Page 249 - Antennas for Base Stations in Wireless Communications

222 Chapter Six

z
y
Conical monopole
x
Slot

Ground plane
Cavity

Excitation

To SMA
launcher
Figure 6.15 Configuration of a monopole-slot antenna 53

6.3.4 New Wideband Unidirectional
Antenna Element
A novel wideband antenna with unidirectional radiation pattern is pre-
sented in this section. This newly designed antenna has many advantages,
including a simple structure, wide bandwidth, low cross-polarization,
a symmetrical radiation pattern, and in particular, extremely low back
radiation. The gain and beamwidth of the antenna are almost constant
over the operating frequency band. The antenna can be simply devel-
oped by using two metallic L-shaped plates with a Γ-shaped feed. Such
architecture offers an advantage of forming a vertically oriented shorted
patch antenna and a planar dipole. With the presence of the shorted
patch, a magnetic dipole can be implemented, while an electric dipole
can be realized together through the planar dipole. In this design, the
antenna has a wide impedance bandwidth due to the Γ-strip coupled
line and the double resonance from the planar dipole and the shorted
patch antenna. Because the Γ-feed consists of an L-strip coupled line,
which is a well-known bandwidth-broadening technique for microstrip
antennas and short-circuited patch antennas, achieving a wide imped-
ance bandwidth for the proposed antenna is easy.

6.3.4.1 Complementary Antennas Composed of a Vertically Oriented Shorted
Patch and Planar Dipole The proposed design is based on the approach of
combining an electric dipole antenna and a magnetic dipole antenna. From
among many choices of electric dipoles, a planar dipole antenna has been
chosen, as shown in Figure 6.16a, whereas a wideband short-circuited