Richard Clark wrote:
On Tue, 11 Oct 2005 09:11:19 +0100, Ian White G/GM3SEK
wrote:

The subsequent conversion to VSWR is a mathematical relationship only.

Hi Ian,

This seems to be a particularly notable difference - to which
absolutely NO ONE has ever deviated from in ANY determination of SWR!

That is to say, this "mathematical" distinction that some rely on to
differentiate their arguments has not got one scintilla of difference
over any other method.

The only way to claim you "directly" measure SWR is to find some way
to place two probes of a meter along the line such that one probe goes
into the trough and the other into the peak and the meter reads SWR
directly. Unfortunately for rhetoric's sake, this STILL renders the
determination in terms of a mathematical relationship. It cannot be
escaped.

Thank you, you're right. The key difference between direct and indirect
measurements is not about the need for mathematics; it's about the need
for additional input from theory.

What I should have said is:

When you calculate the VSWR from measurements of maximum and minimum
voltage on the line, that simple division formula comes directly from
the definition of VSWR. The measurement is direct and completely
self-contained, needing no further input from transmission-line theory.

But you cannot calculate VSWR from a single-point measurement of
reflection coefficient unless without some additional input from
transmission-line theory which connects them together. This dependence
on additional theory is what makes the measurement an indirect one.


Why this keeps on being revisited must be to allow the new lurkers to
observe my correction.

Yes, by all means.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek