Page 103 - Antennas for Base Stations in Wireless Communications

76 Chapter Two

A 10l array with downtilt adjustable between 0° and 10° requires a
total phase shift across the array of 562°, so the outer arc phase shifter
needs a length of about 280°. If the radiating elements are connected
to the phase shifters using lines of equal length, the available tilt will
be ±5°, so to provide a beamtilt of 0°–10° an intrinsic 5° tilt must be
provided by the fixed lines in the feed network.
There is no single optimum design—each variant provides different
technical and economic trade-offs. An array with uniform phase shift
between every element at all tilt angles requires more phase shifters but
can provide better-shaped elevation patterns and higher gain at extreme
tilt angles. A design with concentric phase shifters will be mechanically
less complex but may provide less control of the current amplitudes
in the inner and outer element pairs—and so may have less ability to
shape the elevation pattern. The use of subarrays of two or three ele-
ments simplifies the design and reduces cost, but at some compromise
to performance over the range of supported tilt angles.
A microstrip design for a variable phase shifter is shown in Figure 2.18.
A successful design must provide a low-input VSWR over the whole
operating band and for any selected value of phase shift.
A dual-polar 10l antenna covering one frequency band (including
wideband antennas covering 1710–2170 MHz) requires two sets of phase
shifters, one for each polarization. Both phase shifters are usually oper-
ated from a single drive, and the beamtilts on both polarizations must

Position of wiper adjusts
Large arc gets largest phase distance between outputs,
shift and is connected to adjusting phase.
outermost radiating subarrays.
Smaller arc causes smaller
phase shift and is attached
to inner subarrays.

Capacitive joint couples
energy while allowing DC ground for Fixed phase output
movement for center subarray
lightning protection
Figure 2.18 Microstrip phase shifter with two concentric arcs (Photo courtesy of
Andrew Corporation)