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

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

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

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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