"Frank" wrote in message
newsNOae.62351$VF5.45329@edtnps89...
"Reg Edwards" wrote in message
...
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.
----
Reg, G4FGQ

Interesting topic Reg. I have always been concerned with uncertainties
involved in antenna measurements. ATR antennas do not always provide the
source for calibration, but assume it could be ANSI/IEEE Std 149-1979.
ETS-Lingren quotes, for their conical log spiral, model 3102 antenna
factor uncertainty as SAE, ARP 958 1M (With which I am not familiar). The
antenna factor uncertainty is specified as +/- 0.8dB from 1 - 10 GHz.
More data, including other conical log spirals, biconicals etc. is
available on ETS-Lingren's web site at www.ets-lingren.com if anybody is
interested.

One company I worked for was making measurements in a 3 meter shielded
ATR. Their distance measurements from the source were measured from the
support pole of a conical log spiral, when it should have been measured
from the tip. With the 3102 antenna this introduced a 3 dB error.
Radiated spurious limits quoted in V/m were also assumed to be peak, but
not specified. Some research indicated that these limits are in fact
RMS -- another 3dB error! When used for linear field measurements the
conical log spiral gain is 3 dB below that for circular polarization.
Conical log spirals are calibrated with a circularly polarized signals.

A conical log spiral antenna's radiating plane moves along it's axis with
frequency. Various models place the support pole at the rear or at the
center of the radiating axis. In any case, use this class of antennas was
strongly discouraged after 1996 by MIL-STD-461D.


Agilents 11940A has an antenna factor uncertainty of +/- 2dB from 30 MHz
to 1 GHz, and is calibrated in the far field, for near field measurements.
ETS-Lingren's antennas are all calibrated at 1m irrespective of frequency.

Your should always calibrate your measurement antenna in accordance with the
applicable testing standard. For MIL-STD-461E, this means a 1-meter
distance. For commercial emission testing, that means separate calibration
tables for 3-meter, 10-meter & 30-meter ranges. And for some conditions,
like FCC Part 18 or broadcast station field-strength "footprints", you
should obtain a true far-field calibration. Calibration at any distance
other than the actual use distance is just not enough.

When questioned about accuracies of measurements people usually say "This
is the way we have always done it". I have very little experience with HF
outdoor measurements, but have heard of sites using high wooden towers to
minimize ground reflection effects. A helicopter is required to plot the
field strength. Very few EMC antenna manufacturers seem concerned with
low frequency far-field measurements. Siemens was about the only company
I could find for such measurements.

Regards,

Frank (VE6CB)


Perhaps the lack of interest in "low frequency far-field" measurements is
driven by an absence of any "low-frequency, far-field" compliance
requirements? OTOH, MIL-STD-461E is quite concerned with radiated E-field
emissions right down to 10 kHz, but at a 1-meter separation distance, this
is decidedly near-field! BTW, calibration of this standard's defined 10 kHz
to 30 MHz test antenna (an electrically short 41" monopole standing above a
small ground plane) is not done on an antenna range! The calibration
technique is all conducted, with a known signal being applied by coax,
through a shielded 10 pF capacitor, to the antenna input point of the
matching network (a box at the base of the 41" rod). The accuracy of the
calibration is dependent only on the test lab's ability to read the RF input
& output voltages.


--
Ed
WB6WSN
El Cajon, CA USA