Page 66 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 39

PIMs, is a severe test of the mechanical design and construction of any
antenna and is only achieved by rigorous attention to detail at every
stage. The practical implications of this requirement are discussed later
in this chapter.

2.2.1.6 Grounding for Lightning Protection Base station antennas are
often mounted on tall structures, and they are likely to suffer the effects of
direct or nearby lightning strikes. To minimize the probability of damage
to the antennas or the systems connected to them, careful attention
must be paid to bonding of conducting parts of the antenna. Antennas
are sometimes protected by a lightning spike mounted to the support-
ing structure, projecting some distance above the top of the antennas,
but sometimes the antennas themselves form the highest point on the
structure, in which case they must be fitted with a lightning spike and
a solid connection to ground, independent of their connecting cables.

2.2.1.7 Mechanical Design A base station antenna is a complex device
that must provide highly reliable service over a period of many years.
The achievement of stable operating characteristics despite the effects
of wind, rain, and pollution requires careful attention to the selection of
materials, the design of joints, and the arrangements for the exclusion of
water from critical areas. This requires some careful design judgments
because the cost and weight of antennas is tightly constrained. Antenna
2,3
specifications often cite international test criteria typically relating to
operation at high and low temperatures, thermal cycling, driving rain,
salt mist, and mechanical vibration. Specifications also constrain the
maximum permitted lateral wind thrust for an antenna and its mount-
ing hardware, for wind in any azimuth direction at a specified velocity,
typically 45 m/s (100 mi/h), but varying with local conditions.
The design of mounting hardware to support an antenna, which may
be up to 2.5 m in length and weigh 20 kg, is complicated by the wide
variety of mounting poles to be accommodated and the requirement to
provide for mechanical tilting of the antenna. The unpredictable veloc-
ity and turbulence of wind flow over the roofs of buildings has led to
occasional unexpected structural failures of antenna mountings. This
has resulted from fatigue failure of the mounting hardware or its points
of attachment to the antenna, induced by the coupled resonant behavior
of the antenna, its mountings, and the supporting structure. In conse-
quence, some operators now wisely specify that antennas mounted in
locations where injury could result from a mounting failure—for example
where antennas could fall into a street—are fitted with a secondary
method of restraint such as a flexible stainless steel wire rope, firmly
anchored to the antenna and the supporting structure.