Antenna System Selection

This chapter will briefly discuss some of the more important issues associated
with an antenna system regarding 3G applications. The selection of
the antenna type to utilize for base station whether it is for a macro, micro,
or pico cell is similar for all the technology platforms. There are of course
some differences related to the different design issues associated with a 1G,
2G, 2.5G, or 3G system. The key difference in the antenna design issues lies
in the desire to keep the systems either separate or unified depending on
the underlying technology platform the new system is being overlaid upon.
The antenna system for any radio communication platform utilized is
one of the most critical and least understood parts of the system. The
antenna system is the interface between the radio system and the external
environment. The antenna system can consist of a single antenna at the
base station and one at the mobile or receiving station. Primarily the
antenna is used by the base station site and the mobile for establishing and
maintaining the communication link.

There are many types of antennas available, all of which perform specific
functions depending on the application at hand. The type of antenna used
by a system operator can be a collinear, log periodic, folded dipole, or yagi to
mention a few. Coupled with the type of antenna is the notion of an active
or passive antenna. The active antenna usually has some level of electronics
associated with it to enhance its performance. The passive antenna is
more of the classical type where no electronics are associated with its use
and it simply consists entirely of passive elements.

Along with the type of antenna there is the relative pattern of the
antenna indicating in what direction the energy emitted or received from it
will be directed. There are two primary classifications of antennas associated
with directivity for a system, and they are omni and directional. The
omni antennas are used when the desire is to obtain a 360 degree radiation
pattern. The directional antennas are used when a more refined pattern is
desired. The directional pattern is usually needed to facilitate system
growth through frequency reuse or to shape the system’s contour.

The choice of which antenna to use will directly impact the performance
of either the cell or the overall network. The radio engineer is primarily concerned
in the design phase with the base station antenna because this is
the fixed location, and there is some degree of control over the performance
criteria that the engineer can exert on the location.

The correct antenna for the design can overcome coverage problems or
other issues that are trying to be prevented or resolved. The antenna chosen
for the application must take into account a multitude of design issues. Some
of the issues that must be taken into account in the design phase involve the antennas gain, its antenna pattern, the interface or matching to the transmitter,
the receiver utilized for the site, the bandwidth and frequency range
over which the signals desired to be sent will be applicable, its power handling
capabilities, and its IMD performance. Ultimately the antenna you use
for a network needs to match the system RF design objectives.