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We agree then for, as I said , it cannot be _measured_ at one
point.
The loosely called "SWR meter" sold as such in ham radio shops
is badly named , and "SWR Comparator-Deducer" would be
better. But then ammeters and voltmeters and speedometers dont measure volts and
amps and speed, not even torque, what is measured is the angular distance
of the pointer. As long as the user knows what he is looking
at all is well.
Long long ago in a faraway land now called 9M2 , I stood
looking at a pile of scrap metal while a sincere stationary diesel engine
minder told me,,,,,,,
.."At 4. o'clock in the morning I saw that the oil pressure gauge was not
working because it was showing nothing , I knew of course the faulty guage
would need to be replaced but that this could not be done until the
storekeeper opened at 8 o'clock.........."
Bryan G3GVB
----- Original Message -----
Sent: Sunday, January 16, 2005 11:14
PM
Subject: Re: LF: Re: SWR-off topic
In reply to:
I do not think there is a "true" SWR meter which can work by
measuring
things at only one point on a transmission line.
SWR is a
ratio and can only be indicated by making a set up which responds
to it
and even then only by empirical calibration - I
think.
With all due respect, there's nothing magical about SWR. If the
voltage is uniform along the length of a line (ratio 1:1), it is because the
line is terminated with a load that matches its characteristic
impedance. If not, then reflections exist because of a mismatch.
The extent of that mismatch will be reflected-- no pun intended-- in
the ratio of the voltage at a location where it is maximum along the
line (constructive interference between the forward and reflected wave),
versus a location where it is at a minimum (destructive interference).
The degree of cancellation at the minima is dependent upon the
percentage of power being reflected, and the phase relationship between the
two waves. Thus, "true" SWR can only exist where a transmission line is
involved. But even so, it is still only a tool for measurement, an
*effect* arising from precisely the same phenomena that are
measured by a well constructed reflectometer setup or the power and phase
detectors in a network analyzer.
When measuring VSWR on a physical transmission line, you need
only read the voltage at one of the maxima and the voltage at one of the
minima along the line; whereas, with a reflectometer, the sampler
(be it terminated loops or bridge) must match the intrinsic impedance of
the line to discriminate the direction of power
flow accurately, and you must reference the reflected power as a
percentage of the forward power (PSWR, if you will) then convert the
power ratio to a voltage ratio (VSWR) before you can arrive at a result
expressed in the same units.
Rigorously speaking, without a transmission line as in Peter's example,
it shouldn't be expressed as "SWR" at all; it should
be expressed in terms of "return loss." And yet one is
measuring the same fundamental condition either way, so the common
nomenclature from the days of measuring "true" VSWR has stuck.
John
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