[Elecraft] Antenna Analyser - Phase Angle Sensor

[Larry Phipps]

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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