On Sun, 24 Apr 2005 05:44:34 +0000 (UTC), "Reg Edwards"
wrote:

All electrical calibration and testing laboratories issue tables of
claimed accuracies of measurements. Measurement uncertainties stated
on calibration certificates are legally binding. All stated
measurement results must be traceable to International Standards or a
laboratory or testing station loses its status.

Consequently there is no incentive for a laboratory to overstate its
capabilities in its sales literature. Indeed, it is dangerous,
illegal even!

Naturally, laboratories can differ widely, one from another.

It would be interesting to compare laboratory uncertainties with
performance figures claimed by antenna manufacturers. Or anyone else.

Does anyone have typical examples of measurement uncertainties claimed
by antenna testing stations? Answers in decibels please.

A reply from a testing station, at HF or VHF, would be specially
appreciated.

As stated by Ian, there's no simple answer. The bane of antenna
testing is reflections reflections reflections.

It may come as a surprise to our correspondent who likes to disparage
"gurus" that "standard-gain" antennas are widely used as reference
standards. To head off the question of how the standard gain is
determined, that is done by testing three "identical" antennas in
pairs; each one against the other two, with one the source and the
other the receiver. A bit of algebra and you have the gain of each
one individually.

http://www.mi-technologies.com/literature/a00-044.pdf

The foregoing paper might help answer Reg's question about achievable
accuracy.

While not addressing hf and vhf measurements, some of the following
might be of interest.

Indoor measurements are usually conducted in anechoic chambers where
the shape is often tapered to control reflections and the walls are
covered in absorber material. A chamber will have a "quiet zone"
where the reflections are specified to be X db down. Very often the
antennas under test are being characterized for side lobe levels or in
the case of monopulse radar, the null depth of the difference
pattern(s). If you're trying to measure a 60 dB null, it doesn't pay
to have a quiet zone of -40 dB.

These measurements also require an amplitude and phase front that
mimics a source at infinite distance. This used to require huge
chambers, often hundreds of feet long. A new way to accomplish this
is to "fold" the range by using specially shaped reflectors to flatten
the amplitude/phase across the test aperature. This has the added
benefit of shorter cables between sources, DUT and measurement
receiver. At X and K band, cable loss can be a killer. Likewise
moving cables around and even temperature changes can affect the
measurments.

I have used such a range to measure antennas from L to Ka band.

Outdoor ranges often "feature" the ground reflection, since it is
difficult to eliminate it physically. This is particularly true at
hf/vhf. I have used a technique that utilized the time-domain
capability of a modern network analyzer (HP-8510) to identify the
reflection and then place absorber material to attenuate it.
Similarly, a frequency-domain measurement, that includes ground
reflection, can be transformed to the time domain where the reflection
is gated out and then transformed back to the frequency domain for
"reflection free" analysis.

See also:

http://www.lehman-inc.com/pdf/mag2.pdf