Page 99 - Antennas for Base Stations in Wireless Communications

72 Chapter Two

Although these tools are valuable aids to a competent antenna design
engineer, their use is no substitute for adequate understanding of the
processes at work in the physical antenna. All the available tools are
far better at analysis than at synthesis—given a complete description of
an antenna structure, the tools will compute its electrical behavior with
considerable accuracy. Optimization routines are usually provided, but
these are comparatively unintelligent processes that allow the designer
to explore a range of input parameters and compute the corresponding
performance of the model, seeking an optimum within a defined search
area.
Simulation is powerful in predicting the radiation patterns that will
be obtained from a specified structure and its use can significantly
shorten the time needed to develop a radiating element and reflector
structure with the needed characteristics (VSWR, beamwidths, F/b
ratio, and polarization). The element currents required to provide a
specified elevation pattern can be obtained either by an interactive or
a self-optimizing routine. Having decided on the required radiating cur-
rents, simulation is almost essential for the rapid optimization of array
feed networks following a first-pass design from basic principles.
One of the most useful aspects of simulation is the way in which the
design engineer can examine fields and currents in the antenna. This
knowledge provides a key to understanding how the antenna operates,
and this in turn helps in the understanding of the performance deficien-
cies that usually beset a new design. Useful insights are often obtained
by comparing the operation of the computer model with that of a physi-
cal prototype.
There is a constantly changing point of balance between computer
simulation and practical experiment. Dramatic increases in the speed
and capabilities of physical measurement systems have provided a much
better insight into the performance of antenna prototypes, but neither
impressive measurement systems nor the latest simulation tools can
replace the competent design engineer who can engage in the creative
process of new design and can understand what needs to be done when
problems arise.

2.3.9 Arrays with Remotely
Controlled Electrical Parameters
Electrical beamtilt has been used since the inception of mobile radio
systems as a means of controlling the range of communication from a
base station while maintaining near-in coverage in shadowed regions
and inside buildings. Its application to reduce the interference to and
from users outside the intended coverage area of a mobile radio cell, and
so to improve frequency re-use, was described in 1981 by Lee. 20