Page 241 - Antennas for Base Stations in Wireless Communications

214 Chapter Six

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capacitance induced by the patch slots, can effectually compensate for
the probe inductance, resulting in wide impedance bandwidth. In the
meantime, the unwanted cross-polar radiation excited by the feeding
probes is suppressed due to the anti-phase current flow on the probes.
Subsequently, as a refinement of the concept of the DFPA for further
bandwidth broadening, a new approach designated as a vertical-plate
48
pair as a differential feeding scheme has been presented. This tech-
nique is to modify the feeding probe into the vertical plate. As a result,
impedance bandwidth is significantly enhanced. Moreover, by virtue of
the differential feed, excellent radiation performance is attained within
the whole operating band.
The basic geometry of the broadband patch antenna with a vertical plate
50
pair is illustrated in Figure 6.7. The proposed antenna is designed with
the center frequency chosen at f o = 2.17 GHz (l o = 138.25 mm). It consists
of a planar patch, a ground plane, and a pair of vertical plates connected
to the feeding probes of the 50 Ω SMA launchers. The copper patch is
rectangular in shape with P L = 68 mm (0.492l o ) and P W = 60 mm (0.434l o ).
It is located over the ground plane at a height of h p = 16 mm (0.116l o ).
The patch is directly driven by a pair of planar plates that are vertically
placed beneath the patch. The feeding edges of the plates, with L = 43 mm
(0.311l o ), are symmetrically located on two sides of the central line (y-axis)
along the nonresonant direction of the radiating patch. The separation
between the feeding edges is selected to be s = 26 mm (0.188l o ).
Slightly different from the aforementioned vertical-plate fed patch
antenna, this DFPA employs feeding plates folded at the bottom. Each
folded plate is composed of a vertical part and a horizontal part. The
capacitance due to each horizontal part and the ground plane offers an
additional design flexibility to accomplish good impedance matching. By
properly selecting the lengths and widths of both parts, performance
can be optimized. The dimensions of the two sections are L = 43 mm
(0.311l o ), W V = 13.5 mm (0.098l o ), and W H = 7.5 mm (0.054l o ). A gap
is formed between the two horizontal portions, in which the gap size,
denoted as g, is determined by W H and s. Having one edge connected
to a vertical part, each horizontal part, with the height of t = 2.5 mm
(0.018l o ), is driven by a probe.
Resorting to the use of a differential feed, two probes, having radii
of 0.5 mm, are directly connected to the horizontal parts of the folded
plates. Each contact point locates at the center of the corresponding
horizontal part. The distance between the two probe centers is d =
20 mm (0.145l o ). A 2-mm thick aluminum ground plane in a square
shape with G L = G W = 250 mm (1.808l o ) is utilized. The center of the
antenna is aligned with that of the ground plane, resulting in the
symmetric configuration with reference to both the x-axis and y-axis.
Detailed dimensions are depicted in Figure 6.7.