Page 94 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 67

tower-top equipment including antennas, brackets, and headframes,
and the strength, weight, and visibility of the necessary supporting
structure. The downside is that a six-port antenna (three bands, two
polarizations) is a very complex device and failure in any of its six con-
stituent arrays will require its replacement, potentially putting three
networks off the air, possibly for several hours until rigging operations
are complete.

2.3.5 Feed Networks
A typical feed network is shown in Figure 2.12. In this example, the
input signal is divided in three stages. The primary power divider is a
two-way splitter feeding the upper and lower halves of the array; the
tertiary dividers each feed a pair of radiating elements; and the second-
ary dividers have the appropriate number of ports, depending on the
number of elements in the whole array. In an array with a number of
elements that is not a power of 2 (2, 4, 8, or 16), an additional stage of
power division is sometimes used.

2.3.5.1 General Design Considerations The first step in feed network
design is to compute the required power division ratio needed at each
junction. As noted, it is common for adjacent elements to be assigned
currents with equal amplitudes, and this adjustment should be carried
out before designing the network as it reduces the number of different
power ratios needed.
Power dividers often have only two output ports, especially if cable
impedance matching sections are used. If a power divider feeds a dif-
ferent number of elements from each output, then the elements with
smaller currents are grouped together so that every power divider has
output powers that are as equal as possible; in general, it is easier
to ensure a constant ratio from a wideband divider when the output
powers are comparable.
The effects of mismatch at junctions can be mitigated by using hybrid
or Wilkinson power dividers, but these are expensive and may not be
practicable. Not only must the element impedances be well matched,
but also any transformation ratio needed between the junction and the
transmission line feeding the elements must be achieved in a manner
that provides stability over a wide bandwidth. To achieve this, it is
common to employ multisection impedance transformers that can be
designed initially by using a Smith Chart and then optimized using a
suitable simulation program. A high VSWR at the input of the second-
ary dividers will cause phase errors in the primary divider, resulting
in an unstable elevation beamtilt—a problem that must be avoided to