Page 246 - Antennas for Base Stations in Wireless Communications

New Unidirectional Antennas for Various Wireless Base Stations 219

that the radiation characteristics are stable across the band. Obviously,
the measured cross-polarization levels are around 20 dB less than the
co-polarization levels. The simulated cross-polarization in both E-plane
and H-plane is vanishingly small across the operating band under the
ideal conditions and is thereby not able to be observed. In addition, taking
advantage of the structural symmetry, the co-polarized radiation patterns
in both E-plane and H-plane are symmetric with respect to the broadside
direction within the band. Noteworthy, the measured maximum power
is always in the broadside direction, which offers benefits to many wire-
less communications systems, in which stable radiation patterns and the
purity of wave polarization within operating bandwidths are required. In
addition, the backlobe radiation level is less than –15 dB.

6.3 Complementary Antennas Composed of
an Electric Dipole and a Magnetic Dipole
After several decades of developing wideband antennas, many simple
and wideband antenna elements have flourished and contributed to real
applications in many wireless communication systems. Among many
categories of antennas, dipole and patch antennas are the two popular
types. They have been diversified into many wideband antenna designs.
The basic structures of these two types of wideband antennas are low in
profile, easily fabricated, and convenient for having directional radiation
patterns. Nonetheless, they have the disadvantages of large variations
in gain and beamwidth over the operating bandwidth, different beam-
widths in the E- and H-planes, and strong radiation in the backlobe.
In this section, the design of wideband unidirectional antennas with
low cross-polarization, low back radiation, symmetric E- and H-plane
patterns, and stable gain is introduced. To accomplish this mission,
a wideband complementary antenna consisting of an electric dipole
and a magnetic dipole has been developed. This concept of exciting two
complementary sources was first initiated in 1954 for achieving equal
E- and H-plane patterns. To realize this idea, much subsequent research
has been done and has resulted in similar antenna performances.
Most of these suggest using a combination of either slot-dipole or slot-
monopole to form complementary antennas with a directional pattern
that is equal in the E- and H-planes. However, they are still compli-
cated in structure or are narrow in bandwidth. To overcome these prob-
lems, a new wideband antenna element that is formed by combining a
planar dipole antenna and a shorted patch antenna is acquainted. This
new wideband element achieves excellent performance in all electrical
parameters. In particular, its low back radiation characteristic makes
it highly attractive for developing various kinds of indoor and outdoor
base station antennas for modern cellular communications. This is