08-06-2013, 04:56 PM
A PRACTICAL MINIATURIZED U-SLOT PATCH ANTENNA WITH ENHANCED BANDWIDTH
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Abstract
In this paper, an asymmetric U-slot patch antenna with
low probe diameter is presented. It will be shown that reduction
in probe diameter causes in reduction in bandwidth. One of the
characteristics of this antenna is keeping the bandwidth in 30% in spite
of reduction in antenna size and use of low probe diameter compared to
antenna presented in [1]. The presented antenna in this paper has been
fabricated by pcb technique and tested. The far-field results have also
been presented based on simulation and measurement. Although the
antenna has high cross polarisation level, in the case of using circular
polarisation, the use of this antenna can be recommended because of
its reduced size, high impedance bandwidth, high total gain in spite of
having low size, and ease of fabrication.
INTRODUCTION
The major disadvantage of microstrip antennas is their low bandwidth.
One of the methods to increase the bandwidth, which has lower
increment in antenna volume behind, is the use of a U-slot in the
single layer single patch antenna. In 1995 a broad band single layer
probe fed patch antenna with a u-shaped slot was presented by Huynh
and Lee [2] and [1]. In [1] a symmetric type of this antenna with 30%
bandwidth and probe with 1.27mm diameter and center operating
frequency of 4.5 GHz with Foam substrate has been presented. Since
probes that are available for us have 1.27mm diameter, in the case of
designing the antenna for this probe and lower frequencies the ratio
of probe diameter to wave length will be reduced. According to [3],
the reduction in probe diameter results in the increment in the fringe
inductive at the feed. Through an example we will show that this
phenomenon causes in reduction in the bandwidth.
SYMMETRIC U-SLOT PATCH ANTENNA
Previous Works
Here, we investigate the antenna mentioned formerly, from [1]. In order
to reduce the operating frequency to a center frequency of 2.25 GHz,
we scaled its structure two times larger, and plotted in Figure 1. This
figure shows the dimensions of patch and U-slot. The U-slot is placed
in the center of the patch and altogether lays on a foam substrate with
permittivity equal to 1 and thickness of 10 mm, then a finite ground
plane with sufficiently large size, approximately two times larger than
the patch. The feed probe has a diameter equal to 2.5 mm. This
antenna has been simulated using HFSS software, and its return loss
plot with a solid line is shown in the Figure 2. It can be found that
this plot in Figure 2, matches with the indications in Section 1, and
shows approximately 30% bandwidth from 1.97 to 2.7 GHz with VSWR
lower than 2 or return loss lower than about −9.5 dB. The probe of
this antenna which has 2.5mm diameter is thicker than the available
probes. The simulation result of this antenna only with changing the
probe diameter to 1.27mm and keeping other parameters fixed, is
shown in dashed line in the same figure. From Figure 2 altogether,
it is presumable that reduction of probe diameter causes in reduction
in bandwidth.
THE ASYMMETRIC U-SLOT PATCH ANTENNA
STRUCTURE
Figure 4 shows the layers of modified U-slot patch antenna with two
layer substrate. According to the Figure 4 a ground plane with 1mm
thickness and dimensions of 10×10 cm connects to the connector shield,
up to it, there is a 8mm thickair layer, and then, a Fr4 substrate
with permittivity of 4.4 with dimensions 65 × 65mm and thickness of
1.6mm with the patch conductor on it. The patch is also connected
to the probe at the feed point. The patch dimensions and the place of
the feed point are shown in Figure 5.
The Simulation and Measurement Results
This antenna has been fabricated (Figure 19), and then tested in
antenna lab of Khajenasir University. The test results together with
simulation results using HFSS has been shown in the Figure 6.
In this figure we see a measurement bandwidth from 1.9 to
2.6 GHz or 31%. This bandwidth is a result of 3 resonant frequencies
in passband that indicates a principal difference with symmetric Uslot
patch antenna, which has 2 resonant frequencies [1]. The probe
used in this antenna is narrower than the corresponding probe in [1]
and according to [3] has higher inductance. But although we have
used narrower probe contrary to the antenna presented in section 2.2
bandwidth does not decreases.
The Antenna Size
The total thickness of this antenna, including Fr4 layer and air layer,
is 0.07 of passband center frequency wave length. This is a little lower
than the antenna thickness in [1] equal to 0.08 of corresponding wave
length. The reduction in the thickness of antenna by decreasing the
air layer thickness causes in increment in quality factor of the patch
resonator [3]. According to Smith Charts plotted in Figure 9, this
change in the air layer thickness, causes an increment in loops size in
the smith chart of the antenna.
CONCLUSION
In this paper a broadband asymmetric U-slot patch antenna with
narrow probe presented. From this antenna capabilities compared
to symmetric U-slot patch antenna, it can be indicated that, it has
reduced size, lower probe diameter, without reduction in bandwidth,
compared to antenna in [1], and being possible to have more bandwidth
than [1] with similar conditions, although it uses a simple structure.
Its low size and being possible to fabricate using pcb technique, can be
used in array applications. This antenna can be recommended when a
circular polarisation is used, but in linear polarisation applications,
it has high cross polarisation level, which should be noticed in its
application. designer should note that the resonant frequencies should
be placed the most far apart each other, but the broad band conditions
also should be verified.