Page 135 - Antennas for Base Stations in Wireless Communications

108 Chapter Three

They are supported by plastic posts above the ground plane. The lower
patch is proximity-coupled by an L-shaped probe located under its edge.
In order to reduce the cross-polarization of the array, which is mainly
contributed by the vertical portions of the two L-shaped probes, the
horizontal portions of these probes are located at the opposite edges with
out-of-phase excitations. Thus, a wideband two-way power divider with
an output phase difference of 180° is needed to feed these probes. The
short-circuited stub of the power divider is used to improve impedance
matching and provide a dc ground to the antenna. This divider is
fabricated on a microwave substrate with a dielectric constant of 2.65.
Each L-probe is made from a copper strip with a width of 4.13 mm
and a thickness of 1 mm. Four vertical sidewalls are mounted on the
edges of the ground plane for the purpose of reducing the backlobe
level. To further enhance their effects, a novel backlobe suppression
technique is employed, which is implemented by cutting slots on the
two sidewalls parallel to the axis of the array periodically. The antenna
dimensions are given in Figure 3.7, where l 0 is the free space wave-
length at 0.89 GHz.

3.2.2.2 Measurement Results In order to evaluate the effect of the two
slotted sidewalls on the standing wave ratio and backlobe level of the
array, an antenna array without slotted sidewalls (shown in Figure 3.8)
was also fabricated and tested. Its standing wave ratio against fre-
quency is shown in Figure 3.9. Clearly, the antenna array with slot-
ted sidewalls located parallel to the array axis has a wider impedance
bandwidth of 18% for a SWR < 1.5, ranging from 0.81 to 0.97 GHz. The
radiation patterns of these two arrays at several frequencies within
the operating band—0.824 GHz, 0.89 GHz, and 0.96 GHz are shown in
Figures 3.10 and 3.11. We observe that the array with slotted sidewalls
has a better backlobe level than the case without slots. Also, both of
them have a wide 3-dB beamwidth of more than 90°. The measured gain
of the array is about 9.5 dBi, the gain against frequency is not shown
for brevity.

3.2.2.3 Summary A 90° linearly polarized antenna array was designed
and tested. Measured results demonstrate that it has a standing wave
ratio of less than 1.5 over 0.82 to 0.96 GHz, which covers both CDMA
and GSM mobile communication frequency bands. Within this frequency
range, it has a wide 3-dB beamwidth of more than 90° in the horizontal
plane. Also, it can achieve low backlobe radiation in both planes if slot-
ted sidewalls are included.