Page 233 - Antennas for Base Stations in Wireless Communications

206 Chapter Six

However, wideband dipole antennas do not have a very stable radiation
pattern over the operating bandwidth. Their radiation patterns can
change substantially depending on the frequency.
The second popular unidirectional antenna is the microstrip/patch
antenna. Many publications are available regarding the designs of wide-
band patch antennas, for instance, a patch with an L-probe feed 9–13 or an
aperture coupled feed, 14–16 double resonances by stacked patches 17–21 or
a U-slot patch 22–27 , and so on. These techniques give the patch antenna
a very wide impedance bandwidth (from 20% to 40%) enhancement,
which is sufficient for covering many wireless communication systems.
However, this class of wideband antennas 9–27 has several weaknesses:
high cross-polarization and large variations in gain and beamwidth
over the operating band. Although some techniques, such as anti-phase
29
28
cancellation, a twin-L probes coupled feed, an M-probe feed, 30–34 and
so on, were suggested for suppressing cross-polarization, these antennas
still are weak in terms of gain and beamwidth variations with frequency
as well as different beamwidths in the E- and H-planes.
In order to achieve stable radiation over the entire operating band, the
third approach of using complementary antennas combining an electric
dipole and magnetic dipole should be explored. This idea for a comple-
mentary antenna with equal E- and H-plane patterns was revealed in
35
1954 by Clavin. An electric dipole has a figure-8 radiation pattern
in the E-plane and a figure-O pattern in the H-plane, whereas a mag-
netic dipole has a figure-O pattern in the E-plane and a figure-8 in the
H-plane. If both electric and magnetic dipoles can be excited simultane-
ously with appropriate amplitude and phase, a unidirectional radiation
pattern with equal E- and H- planes can be obtained. A practical design
36
was proposed by Clavin again in 1974. Another design, which consists
37
of a passive dipole placed in front of a slot, was also reported by King.
Similarly, this idea, based on a slot-and-dipole combination, 38–40 was
realized by other investigators; nonetheless, all of these designs 35–40 are
either narrow in bandwidth or bulky in structure. They may not fulfill
the requirements for current wireless communication systems. Recently,
a new wideband unidirectional antenna element composed of a planar
dipole and a vertically oriented shorted patch antenna has been pre-
41
sented. This antenna is based on the complementary concept of exciting
the electric dipole and the magnetic dipole simultaneously. And it has
many advantages, including a simple structure, wide bandwidth, low
cross-polarization, a symmetrical radiation pattern, and in particular,
very low back radiation. Because of the low back radiation, the gain and
beamwidth of the antenna are not noticeably changed with frequency,
and the gain and efficiency of the antenna are higher than that of many
other antenna elements available in the literature. This antenna finds
numerous applications in modern wireless communications.