On Tue, 11 Oct 2005 21:11:55 +0100, Ian White G/GM3SEK
wrote:

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:

Hi Ian,

Remarkable touch of admission - especially to my over-arching method
of criticism.

Another point needs to be attended; the discussion of the measurement
of SWR seems quite, and absurdly, drawn in kindergarten terms of
mathematics as if the determination were king and the numbers simply
fell out be virtue of cranking the equation.

Nothing could be further from the truth.

As much as Reg pines away about what a SWR does not measure,
absolutely the same could be said for his unspoken inference that
probe measurement along a line does measure it.

True, probing the line reveals a pattern that in the mind conforms to
the expectation of standing waves, but this is simply trying to
measure your own shadow when each time you stretch out the rule your
shadow moves further out. What size is your shadow - when?

Using the formula everyone here leans upon as the archetypal equation
for SWR, and claiming they've measured at the appropriate points along
the line (undoubtedly only in their imagination) easily leads to
errors above 20%. Worse yet is that this same formula fails utterly
at the bench (am I embarrassing anyone?) for any but the most
pedestrian of SWRs which are easily resolved by a SWR meter in the
first place.

Beyond this issue of accuracy (certainly no one is interested in that
are they?) stands the fictions of requiring slightly more than a
quarterwave length, or access along the line to both the trough and
the peak. Anyone so hamstrung to NEED these criteria, hasn't ever
really faced the problem of measuring SWR on an open line in the first
place. The double-minima method offers an exceptional accuracy for
high SWRs and occupies a smaller region of line than otherwise
demanded. Measuring at the minima also reduces the error introduced
in the very act of measuring SWR.

To this last, how many here can guarantee their probes will approach
the line with the same offset? There's a very good reason why SWR
probes are mounted on vernier carriages. How many would recognize
when the probes were too deep, or not deep enough to justify the
measurement? To imagine any approaching the line with hand held leads
raises the prospect of shooting marbles without thumbs.

As to these meters that everyone is rushing to use - Square Law or
Linear? Don't know the difference? You don't know accuracy or how to
obtain it when you have no choice. How do you render a Square Law
detector linear? How do you linearize a Square Law detector
measurement? No concern? You aren't measuring SWR then either. The
method of measurement for low, medium, and high SWRs is not the same.
One size does not fit all as the discussion in this group might imply
(from that same lack of actually having done it). Even the math is
different - and if any argue that this observation flies in the face
of simple transmission line equations, then these casual tourists are
comfortably remote from actually measuring the rough terrain of SWR.

However, none of this practicality is going to disturb the armchair
SWR analyzer. It comes to their great fortune that one simple
instrument will probably offer far more accuracy than they could ever
obtain by trying to be literal about SWR "on the line."

73's
Richard Clark, KB7QHC