Page 70 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 43

device and suffers a tendency to squint in its E-plane. In a slant-polar
array, this is in the 45° diagonal plane because of the orientation of
the elements. In a vertical array, it has the rather unexpected effect
of creating an azimuth squint that becomes larger as the beamtilt is
increased—an example of the coupling of parameters between planes
seen in these arrays. The squint is usually frequency dependent and so
is most troublesome when the patch is used in wideband arrays; it can
be rectified by using a balanced feed system for the patch, either by
driving it in antiphase from opposite edges or by exciting the radiating
patch from a balanced structure below it.
In order to recognize the practical limitations of array design, as well
as reflecting the significance of potential coverage and handoff defects,
the permitted tracking error is usually specified separately between and
beyond the nominal –3-dB points of the azimuth pattern. There is prob-
ably no need for the related contributory parameters to be separately
specified—although they often are.
The elevation radiation patterns in each polarization should match
as closely as possible, and because of the high rate of change of signal
level with elevation angle below the main beam, the beam tilts should
remain equal across the operating frequency band.
Cross-Polar Discrimination Most dual-polar antenna systems are required
to have a high degree of cross-polar discrimination (XPD) so each port
receives signals only with the designated polarization. It can be shown
that the XPD needed to provide effective polarization diversity is not
large; this is fortunate because achieving a constant polarization angle
over a wide range of azimuth bearings is not easy. A diagonal patch or
crossed dipole has a polarization angle of 45° in the boresight direction,
but as the angle off boresight increases the polarization angle tends
towards 90° (vertical) simply because of the geometrical arrangement.
The XPD in the boresight direction is typically around 23 dB, whereas at
the edges of a 120° sector it is likely to fall to around 10 dB. The practi-
cal result of this is a progressive fall in diversity gain as a distant MS
moves off the array axis. To some extent, this behavior matches that of
a pair of spatial-diversity antennas where the lateral antenna spacing
falls with the cosine of the angle from boresight.
The incoming signal from an MS generally has elliptical polariza-
tion with an arbitrary axial ratio and polar angle. Because of this, the
diversity behavior of a BS antenna depends on the orthogonality of the
polarization responses of the two receiving arrays rather than the scalar
XPD; the computation of orthogonality requires the measurement of
and E , i.e., when
the complex radiation patterns of both arrays for E V H
the antenna is separately illuminated by plane-polarized signals with
vertical and horizontal polarization.