Page 109 - Antennas for Base Stations in Wireless Communications

82 Chapter Two

With some near-field systems, measuring two components of the
E-field (E V and E H ) may be necessary in order to synthesize a pattern
with ±45° polarization; with other systems, illuminating the AUT
directly with the required polarization may be possible, so it may take
only half the time to measure a single pattern—often this is all that is
needed to compare results during the development process. A wideband
illuminating horn allows measurements on both low and high bands
to take place during one measurement process. The use of a dual-polar
horn with electronic switching is an effective way to accelerate mea-
surements.
An ideal arrangement is a combination of both types of range as
there will always be some need for the measurements that can only
be performed on the far-field range, whereas the ease of use and
accessibility of the near-field range is a huge bonus, especially in the
winter!

2.3.11.2 Gain Measurements The internal complexity of a base station
antenna and the long electrical length of the transmission lines con-
necting its components make it possible that, in some regions, the gain-
frequency curve has minima affecting relatively small frequency ranges.
This possibility makes continuous frequency-swept gain measurements
important, at least during product development and until confidence
is established that a product has stable and substantially frequency-
independent characteristics.
Comparing the gain of a base station antenna with a standard
gain horn is prone to errors caused by the very different ground
reflections that affect measurements with antennas with such differ-
ent beamwidths (say 6° and 90° in the vertical and horizontal planes
for the BS array and around 20° in both planes for the standard gain
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horn). A better procedure is to use the three-antenna method to
calibrate a base station antenna as a secondary standard. By this
method, site reflections affecting the (secondary) gain standard and
the AUT are similar in magnitude, so more accurate results can be
obtained.

2.3.11.3 VSWR and Cross-Polar Isolation Measurements VSWR and XPI
measurements are normally made using swept-frequency network ana-
lyzers. In development, these will usually be vector network analyzers
(VNAs). In production, vector or scalar analyzers may be used, and it is
common for the test gear to be controlled by a computer that, following
a scan of a barcode on the AUT, sets up the correct frequency bands and
test limits, prompts the test technician to make the appropriate con-
nections for each measurement, records the result to an archive, and
prints a test certificate.