Page 108 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 81

an azimuth-over-elevation mount for the AUT or a high mounting loca-
tion for the AUT with the illuminating antenna at a lower elevation. For
slant-polar antennas, the AUT must be illuminated in turn with plane
waves polarized at ±45°.
Some facilities are equipped with very large anechoic chambers, but if
valid far-field measurements are to be obtained, their use requires the
application of correction techniques. These may include the installation
of movable or multiple source antennas and the recording of complex
field values for later computation. Such systems can be considered as a
class of very large near-field ranges.
Far-field measurements are fast to perform, and data for a large
number of frequencies can be gathered in a single rotation of the AUT.
It is also easy to conduct a swept gain/frequency measurement that
provides confidence that there are no frequencies at which the gain of
the antenna is unexpectedly lower than elsewhere. The main drawbacks
are the need for land for the facility, the cost of handling antennas, and
problems caused by inclement weather.
When measuring the radiation patterns of a dual-polar array, it is
important to appreciate that the radiation in the forward direction has
the opposite polarization sense to that of the rearward radiation (seen
from behind, the radiating element appears to have a mirror image ori-
entation compared with when it is seen from the front). This means that
to assess the F/b ratio, we must compare the forward radiation of one
polarization with the maximum rearward radiation of the other polar-
ization (or both of them to be certain of capturing the largest value).
Sometimes a total rearward power is computed by adding both polariza-
tion contributions power-wise.
Near-field measurements are usually carried out in a screened anechoic
chamber; they have the advantage of ease of accessibility and use and
being unaffected by weather conditions. The field radiated by the AUT
is probed by a small horn or open-ended waveguide. Because the main
area of interest in the radiation pattern of a BS antenna is within a
few degrees of the horizontal plane, a cylindrical near-field system can
be used rather than a full three-dimensional system—although high-
angle grating lobes will not be seen. The main constraint of a near-field
system is that gain measurements are only available at those frequen-
cies at which a pattern measurement is made, so no frequency-swept
gain measurement is possible.
Some near-field ranges are provided with multiple illuminating
horns, allowing electronic switching of the source antenna rather than
much slower physical manipulation. A system of this type provides fast
measurements allowing more frequencies to be measured in a given
time, but the security of a continuous gain/frequency measurement is
still missing.