Microstrip patch miniaturization by slots loading
Hung Tien Nguyen, Sima Noghanian and Lot Shafai
University of Manitoba, Winnipeg, MB, Canada, R3T 5V6
htnguyeneee.umanitoba.ca, sima@ee.umanitoba.ca, shafai@ee.umanitoba.ca
The effect of slot loading on microstrip patch antennas is investigated. Initially, Koch
island fractal and H-shape slots are introduced to microstrip patch antennas and their
effect on reduction of the resonant frequency is determined. Then, additional slots of
more complex geometry are implemented on the H-shaped patch to further reduce its
resonance frequency. Their effects on the impedance bandwidth are also investigated.
Introduction
The demand for low profile, portable and compact wireless systems, has increased
research interests in miniature antennas. There are many different approaches to meet
specifications of small size antennas. Koch island fractal and H-shape slotted microstrip
patches are good candidates for miniature antennas, due to their simple geometries.
In above two antenna types, the resonance frequency reduction is achieved by increasing
the electrical length of the antennas. In other words, the surface current is redirected so
that it has to take a longer path length on the microstrip patch. By using this concept,
more slots are implemented on the existing H-shape patch antenna to further increment
its surface currents path length. In the following sections, the Ansoft Designer simulation
results of above antennas are presented. The experimental results for the H-shape and
multi-slotted antennas are also included and discussed.
Effects of Slot Dimensions on Miniaturization of Microstrip Patch
A 65.33 mm square patch with a substrate thickness of 1.6 mm and relative permittivity
of 2.2, fed by a coaxial probe, is used for investigation. Then slots, are introduced on the
patch to generate Koch island (with iteration ratio of 0.25) and H-shape patches. Fig. 1
shows the patch shapes. The resonance frequencies for the original patch, Koch island, Hshape with wide slot and H-shape with thin slot are 1.51 GHz, 1.158 GHz, 1.127 GHz
and 855 MHz, respectively. We can see that the top and bottom slots in the Koch island
patch do not effect significantly the resonance frequency reduction. This is clear by
noting that, the resonant frequency of the H-shape patch with a wide notch (notch length
and width are both 16.33mm) in Fig. Ic is about the same as that of Koch island patch
shown in Fig. lb, where the top and bottom slots are removed. In Fig. Id, the notch
length and width are 26.33mm and 8mm respectively.
To investigate the relation between the notch sizes and resonant frequency we performed
a parametric study. In the first set of simulations, the notch length is changed from 10mm
to 26.33mm, while the notch width is kept at 5mm. The resonance frequency decreases as
the notch length increases. In the second set of simulations, the notch length is kept at
26.33mm and the notch width is changed from 1mm to 20mm. The narrower notch
0-7803-8883-6/05/$20.00 ©2005 IEEE
215
provides a smaller resonance frequency. The relationships between the effective total
edge length (L+21+2AL), the resonance frequency and half wavelength in the dielectric
and directivity of the patches are shown in Table I and Table 2. It can be seen that, as the
notch becomes longer, the total effective edge length becomes closer to the half
wavelength in dielectric. This can be used to define the notch length based on the desired
frequency, then the notch width can be changed to fine tune the resonant frequency.
')
IX
b)
c)
)
Fig. 1: Fractal microstrip patches (a) square patch (b) Koch Island patch, (c) H-shape
with thick notches, (d) H-shape with thin notches
..
j.
i
a
-~
.
T
I_ 11
I
Fig. 2: Retum loss for square patch, Koch island and H-shape microstrip patch antennas
Table 1: H-shape patch resonantfrequencie and effective patch length relationships with
different notch lengths (eff = 2.1274, AL = 0.8449 mm, Le= L+21 + 2AL mm)
(GHz)
(GHz)
x (mm)
1.37
150.133
1.24
165.87
,1d (mm)
Y2
(d)
10
87.0198
1.1577
7.3742
15
97.0198
1.1698
7.2707
I(mm)
1.10
186.98
20
107.0198
1.1447
7.1573
0.887
231.71
26.33
119.6S98
1 .0330
7.0701
216
Table 2: H-shape patch resonantfrequencies and effective patch length relationships
with different notch widths
A
Y2()
(dB)
1
1.117
7.1102
5
1.033
7.0701
8
0.990
7.0791
15
0.927
9.3749
0.901
7.0625
f, (GHz)
K( )
0.96
214.2523
0.87
231.7102
0.85
241.7885
0.80
258.1774
0.77
265.5677
20
In order to increase the physical edge length, smaller slots were added to the notches.
These modifications are shown in Fig. 3. In Fig. 3a the H-shape patch with w = 5mm and
1=26.33m is shown. Fig. 3b shows four additional vertical slots, each with a length of 10
mm width of 5 mm. In Fig. 3c four more horizontal slots with a width of 5 mm and
length of 15 mm are added and finally, in Fig. 3d eight vertical slots with a width of 3
mm and length of 10 mm are added. As it is expected, additional slots reduce the
resonance frequencies to 888 MHz, 801 MHz, 680 MHz and 570 MHz for configuration
(a), (b), (c) and (d), respectively. The multi-slot configuration shown in Fig. 3d shows the
resonant frequency reduction down to 0.38fo.
b) s
4
aN
e
Fig. 3: Notch modifications in H-shape antenna to reduce its resonant frequency
T
4W
Y
4 7
_
,
.5I .1.
O-
I
,L
d.
-.--,,Lsdb
J,
Ias~dC
.na ctdd
Fig. 4: Return loss for different patch configurations shown in Fig. 3
217
Experimental Results
Two patch antennas were fabricated. The first patch was H-shaped with w = 5 mm and
1=36.33 mm. The second was a multi-slot patch with the configuration shown in Fig. 3d.
However, the permittivity of the substrate was 2.5 (the above simulations were for 2.2)
and all dimensions were the same as above. The simulated return losses for the H-shape
and multi-slot patch (Er = 2.5) are 835 MHz and 540 MHz. The corresponding
measurement results are 851 MHz and 551 MHz. Fig. 5 shows the simulation and
measurement results for these two antennas.
Fig. 5: Measured and simulated resonance frequencies for H-shape and multi-slot
antennas
Conclusions
Different methods for miniaturization of a square microstrip patch were studied and a
novel fractal patch with multiple-slots was developed. A parametric study of the H-shape
slots was performed and shown that, its resonant frequency decreases as the apparent
current path on the patch is increased due to the slots
Acknowledgement
The authors acknowledge the financial support by the Natural Sciences and Engineering
Research Council of Canada.
[I]
[2]
References:
I. Kim, J. Yook and H. Park, "Fractal-Shape Small Size Microstrip Patch
Antenna", Microwave and Optical Technology Letter, Vol. 34, No. 1, July 2002,
pp. 15-17.
J. Anguera, L. Boada, C. Puente, C. Borja and J. Soler, "Stacked H-shaped
Microstrip Patch Antenna", IEEE Trans. Antennas and Propagation, Vol. 52,
No. 4, pp. 983-993, 2004.
218