Page 234 - Antennas for Base Stations in Wireless Communications

New Unidirectional Antennas for Various Wireless Base Stations 207

This chapter focuses on the development of wideband directional
antennas with low cross-polarization, a stable radiation pattern, steady
gain, and a simple architecture. First, microstrip/patch antennas fed
by a twin-L probe design and by an M-probe design are introduced.
Then, a differential-plate fed patch antenna is demonstrated. Finally, a
wideband complementary antenna composed of a planar dipole and a
shorted patch antenna is discussed.

6.2 Patch Antennas

6.2.1 Twin L-Shaped Probes
Fed Patch Antenna
One popular technique for enhancing the bandwidth of patch antennas
17
is to use parasitic elements, as in co-planar 42–44 or stacked geometry.
Another common method for enhancing bandwidth is to employ a coax-
ial probe to feed a slotted patch on an electrically thick substrate (0.08–
0.1l) of low dielectric constant. 22,45 Such designs yield an impedance
bandwidth up to 30% (SWR ≤ 2). However, no noticeable improvement
in gain is seen. On the other hand, although the gain can be improved
18
to around 10 dBi (estimated) by enlarging the separation between the
driven patch and the stacked parasitic patch to approximately 0.3l, the
trade-off is narrow in bandwidth, around 2–3%. The gain can also be
increased in co-planar geometry by placing the parasitic patches adja-
46
cent to the fed patch to form an array, but its gain-bandwidth is not
wide enough for most applications. In addition, the major disadvantage
of using parasitic elements is an unfavorably large antenna size. To
enhance both bandwidth and gain for a single antenna element, a twin
29
L-shaped probe fed patch antenna is suggested. This idea of using
47
parallel feeds is a simple twin-feed structure with two in-phased L-
9
probes to further enhance the gain to 10 dBi while maintaining wide-
band performance. More importantly, the achieved 1-dB-gain bandwidth
is wide enough to cover the operating bandwidth.
A twin L-shaped probe fed patch antenna that operates at a center
frequency of 5 GHz is shown in Figure 6.1. The copper patch has a thick-
ness of 0.3 mm (0.005l), a width W = 44 mm (0.733l), and a length L =
W
22 mm (0.367l). The aspect ratio of the patch is equal to 2.0. The patch
L
is supported by two small cubic foam-spacers (ε r ≅ 1) of thickness H =
6 mm (0.1l). The fundamental mode (TM 01 ) of the patch is simultane-
ously excited by the two in-phased L-shaped probes (with probe radius =
0.5 mm), which are separated by S = 28.6 mm (0.477l) and connected
to the microstrip feed network mounted on the other side of the ground
plane. The simple T-shaped power divider is etched on a Duriod substrate
= 2.33 and a thickness of 1.5748 mm (0.062 inch). The square
with ε r