Page 82 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 55

f. This net directivity of the array is the upper limit to the gain that we
can achieve, even if the antenna were lossless (f = a – b – c – d + e).
g. Losses are inevitable, and will tend to be higher for elements and
feed structures etched on low-cost PCB substrates, whereas fabricated
metal designs generally have low losses. Unfortunately the estima-
tion of losses by many simulation programs is not accurate and is
often optimistic. Element efficiency can be measured by comparing the
input power with the integral of the radiated power measured over a
sphere in an anechoic chamber. As we have seen, the input VSWR of
individual elements should be kept below 1.2:1 for other reasons; at
this level the reflection loss is only 1% (0.04 dB) and can be ignored.
h. For an antenna with length 8l or more, the attenuation of the feed
system is considerable, especially in the high band, and the need to
control it determines the diameters and types of coaxial cables, and
the dimensions and materials of microstrip lines. Losses also cause
heating of the feed system of transmitting antennas and limit their
maximum power rating, so in a mobile base station antenna both
attenuation and heating effects must be considered. Requirements
for elevation pattern shaping and the need to control currents to
avoid excess shaping loss mean that the feed system must have pre-
dictable and repeatable phase characteristics. Because feed networks
are usually deployed in a very limited space, cables are formed on
assembly with bend radii often close to their permitted minimum
limit. Despite this they must accurately maintain their phase lengths;
for this reason, cables with solder-dipped braid have become popular
because they are reasonably flexible and retain their position and
electrical properties after bending. An array design with excessive
internal loss will be suboptimal in several ways: for a given gain, it
will be longer than it needs to be; it will have a narrower elevation
beamwidth than would normally be associated with an antenna of
the specified gain; and it will have a larger visual profile, a higher
windload, and higher cost.
i. Tower-mounted antennas can be fitted with an external connector at
the rear midpoint of the antenna, so the internal cables extend only
from the midpoint to the upper and lower ends of the array. Antennas to
be pole-mounted are usually fitted with bottom-mounted external con-
nectors, so internal cables must extend from the bottom of the antenna
to the midpoint and from there to the ends of the array. This arrange-
ment produces higher internal losses, especially on high band.
j. A radome causes both reflection and absorption of energy radiated
by the antenna elements. The extent of these effects depends on the
dielectric constant, dielectric loss factor, and thickness of the radome.