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