Page 251 - Antennas for Base Stations in Wireless Communications

224 Chapter Six

50 Ω air microstrip line for transmitting electrical signal from the coax-
ial launcher to the second portion of the feed. The second portion, which
is located horizontally, is responsible for coupling the electrical signal to
the planar dipole and the shorted patch antenna. The input resistance
of the antenna is controlled by the length of this portion. This portion
is equivalent to an inductive reactance, which causes the antenna to
be totally mismatched. The third portion incorporated with the second
vertical plate forms an open circuited transmission line. The equivalent
circuit of this line is a capacitor. By selecting the appropriate length for
this portion, its capacitive reactance can be used to compensate for the
inductive reactance caused by the second portion.
In the conventional designs of complementary antennas, 35–40,52,53 both
electric dipoles and magnetic dipoles are excited and separated with a
specific distance (around 0.25l) for controlling the proper amplitude
and phase of the two radiating sources. On the contrary, in our proposed
design, two dipoles are presented with a zero separation, as shown in
Figure 6.16. The unique structure of this proposed antenna results in
forming an inverted-U equivalent magnetic current from the aperture
of the vertically oriented patch antenna. Therefore, the width of the
antenna, W, is one of the key parameters for adjusting the proper ampli-
tude and phase of the two current elements for achieving equal E- and
H-plane radiation patterns.
A comparison of the simulated radiation patterns of a thin dipole,
a planar dipole, and a wideband complementary antenna is depicted
in Figure 6.18. The three antennas have the same ground plane size
(160 mm × 160 mm) as well as the same antenna height (30 mm, 0.25l).
The length of the dipole, L, for the three cases is chosen to be 60 mm,
which is equal to 0.5l at the operating frequency of 2.5 GHz. For cases
Figure 6.18a and b, the antennas are excited by a conventional coax-
ial cable with a balun; for Figure 6.18c, the antenna is excited by a
Γ-shaped strip feed.
The simulated results demonstrate that when the conventional thin
dipole, as presented in Figure 6.18a, is modified to become a planar
dipole (Figure 6.18b), the radiation pattern does not change much.
However, when the planar dipole is combined with the open end of a
vertically oriented shorted patch antenna, as shown in Figure 6.18c,
the beamwidths in the E- and H-planes become similar. Moreover, the
level of back radiation is also smaller than the cases without the shorted
patch antenna by about 10 dB. In addition, the three antennas also
have low cross-polarization due to the symmetry in the geometries of
the antennas. The level of the cross-polarization among the three cases
is less than –40 dB, thus they did not appear on the graphs presented
in Figure 6.18.