TopBand: Them Cheatin' Meters Will Tell on You

[Ward Silver]

If Ireland has sectarian violence, and the Balkans have ethnic cleansing,
then ham radio must have SWR arguments.

First - the meter does not "lie"...it tells you exactly what it is capable
of telling you.  No more and no less.  Like all test equipment, SWR meters
can be misapplied and will give inappropriate results in environments for
which their design is inadequate.  If you want accuracy at high SWR, it
takes a different sort of sampling circuit than for high precision at low
SWR.

Second - observe the Smith Chart.  There is one, and only one, point on
the chart at which SWR is 1.0:1.  That is at the point where the impedance
of the load is entirely resistive and exactly matches the characteristic
impedance of the attached lossless transmission line.  If the line is
50-ohm line, then the load must be 50 + j0 ohms.  A reactive load of 0 +
j50 ohms gives an infinite SWR because the purely-reactive load can not
absorb any real power and reflects it all back to the source.  Even
complex loads with a magnitude of 50 ohms will not give an SWR of 1.0:1.

I'm sure that I'm not telling KM1H and W8JIT anything they don't know
well, but I thought I'd interject these points into the discussion...er,
debate...er, knock-down drag-out.  How interesting...a cross between the
World Wrestling Foundation and engineering school!

73, Ward N0AX



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