In article ,
Reg Edwards g4fgq,regp@ZZZbtinternet,com wrote:
If one knows what they are doing, SWR can always be measured.
NOT on a line which isn't there. QED.
In the strong sense of the definition ("A VSWR meter is a meter which
measures, literally, the ratio between the voltage maxima and minima
present on a transmission line"), Reg is correct. This can be done
with a section of slotted line and a probe, of course, tapped into a
section of a transmission line having the same characteristic
impedance.
That's not how a typical amateur "SWR" meter works, though - it's not
either locating or measuring either the maxima or the minima of either
voltage or current on the line. So, the strong, literal, pedantic
sense of the term, I agree with Reg that a standard "SWR" meter is not
truly measuring SWR, and that he's correct in his objection.
However, I also think he's overstating the case.
An "SWR" meter circuit, in the usual sense (e.g. a Monimatch or
similar) can provide an accurate *indirect* measurement of SWR, *if*
the conditions under which it is used are appropriate. It measures
the currents and voltages flowing through it, and derives
(electrically and mathematically) a number which correlates extremely
well to what a true SWR measurement on a T-line of specified impedance
would say. If you build and calibrate this sort of circuit accurately
enough, and then put it in the middle of a section of 50-ohm line (or
whatever it's calibrated for), it'll give the same numbers as a true,
direct measurement of VSWR on a length of T-line at that point.
Now, it's true (as Reg says) that a Monimatch or similar
indirect-measurement meter can give you inaccurate or misleading
numbers, if used in an environment other than what it's designed for.
If you stick a Monimatch at the output of a transmitter or transmatch,
with its output looking into a high-impedance balanced line, then the
numbers it displays won't equal the actual VSWR on the line. You
might be able to come up with a correction factor / curve for it,
though. If you stick it right at the transmitter output, and it reads
1.0:1, then you can be confident that your transmitter is seeing the
load that it wants to see... hence Reg's desire to have it renamed as
a "TLI".
Seems to me, though, that the same is true of a real "VSWR"
measurement system (e.g. a slotted line) if you use it under
inappropriate test conditions. If you take a slotted-line-and-
probe measurement device whose internal line is 50 ohms, and stick it
in the middle of a 75-ohm line, and measure the VSWR on your slotted
line, you'll come up with a number which *does* equal the VSWR in
the slotted line but does *not* equal the VSWR on the actual
transmission line. Same problem, really.
In that sense, even a "real" VSWR meter isn't a "useful" VSWR meter,
if you use it inappropriately.
All that being said: I conclude that there's nothing wrong with
calling a Monimatch (or similar) current/voltage measurement circuit a
"SWR" meter, as it *will* display correct and accurate numbers for the
SWR on the line when used appropriately (within the limits of its
calibration, of course). It's up to the user to understand the
conditions under which this sort of measurement can be made accurately
and usefully... just as it is with every other sort of test instrument
I know of.
Reg, I think you're tilting at windmills.
--
Dave Platt AE6EO
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http://www.radagast.org/jade-warrior
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