"SWR" has two different definitions, and Cecil is switching between them
with his usual facility.

Definition 1: the ratio of maximum to minimum voltage on a transmission
line. To measure that, you obviously need a significant length of
transmission line "for the wave to stand on" - depending on there the
maximum and minimum are, you could need anything between an electrical
quarter-wave or an electrical half-wave to locate both points with
certainty.

Cecil wrote:

The consensus of opinion over on science.physics.electromag is
that a two foot long section of 50 ohm coax is all the length
needed to force the V/I ratio to be 50 ohms at HF - something
to do with the length Vs separation between conductors ratio.

This is a side-issue, not relevant to the main discussion. I'm not sure
whether that distance should be in units of wavelengths, line diameters
or a function of both - but definitely not a simple length in feet or
metres. However, the line length required for the V/I ratio to come very
close to its characteristic value is certainly a lot less than the
length required to measure an SWR under Definition 1, so it's a
completely separate side-issue.


Cecil again:
That V/I ratio = 50 is the assumption made by the SWR meter
designer when the meter is calibrated.

That *is* relevant - but it's relevant to a different definition of SWR!

Definition 2: a mathematical function indicating how closely a given
impedance is matched to a given system reference impedance.

Under definition 2, you can measure SWR at a single point in the line.
The "given impedance" whose SWR you are measuring is the impedance
connected to the Output (or "Antenna", or "Load") side of the meter. If
50 ohms is the chosen system reference impedance, then the SWR meter is
designed, built and calibrated to indicate SWR=1 when it's terminated in
an accurate load of that impedance.


Everybody lives very comfortably with those two definitions of "SWR"
that exist side by side.

And that includes Reg - he understands transmission lines inside-out,
and two definitions of "SWR" don't trouble him in the slightest.

What Reg can't live with is that uncontrollable itch to make people jump
through hoops.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek