Roy Lewallen 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.

That's not the question. The question is: Does it affect the SWR meter?
Does the localized energy exchange between two reactances (your own "third
power" term), as alluded to by Chipman, cause erroneous SWR readings from
a directional wattmeter?
--
73, Cecil, W5DXP

Roy Lewallen wrote:
People who believe that SWR is affected by source impedance have either
rejected established theory, or don't have the background or interest to
read and understand what we consider to be very simple equations. So I'd
hardly expect them to be impressed by someone pointing out what the
equations and established theory say clearly and unambigously. You can't
fight Ouija boards with math.

The question remains: Does the localized exchange of energy between
reactances, as presented by Chipman and by you as a third power term,
cause a directional coupler error?

Source---w---(-j500)---x---(+j500)---y---50 ohm load
| |
+---braid---------braid--------braid------+

Given the phasor addition that happens in a directional coupler,
does it handle all cases of voltage and current properly? In the
above example, the measured SWR is 1:1 at 'w' and 'y' but not
at 'x'.

I remember someone saying that an SWR meter reading is correct only
when the SWR is 1:1.
--
73, Cecil, W5DXP

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

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

If reflections are not allowed to reach the source (typical of
most ham installations) the source impedance cannot have any
effect on the SWR meter readings. Most ham installations are
Z0-matched to 50 ohms thus eliminating reflections between the
Z0-match point and the source.

You are describing systems where reflections are allowed
to reach the source which is atypical of ham installations.
--
73, Cecil, W5DXP

On Fri, 17 Oct 2003 09:11:42 +0000 (UTC), "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!

I hope you meant to say that the meter "expects" to see the correct
line Zo.

Rho, SWR, RL, etc. are figures of merit for how well the load matches
the transmission line impedance (Zo), thus to derive this figure of
merit the meter's internal reference should be the same as Zo not Zs.

Your "50" in the equation above is simply a special case. Let Zs, Zo
and Zl(Zt) all equal 75. The line is perfectly matched, but plug 75
into your formula and see what happens.

This brings up an interesting paradox: all real lines have some loss,
thus Zo = Ro-jX. Unless Zl = Zo = Ro-jX the line is mismatched.

Likewise, the meter reference should also equal Ro-jX if the actual
line condition is to be measured.

As far as the source Z having any influence on SWR, Roy is (as usual)
exactly correct.

Wes N7WS

"Cecil Moore" wrote in message
...
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.
--
73, Cecil http://www.qsl.net/w5dxp
Cecil,

I did that, and the SWR did not change. But, then you could say this was by
definition because the meter measured RL/Z0.

Tam/WB2TT

Tarmo Tammaru wrote:

"Cecil Moore" wrote:
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.

I did that, and the SWR did not change. But, then you could say this was by
definition because the meter measured RL/Z0.

Chipman alludes to the problem not occurring with a resistive load
or with lossless lines. If the load is 50+j500 and the impedance
looking back into the lossy line is 50-j500, this seems to be
the correct conditions to cause the localized resonant energy
transfer problem.
--
73, Cecil, W5DXP

Cecil,

You saw my simulations of your example. I got an SWR of 66.3 for ZS=0 and
69.1 for ZS= j400. That is as close as I can resolve. I had thought that if
it was going to change, I would get an SWR of 1:1 for the conjugate matched
case. What I had neglected, and maybe you also, is the fact that when you
connect an SWR meter into the middle of a resonant series tuned circuit, the
current is 90 degrees out of phase with the voltage.

Tam/WB2TT
"Cecil Moore" wrote in message
...
Tarmo Tammaru wrote:
It seems fair to say that if the source impedance
determines V+, clearly it has an effect on V-. But, that does not mean
it
has anything to do with rho.

Chipman seems to say that an SWR meter can be disturbed by a localized
energy
exchange between reactive values with opposite signs. The impedance of the
source has an effect upon where in the transmission line those localized
energy exchanges occur.
--
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! =-----

Huh?

Humor me, in what way does that equation constitute proof? Where is the
source impedance in that equation?

An SWR meter will correctly indicate the SWR on a transmission line if
the transmission line connected to its output equals the design
impedance of the meter, regardless of the source impedance. If the
transmission line connected to its output doesn't equal the design
impedance of the meter, the meter won't correctly indicate the SWR on
the the transmission line, again regardless of the source impedance.

Why does this seem so complicated?

Roy Lewallen, W7EL

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!
---
Reg, G4FGQ

Huh?

Reflections reach the source at my station any time the SWR isn't 1:1.
But source reflections have no effect on SWR. I explained why in a
recent posting.

Roy Lewallen, W7EL

Cecil Moore wrote:
Reg Edwards wrote:

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


If reflections are not allowed to reach the source (typical of
most ham installations) the source impedance cannot have any
effect on the SWR meter readings. Most ham installations are
Z0-matched to 50 ohms thus eliminating reflections between the
Z0-match point and the source.

You are describing systems where reflections are allowed
to reach the source which is atypical of ham installations.
--
73, Cecil, W5DXP