I'd be interested in the article if you have it in digital form, Mike.
I implement a sign detection algorithm in the Plot program for my LP-100
Digital Vector wattmeter. It works pretty well most of the time, but I
added editing of curve points in case it makes an obvious mistake so
that the curve can be "fixed" after scanning. I can't do it in the box,
because it requires a number of frequency related datapoints to work.
This would be no problem in an antenna analyzer, since the box controls
the frequency, of course.
I think the Autek VA1 uses a dither scheme with their oscillator to
produce the variable frequency, even though it appears you are only
testing at one frequency. It's narrow, but seems accurate with simple
components like caps and coils, and with antennas which have a simple
series or parallel resonance. The problem comes in when measuring an
antenna at the end of a random length of feedline, where the phase slope
can reverse without changing sign. A typical example of this curve would
be a dipole which uses a "broadbanding" network with a "W" shaped curve
at resonance.
A better method for an analyzer is to use a signal source with
quadrature outputs, so that I & Q samples can be generated. Some of the
more expensive analyzers and VNAs do this.
73,
Larry N8LP
Mike Walkington wrote:
Hi,
When discussing antenna analysers there's always a discussion on the issue
of measuring the sign of X. For some years now I've carted round an article
out of the Power Amplifier Handbook titled High-Speed Microprocessor
Controlled Antenna Matching Unit, in which the author implemented a phase
angle sensor which output a logic one for inductive phase angle and logic 0
for capacitive. The circuit didn't look like it was too complex to
implement, so I've always wondered why such a circuit isn't implemented in
every antenna analyser.
Mike
VK1KCK
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