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

Tarmo Tammaru wrote:

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.

Well, almost 90 degrees. V*I*cos(theta) still has to equal the forward
power minus reflected power even at that point. A quick and dirty phasor
diagram seems to indicate that the SWR meter bridge circuitry would get
pretty screwed up at point 'x' in the following:

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

The SWR meter would indicate close to 1:1 at points 'w' and 'y' but
would detect a forward and reflected power of approximately five times
the delivered power at point 'x', offscale on both needles, which is
what happened when I installed my SWR meter at that point.

The SWR bridge circuit phasor adds the two phasors. Even when they are
90 degrees out of phase, they add up to a large magnitude that gets
rectified and routed to the meter.

You may be correctly predicting the actual SWR but I doubt that you
are correctly predicting the response of the bridge circuitry in the
SWR meter.
--
73, Cecil, W5DXP

Roy Lewallen wrote:
Why does this seem so complicated?

The SWR bridge circuitry may not correctly display the
actual SWR. Please see my latest response to Tarmo.

The SWR bridge samples the current and voltage and
performs a phasor addition and subtraction to get
voltages proportional to the forward and reflected
powers. If there is a high voltage caused by reactive
components, it will be close to 90 degrees away from
the current. But phasor adding these two values gives
something slightly greater in magnitude than the high
reactive voltage. That high voltage gets rectified and
displayed as the forward power when it is not actually
the forward power but reactive power flowing from one
reactance to another.
--
73, Cecil, W5DXP

Roy Lewallen wrote:
Huh?

Huh, indeed. The scope of my statement is less than yours and my
statement is a sub-set of yours. If my statement is wrong, then
so is yours. However, it is within the bounds of logical possibility
that my statement might be correct and yours might be wrong. I'm not
asserting that is the case, just that it is within the bounds of
logical possibilities.

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.

Yes, but you didn't prove that source reflections have no effect
on an SWR meter.

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.

You say source reflections have no effect on SWR. I say if reflections
are not allowed to reach the source, the source impedance cannot have
any effect on the SWR meter readings.

My statement is a sub-set of yours and of lessor scope than yours. If
my statement is wrong, yours must also, by the rules of classical logic,
be wrong. :-)
--
73, Cecil, W5DXP

Cecil Moore wrote:
The SWR bridge samples the current and voltage and
performs a phasor addition and subtraction to get
voltages proportional to the forward and reflected
powers. If there is a high voltage caused by reactive
components, it will be close to 90 degrees away from
the current. But phasor adding these two values gives
something slightly greater in magnitude than the high
reactive voltage. That high voltage gets rectified and
displayed as the forward power when it is not actually
the forward power but reactive power flowing from one
reactance to another.
--
73, Cecil, W5DXP

The power flow fairy sure has a lot of warts. :-)

73, jk ac6xg

Roy, W7EL wrote:
"But source reflections have no effect on SWR."

That`s right. The load is going to take a fixed percentage of energy
imposed on it and reflect the rest regardless of the actual magnitudes.

All the source impedance can do is to affect the magnitude imposed on
the load.

SWR is based only upon percentage of energy reflected regardless of its
actual magnitude, not how big or small the energy making the ratio. 1/2,
2/4, and 4/8 are all the same ratio.

Cecil has a valid point. If Roy had a properly tuned tuner between his
transmitter and his feedline, the SWR seen by the transmitter would be
1:1, and that means no reflected energy reaching the transmitter.
Reflections reaching the tuner are either wasted in the tuner or
re-reflected by it if the tuner is properly adjusted and no reflections
make it to the transmitter.

Best regards, Richard Harrison, KB5WZI

Cecil Moore wrote:
Roy Lewallen wrote:
Why does this seem so complicated?

The SWR bridge circuitry may not correctly display the
actual SWR. Please see my latest response to Tarmo.

The SWR bridge samples the current and voltage and
performs a phasor addition and subtraction to get
voltages proportional to the forward and reflected
powers. If there is a high voltage caused by reactive
components, it will be close to 90 degrees away from
the current. But phasor adding these two values gives
something slightly greater in magnitude than the high
reactive voltage. That high voltage gets rectified and
displayed as the forward power when it is not actually
the forward power but reactive power flowing from one
reactance to another.

Even in this unusual situation, the behavior of the meter is completely
predictable, including the incorrect power indication.

Remember that the meter doesn't actually measure power - it is only
*calibrated* to *indicate* power. When placed in a situation where its
calibration is not valid, then of course it won't indicate power
correctly. But even its wrong indication can be predicted if you know
the detailed values to plug into a circuit model.

There's really no mystery about it.


--
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:
Even in this unusual situation, the behavior of the meter is completely
predictable, including the incorrect power indication.

There's really no mystery about it.

My point exactly! So, in the same vein, can the source impedance
adversely affect the SWR meter reading?
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:
Ian White, G3SEK wrote:
Even in this unusual situation, the behavior of the meter is
completely predictable, including the incorrect power indication.
There's really no mystery about it.

My point exactly! So, in the same vein, can the source impedance
adversely affect the SWR meter reading?

It will affect both the forward and reverse readings, but in equal
proportion, so it won't affect the indicated or calculated SWR (unless
there are nonlinearities in the meter, or the meter is not being used
correctly).


--
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:

Cecil Moore wrote:
My point exactly! So, in the same vein, can the source impedance
adversely affect the SWR meter reading?

It will affect both the forward and reverse readings, but in equal
proportion, so it won't affect the indicated or calculated SWR (unless
there are nonlinearities in the meter, or the meter is not being used
correctly).

How about what Chipman says?
--
73, Cecil http://www.qsl.net/w5dxp



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