 2 3


If you measure the SWR at point 1, you let Z1 (Zload) be 50 ohms,
and Zo is the output impedance of the PA. SWR meter1 should be
characterized to Zo.

I've agreed that SWR meter2s SWR reading (at point 3) will not
change as you change PAs impedance, but the SWR at point 1 definitely
WILL.

So, Does the SWR change as you change the source impedance? IT
DEPENDS ON WHERE YOU MEASURE IT! Right at the output of the source,
it will!

From Pozar's Microwave Engineering (Pg. 606):

Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo)

It should be easy to see from this well known equation that a
perfect 50 Ohm Zl will definitely change SWR as you change Zo away
from 50 ohms.


Slick

Why don't you try it? It is easy enough to add impedance between the
transmitter and SWR meter. Keep the power low, but exactly the same for the
two cases. In fact, all you would have to do is replace the 50 ohm line
between the transmitter and meter with 1/4 WL of 75 Ohm line.

Tam/WB2TT
"Dr. Slick" wrote in message
m...
let me rephrase:

PA--+SWR meter1+--50ohm line--+SWR meter2+--50ohm line--+50 ohm
(same Z as PA) (50 Ohms) dummy
load
1 2 3


If you measure the SWR at point 1, you let Z1 (Zload) be 50 ohms,
and Zo is the output impedance of the PA. SWR meter1 should be
characterized to Zo.

I've agreed that SWR meter2s SWR reading (at point 3) will not
change as you change PAs impedance, but the SWR at point 1 definitely
WILL.

So, Does the SWR change as you change the source impedance? IT
DEPENDS ON WHERE YOU MEASURE IT! Right at the output of the source,
it will!

From Pozar's Microwave Engineering (Pg. 606):

Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo)

It should be easy to see from this well known equation that a
perfect 50 Ohm Zl will definitely change SWR as you change Zo away
from 50 ohms.


Slick

"Tarmo Tammaru" wrote in message ...
Why don't you try it? It is easy enough to add impedance between the
transmitter and SWR meter. Keep the power low, but exactly the same for the
two cases. In fact, all you would have to do is replace the 50 ohm line
between the transmitter and meter with 1/4 WL of 75 Ohm line.



You mean change the coax from point 1-2? that wouldn't be the same
as changing the PAs output impedance and the impedance of the SWR
meter1.


Slick

(Richard Harrison) wrote in message ...
Dr. Slick wrote:
"Does the SWR change as you change the source impedance?"

"SWR" as Reg notes is an indication of match condition when measured at
the transmitter output. There may not be enough line for maxima and
minima.

Loading a transmitter usually increases the current drawn by the final
amplifier. This lowers
its output impedance in most cases. As "SWR" is an indication of
mismatch, you can`t change one impedance without changing the "SWR",
unless you make a corresponding change in the other impedance in the
match. Corresponding is a significant word.

Best regards, Richard Harrison, KB5WZI


Well, like i said, the SWR will change as you change the source
Z, but you have to measure it before the reflection occurs (the SWR
must be the same Z as the source, and must be right at the output), or
you won't measure it "downstream".


Slick