  Two novel decoupling and matching networks (DMNs) in microstrip technology
for three-element uniform circular arrays (UCAs) are investigated and compared
to a more conventional DMN approach with simple neutralization lines. The array
elements are coaxially-fed quarter-wavelength monopole antennas over a finite
groundplane. Three-element arrays are considered since UCAs with an odd number
of elements are able to provide an almost constant maximum array factor over
the whole azimuthal angular range. The new designs are explained from a
theoretical point of view and their implementations are compared to four- and
three-elements UCAs without DMN in terms of decoupling and matching bandwidth
as well as beamforming capabilities. In addition to excellent decoupling and
matching below -16 dB, a broader bandwidth is obtained by the two DMNs. The
reasons for the enhanced bandwidth are similar in both cases: By introducing
several circuit elements offering additional degrees of freedom, matching of
the monopole input impedances at different frequencies becomes feasible. One of
the presented designs offers a larger bandwidth, while the other design is able
to provide a better total efficiency. Scattering parameters, radiation
patterns, beamforming capabilities, and enhanced gain are all verified by
measurements over the operating bandwidth.
