Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to
reflected waves, and therefore DOESN'T affect the SWR.

Once again, here's a way to see why. I'll restrict the discussion to a
lossless line for simplicity. When you first turn the source on, a
forward wave (voltage and current) travels toward the load. The source
impedance does play a role in determining the size of this wave; it can
be determined by analysis of a simple voltage divider circuit, with the
source voltage dividing between the source impedance and the line Z0. A
portion of the forward wave is reflected from the load unless the line
is perfectly matched. The fraction which is reflected has nothing to do
with the source impedance, and in fact it can easily be calculated from
only the line and load impedances. That fraction (magnitude and angle)
is known as the reflection coefficient -- you can find the formula in
any transmission line text, or derive it yourself very easily.

Take a look at the system just before the reflected wave returns to the
source. At each point along the line we have a forward wave and a
reflected wave, which vectorially add. These create standing waves and,
if the line is long enough, we can calculate the SWR directly as the
ratio of maximum to minimum voltage along the line. A little bit of
algebra will show that the SWR is determined entirely by the ratio of
forward to reflected waves -- their absolute values don't matter
(except, of course, as it affects their ratio). Given a reflection
coefficient, you can calculate the SWR.

Ok, now suppose that some fraction of the returning wave reflects from
the source and heads back toward the load. Say, X percent of it. When it
reaches the load, exactly the same fraction of it is reflected as was
the case for the original forward wave. That is, if the new forward wave
is X percent of the original, then the new reflected wave is also X
percent of the original reflected wave. If we add the new forward and
reflected waves to the original ones, and take the ratio of forward to
reverse, we find that the ratio of the new, combined forward wave to the
new, combined reflected wave is exactly the same as it was for the first
forward and reflected waves. It doesn't matter what X is -- no matter
what fraction of the reflected wave bounces off the source, the same
fraction of that new forward wave is reflected from the load. The SWR is
the same as it was for the original pair of waves. Eventually, we build
up a large number of pairs of forward and reflected waves. And the ratio
of each forward wave to its corresponding reflected wave is always the
same -- it's the reflection coefficient of the load. So when we add all
the forward waves into a single forward wave and all the reflected waves
into a single reflected wave, we get the same ratio. And that ratio
doesn't depend in any way on the source impedance or what fraction of
each returning wave is re-reflected from the source.

One of the nice things about this way of looking at it is that it's
entirely supported by the theory and equations describing transmission
line operation which engineers have used to design working systems for
the past hundred years or so.

Roy Lewallen, W7EL

Cecil Moore wrote:
Jim Kelley wrote:

It's hard to imagine how Rs (Zs) could have any effect on that ratio.


Consider a reactive load where energy can be locally exchanged between
the load reactance and the impedance looking back into the feedline.
Zs can certainly affect the impedance looking back into the feedline.

Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to
reflected waves, and therefore DOESN'T affect the SWR.

===========================

But it DOES affect the indicated SWR and so the indicated SWR is incorrect.

It is the meter which is at fault ! It is designed to indicate correctly
only when the source is 50 ohms.

Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen
before.

SWR, of course, is calculated from Rho and the meter scale is calibrated
accordingly.

If the source is not what the meter expects then it gives the wrong answers.
And its faithful worshippers believe it!
---
Reg, G4FGQ

Reg Edwards wrote:
Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to
reflected waves, and therefore DOESN'T affect the SWR.

===========================

But it DOES affect the indicated SWR and so the indicated SWR is incorrect.

It is the meter which is at fault ! It is designed to indicate correctly
only when the source is 50 ohms.

Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen
before.

SWR, of course, is calculated from Rho and the meter scale is calibrated
accordingly.

If the source is not what the meter expects then it gives the wrong answers.
And its faithful worshippers believe it!

Sorry, Reg, for the last few weeks I'd believed you'd been trying to
make some profound point about this. But it's rather the opposite: all
you're saying is that the indication on the "SWR" scale of the meter
depends on the actual power level... which is obvious.

That's why the SWR result *always* has to be based on some kind of ratio
between forward and reflected readings on the meter, to allow for
varying power levels.

1. For a Bird-43 type of meter, you have to read the forward *and* the
reflected indicated "power" levels, and plug *both* of those numbers
into the little formula to calculate SWR... which involves the ratio of
those two numbers.

2. With an ordinary manual SWR meter, you avoid taking a ratio by
*always* tweaking the knob to adjust the forward reading to full-scale
as the first step. That compensates for whatever power level you happen
to be using. Then the SWR indication will read correctly on the reverse
setting.

If you omit that first step, then you're not using the instrument
correctly. Don't blame the SWR meter for that.

3. With an MFJ-259 or similar, the RF output is electronically levelled
to a constant value, so instead of the front-panel pot in (2) above
there is an internal set-and-forget trimpot.

4. A computing SWR meter does the calculation for you, at whatever power
level you happen to be using, so it displays an SWR reading that should
not vary with power (within the design limitations of the meter).

If you RTFM and use the SWR meter correctly, either you or the meter
will *always* compensate for whatever power level you happen to be
using.

As others have said - again and again, and correctly - the source
impedance of the transmitter affects only the power level; it does not
in any way affect the steady-state rho or SWR.


