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