Page 93 - Antennas for Base Stations in Wireless Communications

66 Chapter Two

Compound Arrays In some instances, integrating a dual high-band
array, comprising two separate laterally spaced element columns with a
single low-band array, may be required. As an example, such an antenna
may provide operation on GSM900, GSM1800, and UMTS2100. In this
case, the space is sufficient to place the low-band elements on the center-
line of the array with the high-band elements on either side, so no co-
located dual-band elements are required.
2.3.4.2 Dual-Band Radiating Elements Most dual-band radiating ele-
ments are conceived as combinations of two separate radiators, with
separate feed systems. Combinations include the following:

■ A high-band patch mounted over a low-band patch in such a way that
the low-band patch forms the groundplane for the high-band patch 17
■ A high-band crossed dipole mounted over a low-band patch 18
■ A high-band crossed dipole surrounded by a 4-square low-band sub-
array 19

The design of these element groups must achieve good control of
the equality and stability of the azimuth beamwidth over each oper-
ating band, together with adequate cross-polar isolation, despite the
presence of conductors in the radiating structure of the “other” band,
which act as parasitic polarization-coupling elements. The achieve-
ment of a fully symmetrical operating environment for every element
is more difficult than it is for a standard single-band array, and it is
typical that additional polarization decoupling arrangements will be
needed.
2.3.4.3 Operational Considerations Many mobile network operators now
have frequency assignments in two or three bands having selected their
BS sites based on the planning of their earlier networks. As new tech-
nologies emerge, re-using and adapting their infrastructure to provide
support for new networks is a continual challenge. From an operator’s
point of view, using separate antennas for each system is always pref-
erable, whether or not the antennas operate on different bands, so one
network can be optimized, upgraded, or replaced with minimal impact
on other networks.
A single multiband antenna provides a solution that is lower in
capital, installation, and tower rental costs than an arrangement with
separate antennas for each band. The multiband antenna will gener-
ally have a lower total weight and windload than separate antennas
and will present a much lower visual profile than separate installa-
tions for each band/system, especially when considering the complete