A vector voltmeter is what I used to make my first
complex Z measurements at home. I was doing a
consulting job, so I could purchase a pair of
mini-circuits connectorized directional couplers and
deduct the cost. But, one could built a resistive
return loss bridge, using only one twisted-pair balun
to deal with the "ungrounded leg" issue.
Send the signal through the two dir. couplers, with
one configured to create a "forward" sample, and the
other configured to provide a "reverse" sample.
"Forward" goes to Vector Voltmeter channel A,
"reverse" goes to channel B. With an open circuit,
adjust the phase offset knob on the VVM to show zero
degrees. If you need an amplitude correction, you can
put an attenuator in the line from whichever coupler
sampled port you need to get an amplitude of 1. Now
connect the unknown. The amplitude and phase reading
of the VVM represent the magnitude and phase of the
reflection coefficient. Plot this point on a Smith
Chart and read the complex impedance from there.
I still have the instrument, but along with the rest
of my lab, it's packed away in some unknown storage
place, awaiting the day that I finally select a house.
73,
Dave W8NF
====In-reply-to:====
From: David Kirkby <david.kirkby@onetel.net>
Subject: [Amps] Vector voltmeter on eBay - complex
impedance.
To: "amps@contesting.com" <amps@contesting.com>
<snip>
Some time back there was a discussion on here about
measuring complex
(R
+ jX) impedance. I mentioned a vector voltmeter is a
way I have done it
in the past. There is one on eBay at the minute
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4678&item=7514648769&rd=1
This is not the same model number I used, and I have
not bothered
checking the specs as I don't want one. But something
like this might
be
a better bet than cheap 'amateurish' meters I have
seen around. But you
will need to understand complex numbers to make any
use of it.
I'm not the seller, and have no connection with
him/her, and live on a
different continent.
<snip>
-----------
David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex
http://www.southminster-branch-line.org.uk/
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