Page 111 - Antennas for Base Stations in Wireless Communications

84 Chapter Two

In a dual-band array, the coupling between high- and low-band ele-
ment groups can give rise to high-band radiating currents flowing in
low-band elements, and this causes frequency-dependent ripples or
other deformities in the high-band patterns of the array. The amplitude
of low-band currents flowing in high-band elements is usually small
and is seldom troublesome. In an array in which inputs at both bands
are combined using a diplexer, excessive coupling at the element may
interact with the behavior of the diplexer to cause unwanted ripples in
the input VSWR of the high-band array.

2.3.12.2 Feed Networks and Arrays Before building a prototype array,
ensure the feed network provides the expected complex currents at its
output ports and is well matched when all the output ports are correctly
terminated. Although the network may have been simulated before it
was constructed, coaxial feed systems are likely to show some differences
in the stray reactances associated with cable junctions. Inaccuracy in
modeling of interline coupling in microstrip networks may require some
impedance correction on a physical model, particularly if line spacings
are small to economize on the use of expensive laminate materials.
A simple method for the diagnosis of feed network problems is to
sample the outputs of different elements of an array using a balanced
loop. As the loop is moved to couple to successive elements, the mea-
sured ratio of the currents in the sampled elements should be constant
in amplitude, with a phase difference having the correct nominal value
at midband and a value at other frequencies proportional to f/f 0 . This
measurement is easy to carry out using a VNA, and the result can be
displayed in polar or Cartesian form. A corresponding measurement
can be carried out on the feed network itself, terminated with matched
loads, and the difference between the two results is a clear indication
of the effects of element mismatch (including mutual coupling). The
periodicity of ripples in the response give an immediate indication of
the distance separating the sources of interacting reflections, allow-
ing the matching to be improved to reduce the excursions from the
wanted values. The results become confusing if the ports of the VNA are
not themselves well-matched (as is common), so matched attenuators
should be inserted in series at the VNA ports.
The feed network of a RET antenna can be measured in this way to
provide confirmation that there are no unwanted interactions within the
network and that the output currents are correct for the required range
of tilt angles. As an extension of this technique, it is possible to construct
an anechoic “box,” lined with absorber, with arrangements for a probe
to be scanned along the array to measure the relative complex element
currents. These sampled currents can be used to compute the elevation
pattern of the array, often with surprising accuracy. Measurements of