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

Ian, G3SEK wrote:
"1. For a Bird-43 type of meter, you have to read the forward "and" the
reflected indicated "power" levels, and plug "both" of these numbers
into the little formula to calculate SWR...which involves the ratio of
those two numbers."

Yes. Bird gives:
"VSWR = 1+sqrt Pref/Pfwd / 1-sq rt Pref/Pfwd

To eliminate calculations, the Model 43 instruction book includes charts
which give VSWR when Pref and Pfwd intersect on a chart.

A VSWR slide-rule has been produced by Bird which does the same as the
charts, and more.

Bird has published a useful series of technical papers, "Watts New From
Bird" Revisited. One paper, "The Directional Wattmeter", says:
"What is the effect of load impedance on the accuracy of the THRULINE?
The design formulas show that the only imopedance influeincing the
output voltage is Zo, the characteristic impedannce of the line at the
point of measurement. Since each THRULINE wattmeter is supplied with a
section of 50-ohm line, this Zo is accurately known. The load impedance
only affects the forward and reflected power levels which the THRULINE
measures.

Where should the weattmeter be inserted? Again referring to the
formulas, we see that the elements extract a voltage proportional to
either Ef or Er. While the total E varies along an improperly terminated
50-ohm line, the component voltages do not. This is simply another way
of saying that the energy contained in the forward wave remains the same
from the source to the load where some or all of it is reflected (unless
the load is 50 ohms) and the reflected energy remains the same from the
load back to the source. Our directional power meter can, therefore, be
placed anywhere between the source and the load.

Reg`s statement:
"But it DOES affect the indicated SWR and so the indicated SWR is
incorrect." does not apply to the Bird Model 43 wattmeter.

Best regards, Richard Harrison, KB5WZI

If the reflected and forward powers are known and the Bird is a short or
zero distance from the transmitter, why would anybody want to know the
reflection coefficient and SWR anyway?

Of what use are they except to use the graphical calculator to calculate the
forward and reflected powers?

It seems the Bird designer was wise enough to omit the SWR scale from the
meter.
---
Reg G4FGQ

Reg`s statement:
"But it DOES affect the indicated SWR and so the indicated SWR is
incorrect." does not apply to the Bird Model 43 wattmeter.

===========================

I've never seen one but there's nothing special about a Bird. It behaves
exactly the same as those from other manufacturers.

If the line between generator and meter is NOT 50 ohms then the actual SWR
on that line is NOT the value indicated by the meter. As I said, the
indicated SWR is incorrect.

It is important to distinguish between indicated and actual SWR's. It is
fatal to worship the meter as being error free and so make the same
incorrect assumptions as the meter does.
----
Reg, G4FGQ

I am sorely tempted but will refrain from calling the whole lot of you a
bunch of old wives.

Kindly desist in trying to put words into my mouth.

I have NOT said that changing the source impedance changes the SWR
indication. It doesn't !

I HAVE said that if the line is NOT 50 ohms then the SWR reading, even if it
doesn't change, BECOMES GROSSLY INCORRECT !

So "in practice", even if there IS a line of some sort, as you don't know
what the line impedance or generator impedance is, then the SWR indication
and the reflection coefficient from which it is derived are MEANINGLESS,
USELESS !

As Tarmo must by now be exhausted in repeating, and I have been saying for
years, all the meter tells you is whether or not the complex load on the
transmitter deviates in some unknown diirection from a purely resistive 50
ohms.

That is what the thing is there for. Which is all anybody needs to know
anyway.

Even to mention SWR in the present context dis-orients novices and
earners - which we all were at some time or other.

So change the name of the instrument to TLI. It's not a meter anyway.

I also have reservations about so-called reflected power. But that's another
matter.
----
Regards, Reg, G4FGQ

Reg Edwards wrote:
I am sorely tempted but will refrain from calling the whole lot of you a
bunch of old wives.

Kindly desist in trying to put words into my mouth.

I have NOT said that changing the source impedance changes the SWR
indication. It doesn't !
. . .

I posted:

Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to
reflected waves, and therefore DOESN'T affect the SWR.


To which you replied:

===========================

But it DOES affect the indicated SWR and so the indicated SWR is incorrect.

and:

If the source is not what the meter expects then it gives the wrong answers.
And its faithful worshippers believe it!

=========================== End of quote

Better check on the worm-wood content of that wine you've been drinking.
Another tip is to save a copy of each message you post. Then when you
encounter that blank spot in your memory, you can at least read what you
posted the night before.

Sad, isn't it, when the Old Wives can remember stuff you wrote better
than you can.

Roy Lewallen, W7EL
-- Again confirmed as a Reg's Old Wife

Better check on the worm-wood content of that wine you've been drinking.
Another tip is to save a copy of each message you post. Then when you
encounter that blank spot in your memory, you can at least read what you
posted the night before.

Sad, isn't it, when the Old Wives can remember stuff you wrote better
than you can.

Roy Lewallen, W7EL
-- Again confirmed as a Reg's Old Wife

=============================

Dear Roy, a display of annoyance signifies weakness of argument.
----
Reg

Reg Edwards wrote:
Dear Roy, a display of annoyance signifies weakness of argument.

Reg, do you ever display annoyance? :-)
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----