Page 71 - Antennas for Base Stations in Wireless Communications

44 Chapter Two

Cross-Polar Isolation (Interport Isolation) We noted previously that with
a spatial-diversity antenna system, both antennas are commonly used
for the downlink, with half the transmitters connected to each antenna
through a hybrid/filter network. In this configuration, there is at least 30
dB isolation between the transmitting antennas so this figure was (and still
is) used in the specification of the required intermodulation performance
of BS transmitters—unfortunately, achieving 30 dB isolation between
the two input ports of a dual-polar antenna is not so easy. Dual-polar
antennas usually comprise arrays of dual-polar patches, crossed dipoles,
or square dipole arrays, and significant coupling exists between the +45°
element of one tier and the –45° elements of the adjacent tiers—another
outcome of arraying the elements along their diagonal direction. To avoid
the radiation of intermodulation products generated by the PA stage of
the transmitters, it is necessary to exceed 30-dB isolation, at least when
measured between the transmitter ends of an antenna feed system.
Dual-polar BS antennas provide an example of the fact that the cross-
polar isolation (XPI) of a dual-polar antenna is, in general, not related
to its boresight XPD.

2.3 The Design of a Practical
Base Station Antenna
Engineers unfamiliar with wideband radiating element design should
consult the chapters of this book that describe the basic principles of
many of the designs mentioned next.

2.3.1 Methods of Construction
The first decision in the design of a base station antenna is the type of
construction to be used for both the radiating elements and the feed
system. Both may be created using printed circuit techniques or using
coaxial cables and fabricated radiating elements. The choice between
these methods and various hybrid methods is largely a matter of cost.
The optimum design will depend strongly on the cost of labor and mate-
rials in the location where manufacture and assembly will take place;
where labor is inexpensive a more labor-intensive construction will cost
less than a form of minimized labor-input construction—for example,
an all-printed circuit design that may have relatively high material and
processing costs. There is no optimum design for all occasions. These eco-
nomic considerations are outside the usual remit of the antenna design
engineer, but they will strongly influence the choice of the most economic
engineering design for a particular company or manufacturing unit.

2.3.1.1 Radiating Elements There are many designs for the radiating
elements of base station antennas and the discussion here is limited to