[SOC] measuring load impedance: how?

[JMcAulay]

At 07:18 PM 05/04/2003 +0100, Paul wrote:
>Hi John
>
>> The paper referred by Paul mentioned in-line VSWR meters, which measure
>> only Voltage Standing Wave Ratio.  Thus, they will only indicate whether a
>> good match to a predetermined non-reactive resistance exists (almost
>always
>> 50 ohms).  But for many uses, that's good enough.  They are inexpensive,
>
>I confess that I didn't do the sums but the paper quoted mentions that it's
>good for reactive loads too.
>
>Anyhow, for the dozen or so components required it's a good simple test
>instrument.


Right you are on both counts, Paul.  Let me try to be a bit more clear than
my previous post.

The VSWR meter will indicate zero mismatch, same as zero percent reflected
power, same as 1:1 VSWR, if the load has the same impedance as the
reference in the VSWR bridge. The value is usually 50 ohms resistive,
because the indicator is designed for use with 50 ohm transmission line
(RG-8, RG-213, 50-ohm anything else, and it's close enough for RG-58A,
which is 52.5 ohms).  And for the best power transfer, the load should be
non-reactive, with a resistance equal to the output impedance
(non-reactive) of the source (the transmitter), and that resistance should
be the same as the characteristic impedance of the transmission line.  If
the load resistance is not 50 ohms or the load reactance is not zero, or
both, the VSWR indicator will show a VSWR higher than 1:1.  For example, if
there's no load reactance but the resistance is either 25 ohms or 100 ohms,
the VSWR is 2:1.  If the resistance is 50 ohms but the reactance is 35.3
ohms (which is 50 x sqrt 0.5) either inductive or capacitive, you will also
see a VSWR of 2:1.  And there are an infinite number of combinations that
will result in a 2:1 VSWR.  That's the biggest limitation of this
instrument:  if the load is not perfectly matched it will tell you so, but
it won't tell you exactly what's wrong.  However, all is not lost.  If you
are tweaking the match and the VSWR goes up, just crank whatever it was in
the opposite direction, and the VSWR should go down.  After a while, things
may be just fine.  If you don't mind playing around a bit to get the VSWR
as close as possible to 1:1, the VSWR bridge is the cheapest and least
complicated piece of test equipment to use.

Speaking of RG-58A/U, there is a guy on eBay selling it in 50-foot lengths
with BNC connectors on each end (individually packaged) for $3 a section
plus shipping.  He has lots of them.  Fifty feet of that stuff, nice and
flexible, has less than one dB loss at ten meters.  As a sucker for a
pretty face, and a genuine junkaholic, I bought five.

73
John WA6QPL  SOC 263