Page 77 - Antennas for Base Stations in Wireless Communications

50 Chapter Two

Avoiding these effects requires close impedance matching of individ-
ual tiers of elements over the whole frequency band. For a linear array,
the azimuth beamwidth and its stability with frequency, the control
of squint, and azimuth tracking behavior are all directly determined
by the design of the radiating elements. These are all major factors in
determining their design, and they are often overlooked in proposals
for new broadband element configurations by engineers who have not
encountered the practical aspects of BS array design.
Cross-polar discrimination (XPD) is the ratio of the output of an
antenna to an incoming signal with its nominal polarization relative
to its response to the orthogonal polarization. In the case of ±45° slant
linear polarization, we can easily see that for a simple array such as just
described, there is an intrinsic problem in maintaining a high XPD over
a wide azimuth sector. If we consider an array of dipoles oriented at 45°
to the horizontal and placed in front of a vertical reflecting plane, then, in
the boresight direction, the polarization will conform with the required
45° plane (although the asymmetry caused by a narrow reflector will
limit the XPD). However, as an observer moves away from the array
axis, the angle between the plane of polarization and the horizontal
gradually increases until at 90° from boresight the plane of polarization
is vertical. Unless we design some more elaborate radiating structure,
the XPD for signals with ±45° linear polarization will always fall close
to 0 dB at ±90°. A practical antenna will have an XPD exceeding 20 dB
on boresight, falling to around 10 dB at the –3dB points of the azimuth
pattern. Because the incoming polarization of the real signal is an arbi-
trary ellipse that changes in axial ratio and polarization angle as the
source mobile moves through the environment, the precise polarization
of the receiving antenna in a particular direction is not really important;
what matters is the orthogonality of the polarization responses of the
two sets of elements. Orthogonality may be computed from the complex
12
radiation patterns of the two arrays as follows :

P E1 E2 + E1 E2
= +45ϒ +45ϒ −45ϒ −45ϒ
P max E1 +45ϒ E1 * +45ϒ ϒ + E1 −45 ϒ E1 * −45 ϒ E2 +45 ϒ E2 * +45 ϒ + E2 −45 ϒ E2 * −45 ϒ

where E1 +45° is the +45° field component from Port 1; E2 +45° is the
+45° field component from Port 2; E1 −45° is the −45° field component
∗
from Port 1; E2 −45° is the −45° field component from Port 2; and E is
the complex conjugate of E.
Many arrays are designed to operate over 824–960 MHz (a total band-
width of 15.3%) or 1710–2170 MHz (24%), and over these bands they
must maintain closely constant azimuth patterns, impedance, and cross-
polar isolation.