Page 64 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 37

vertical. A beamtilt of 5° is understood to mean a downtilt of 5°. When
plotting elevation patterns in Cartesian form, the x-axis is arranged
with angles below the horizon to the right of the y-axis and angles above
to the left; although universally labeled angle of elevation, the scale is
really the angle of depression.

2.2.1.2 Gain Gain is a function of both the azimuth beamwidth of the
antenna—which will be selected to suit the manner in which cells are
fitted together to meet frequency re-use requirements—and its electri-
cal length in the vertical plane. The maximum length of a base station
antenna is determined either by the physical size that can be accepted
without becoming too obtrusive (as in the low bands) or by the minimum
acceptable vertical beamwidth, which diminishes as the length and
gain increase. As the antenna length increases, the length and attenu-
ation of internal transmission lines increase; for this reason doubling
the antenna length halves the elevation beamwidth and doubles the
directivity, but does not double the available gain.
A broadside array delivers the maximum possible directivity if all
its elements are excited with equal co-phased currents. In practice the
application of beam-shaping requires nonuniform element currents and
the directivity obtained falls short of that of a uniform array; this short-
fall is known as pattern shaping loss and it increases as the extent of
applied null fill and sidelobe suppression is increased. (The nonunifor-
mity of the element currents results in a wider elevation beamwidth
than would be obtained with a uniform distribution and the reduced
directivity is a direct consequence of the increased beamwidth.)

2.2.1.3 Elevation Beamwidth The elevation beamwidth of a BS array
is a function of its electrical length in the vertical plane. There is a
practical limit to the extent to which the elevation beamwidth can be
reduced in order to obtain increased gain because an antenna with a
very small vertical beamwidth is physically large and, because of its size
and narrow beamwidth, it requires a very rigid support to avoid unac-
ceptable deflection by high winds. In hilly terrain it may not provide
good coverage of the intended service area because users may occupy an
elevation arc of larger angular extent than the antenna can illuminate.
Standard practice is to use antennas up to 8 wavelengths long in the
low bands, and up to 12, or occasionally 16, wavelengths long in the high
bands. These array lengths provide elevation beamwidths of around 7°,
5°, and 3.5°, respectively.

2.2.1.4 Beamtilt For an array with a fixed beamtilt, electrical tilt is
usually effected by adding a linear phase shift across the whole array.
This ensures that only a small change occurs in both gain and elevation