A fourth round with the FA-VA5 antenna analyser

I recently acquired a FA-VA5 antenna analyser.

Before trusting measurements made with any instrument, its behaviour should be validated, and this article documents issues discovered in one thread of tests. The developer does not like the term “defects”, he prefers “issues”, a soft denial of “problems”.

So, the test scenario is the VA5 measuring the impedance looking into a 35m length of RG6 coax with an open circuit at the far end. The VA5 has been SOL calibrated with the higher quality loads sold by SDR-kits, and the test is a 3.5MHz. The firmware is the latest, v1.08 (about 3 months old).

The screenshots are taken with a camera, there does not seem to be a method of uploading screenshots to a PC.

Above is a swept Z measurement just above the half wave resonance of the line section. The impedance at the marker is comparable with that measured using an AA-600, so I would accept that it is probably correct. The graph is another matter.

The image above is the previous image with two areas highlighted in green. In both cases the graph lies, the points plotted are not as scaled, eg at 3500kHz the R component is 536Ω yet it is plotted as 200Ω. Likewise, the X locus does not level off as -210Ω. This is the result of a lazy programmer who has simply truncated data points to the scale limits instead of properly windowing the plot.

A common failure, but a failure.

Above, setting the mode to change the scan width, the display has the text “3500kHz, 200-j139” at lower right. Is this to imply the impedance at 3500kHz? But, the last display with the marker showed 536-j140Ω, X is similar but R is very different… it would appear that the annotation is wrong.. though it does agree with the plotted lines which I have already observed are actually wrong.

The plot above is after adjusting scan width, the plot was redrawn, but with a glitch at the left side which does not appear on the wider scan graph, and is not expected, the glitch is an error.

Above, another glitch. These glitches occur sometimes when changing scan parameter and display modes. If the VA5 is powered off and on again, the plot is correct.

These are just a sampling of “issues” noticed during a 10min validation session.