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.

There is no simple reply, Reg, but you're very welcome to come down and
read three box-files full of references on this subject.

It all depends what you're trying to measu simple forward gain or the
complete directional radiation pattern; absolute or relative gain; and
whether the antenna is a beam or something less directional.

The kind of measurement that is subject to the least errors is a
comparison of forward gain between two or more directional antennas that
are very similar. The more similar the antennas under test (AUTs) are,
the better the errors in each individual measurement will match and
cancel out. The more directional the AUT is, the less its gain
measurement will be affected by unwanted reflections.

The largest source of error in this case is probably in the uniformity
of field strength and phase across the test space where you will
position the AUT. There is no single answer in dB for this: you would
have to estimate the error-bars by modeling on a case-by-case basis.

Amateur measurements, such as those made by VHF Groups in the USA,
typically use a ground reflection range technique that creates a test
volume at a height of about 6-10ft above ground, to make it easily
accessible by standing on a picnic table and waving the antenna about by
hand, but these practical needs will also increase the errors compared
with a professional range with remote-controlled positioning and more
time to do it properly.

However, within their limitations, careful amateur measurements can make
valid better/worse comparisons between very similar antennas.
Reproducibility of gain measurements on the same yagi is within a few
tenths of a dB... and the more similar your AUTs are, the closer you can
approach this limit when comparing different antennas.

Absolute gain measurement is an additional can of worms. The most common
amateur mistake is to attempt to measure gain in dBd by comparing a long
yagi against a reference dipole. BIG MISTAKE! A dipole is so
non-directional, it makes the so-called "reference" measurement very
vulnerable to stray reflections that a sharper beam just doesn't see, so
any so-called "standard dipole" is in fact totally worthless.

Or even worse than worthless, the "results" can be anything you want,
wish or dream of. Amateur antenna literature is full of such examples,
all fueled by over-active imagination.

The solution is to use a reference antenna that is as directional as the
AUTs, and to measure or compute its gain by some other means. For
example, there is an IEEE standard gain reference antenna that has been
designed to be both directional and reproducible (in the sense that its
gain is quite tolerant of construction errors) and the gain of that
antenna has been very carefully measured under the best possible lab
conditions. For microwaves, the usual reference is a standard horn
antenna whose gain can be both measured and computed.

What amateur groups like Scott's tend to do is to keep a "gold standard"
reference yagi that is used for all their own measurement meets - and
above all, to put much more faith in the *relative* gain comparisons
than in the claimed absolute gains.

For HF antennas, the required physical size of the test range scales up
with the wavelength, and all the problems about range reflections and
non-directional of AUTs become impossible for professionals and amateurs
alike. That means even professionals are thrown back to computer
modeling... which amateurs can do equally well.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek