Robert,
there are a couple of points that came up in your posts that I would like to
more fully understand.
1. "...the site under test is verified against the performance of the site on
which the antenna factors are calibrated".
I honestly don't get this point. The applicable standards (ANSI C63.4 and
CISPR22) require that you verify the measured Normalized Site Attenuation (NSA)
against a Theoretical Normalized Site Attenuation (ThNSA) of an ideal site
(infinite perfectly conductive ground plane), NOT against the site on which the
antenna factors are calibrated. Ref. sec.ANSI C63.4-1992 sec.5.4.6.1, CISPR 22 -
1997 (3.ed) sec.10.3.2. This is what the standard says, although its validity
has been questioned by a number of EMC experts.
2. "...a dual antenna factor calibration suffices if the same antennas will be
used in the NSA measurement, there is no accuracy advantage when using a
three-antenna method in this case".
In ANSI C63.5-1988 I read (sec.5.2 page12): "In practice, two antennas are never
identical, and the antenna factor calculated by Eq.8 is the geometric mean of
the individual factor for each of the two antennas.."
That's the reason why I said that the 3 antenna method is more accurate.
Also,your statement: "Antenna factors provided by the manufacturer or measured
by a cal lab are typically not of sufficient accuracy (with a few exceptions)"
reinforces my opinion.
3." There are substantial difference in the antenna factors (and site
attenuation) values at various range distances."
I agree that it's always better to calibrate antennas at the test distance. On
the other end, within the range of 3-10 m distance my experience with broadband
antennas (biconicals and log-periodic) between 30 and 1000 MHz tells me that the
error is well within 1 dB, as long as you are in the far-field at 3 m (which is
the case most of the times using biconicals). I have not direct experience but
my guess is that you may have non-negligible errors for distances < 3m and/or
highly directional antennas (horns & freq.> 1GHz), whereby you can be in the
near field even at 3+ meters distance.
4. I know the article you mentioned (Z. Chen and M.Foegelle: Numerical
Investigations of Ground Plane Effects on Biconical Antenna Factor") exposes one
weakness of the ideal (theoretical) model used for calculating ThNSA (see point
1). The analytically derived model assumes that the antennas used are point
dipoles that behave in a different way than the most used broadband antennas.
They use a numerical analysis to measure the effects of the ground plane on AFs
of biconical antennas (field variations and antenna coupling with the ground
plane). Their conclusions are, in their words, that "if an ANSI C63.5 standard
site calibrated AF is used for ANSI C63.4 (NSA measurements), all the
approximations and ground plane effects exactly cancel".
My understanding is that if you use the same geometry for the calibration of
antennas (3 antenna method) over a ground plane and for the validation of your
site (OATS or SAR), the variations of antenna factors associated with the
coupling antenna-ground plane are compensated and do not add errors to the NSA
measurements. Provided the two ground planes (calibration site and test site to
be qualified) have ground planes that behave in the same way !
So under these conditions, for NSA measurements and site validation you can
limit the job to horizontal polarization (as required by the standards).
The antenna calibration in vertical polarization is useful to improve accuracy
of emissions measurements of products (using vertical and horizontal AFs) other
than in cases - as mentioned by Don Umbdenstock - where the error introduced by
AF variation does not get you over your total budgeted measurement uncertainty.
Any further comment would be highly appreciated.
Regards,
Paolo Roncone
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