DRAFT Recommended Practice for Measurements and ...

1/29/98 53 C95.3-1991 Revision — 2 nd Draft
10/98 Draft
n = λd /a 2 . For horns of a given geometric and electrical design (i.e. a family of ''standard
gain horns'' from a particular manufacturer for use at the various waveguide operational
frequency bands), the ratio b/a and η are approximately constant and, according to Eq
4.3, the power density for a particular value of n is inversely proportional to the aperture
area. It is desirable to have n as large as possible to reduce the gain uncertainty;
therefore, if P T is limited, it is necessary to use smaller apertures in order to achieve the
required calibration field strength.
Hence, if one desires to calibrate antennas at short distances, because a long-distance
range is not available, or to avoid the expense of high-power systems and to avoid the
complications caused by standing waves due to multipath reflections from an imperfect
anechoic chamber, the near-zone gain should be known. Two possible techniques for
determining the near-zone gain follow. If one antenna is small (an open-ended
waveguide, for example) and its far-field gain is known, it can be used to determine the
effective on-axis gain of a larger antenna at relatively short distances by means of Eq 4.2.
The measurements should be reasonably accurate (≈ 0.5 dB) so long as d is greater
than four times a 2 /λ for the small antenna. With respect to distance from the larger
antenna, the primary considerations are that the field gradients be small in the calibration
region and the wavefront should approximate a plane-wave. These conditions will be
satisfied reasonably well at a 2 /λ for the large antenna. The dimensions of the receiving
aperture or the sensing elements of the probe being calibrated should also be less than
the aperture dimensions of the small antenna. This procedure was followed by [B126],
which claims an overall uncertainty in the calibrating field of ±0.5 dB from 1 GHz to 18
GHz and ±1 dB up to 35 GHz.
4.5.1.3 Small Apertures. In view of the discussion in the two preceding paragraphs,
there is no advantage in using a large antenna as a source. In fact, one can operate at
closer distances with less transmitter power if the source antenna is kept relatively small.
Open-ended waveguides are perhaps the smallest practical source antennas. They are
readily available, do not have serious mismatch problems, and yet have sufficient
directive gain to concentrate the energy in the calibration region and facilitate the
suppression of scattered energy in the test chamber. Further, one can easily operate at
distances greater than four a 2 /λ. However, an open-ended waveguide antenna should
consist of a section of waveguide whose aperture end extends several wavelengths from
any flanges or bends. Also, the aperture (radiating) end should be very cleanly cut in the
plane perpendicular to the axis of propagation of the guide. For common open-ended
waveguide apertures with a two-to-one aspect ratio, i.e., a/b = 2, the far-field gain is
approximated by the equation [B79].
G = 21.6 fa (Eq 4.4)
where f is the frequency in GHz and a is the width (larger dimension) of the waveguide
aperture in meters.
When it is necessary to calibrate a large number of nominally identical hazard meters,
the extrapolation method described in [B99] is useful when applied as follows Let B d be
the meter indication with the probe at an arbitrary near-field distance d, and B o the
indication with the probe at a large distance d o where far-field conditions hold. One can
write the relations
B o = KW o
(Eq
4.5)
B d = KW d
Copyright © 1998 IEEE. All rights reserved. This is an unapproved IEEE Standards Draft,
subject to change.