Page 442 - Complete Wireless Design

Wireless Issues

Wireless Issues 441

Figure 10.23 A microwave dish
antenna.

the actual size of the dish, with its diameter being about 10 times its wave-
length of interest for high gain, low loss, and extreme directivity.
Figure 10.24 displays the helical antenna, which is constructed of a noncon-
ducting center strut helically wrapped with wire or tubing. This type of anten-
na is circularly polarized, so the opposing antenna of the system must be
wound with the same sense, or little or no reception is possible. While the heli-
cal is a simple and low-cost antenna to design and build, its gain and beam
width do not compare favorably to those of the dish antenna.

10.8.3 Antenna issues
Antennas are an important consideration in optimizing wireless system per-
formance, especially in a multipoint, sectorized environment, where one type
may act as a sectorized unidirectional antenna, with a different frequency or
polarization allotted for each adjacent sector. The following are some consid-
erations in such multipoint applications:

Horizontal beamwidth should be chosen to reduce overlap into adjacent
sectors in order to increase BER performance.
Front-to-back ratio and sidelobe suppression must be maximized to reduce
cochannel interference, and thus improve the BER (antennas are available
with a front-to-back ratio of up to 40 dB, but this figure can be degraded by
10 dB or more by multipath effects).
VSWR, if at or below 2:1, will not normally be a large concern, since the
resulting system loss will be only 0.5 dB or less.

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