measured frequency response of another filter prototype is shown.
This filter has practically the same physical and geometrical parameters as the one shown in Figure 6, except that it uses a smaller
coupling capacitor, Cc. It is observed that the fractional bandwidth
of the filter is reduced to 9% with a measured insertion loss of 1.3
dB. The increased insertion is expected as the filter bandwidth is
reduced [7], while the resonators’ Qs are maintained.
4. CONCLUSION
Figure 7 Measurement and full-wave simulation results of the thirdorder band-pass filter shown in Figure 6. The filter has a fractional
bandwidth of 19% in the pass-band
Since the value of the desired coupling capacitor, Cc, is small,
it can simply be implemented by creating a gap discontinuity in
center conductor of the CPW line. In the present case, this discontinuity is in the form of a very small interdigital capacitor as shown
in Figure 6. To further conserve space, the transmission lines
connecting the interdigital capacitors to the resonator are meandered as seen in Figure 6. The overall dimension of the basic
third-order filter implemented in CPW technology is 22 mm ⫻11
mm. This corresponds to electrical dimensions of 0.14␭0 ⫻
0.07␭0, where ␭0 is the wavelength at 2 GHz. The third-order filter
is then fabricated using standard lithography techniques and a
photo of the fabricated structure is shown in the inset of Figure 6.
The filter response is measured using a calibrated vector network
analyzer and the results are presented in Figure 7 along with the
simulation results obtained from the full-wave simulations. As
observed from this figure, a good agreement between the measurement and simulation results is observed. The fabricated prototype
has a fractional bandwidth of 19%, measured insertion loss of 1.15
dB, and a center frequency of 1.85 GHz.
The theoretical analysis of the filter reveals that the loaded
quality factor of the resonator, QL, is the main factor which
determines the fractional bandwidth of the filter. Therefore, to
achieve a narrow-band response, Cc must be reduced to increase
QL and vice versa. This is demonstrated in Figure 8 where the
A new filter topology for designing odd-order band-pass filters is
presented. Using this filter topology, the task of designing higherorder band-pass filters is simplified to design and optimization of
the unit cell proposed in this article. It was demonstrated that this
filter topology does not occupy a large area and is amenable to
design of both narrow-band and wideband filters with minimal
design modification. Measurement results of two fabricated prototypes were presented to confirm the design procedure as well as
the operation principles of the proposed filter.
REFERENCES
1. R. Azadegan and K. Sarabandi, Miniature high-Q double-spiral slot-line
resonator filters, IEEE Trans Microwave Theory Tech 52 (2004), 1548 –
1557.
2. F. Aryanfar and K. Sarabandi, Compact millimeter-wave filters using
distributed capacitively loaded CPW resonators, IEEE Trans Microwave Theory Tech 54 (2006), 1161–1165.
3. L. Lin, M. Ho, and W. Xu, Design of compact CPW bandpass filters
with wide stopband, Microwave Opt Technol Lett 49 (2007), 973–976.
4. A. Abbaspour-Tamijani, L. Dussopt, and G.M. Rebeiz,, Miniature and
tunable filters using MEMS capacitors, IEEE Trans Microwave Theory
Tech 51 (2003), 1878 –1885.
5. G. Matthaei, E.M.T. Jones, and L. Young, Microwave filters, impedance-matching networks, and coupling structures, Artech House, Norwood, MA, 1980.
6. F. Aryanfar and K. Sarabandi, Characterization of semilumped CPW
elements for millimeter-wave filter design, IEEE Trans Microwave
Theory Tech 53 (2005), 1288 –1929.
7. I. Hunter, R. Ranson, A. Guyette, and A. Abunjaileh, Microwave filter
design from a systems perspective, IEEE Microwave Mag 8 (2007),
71–77.
© 2008 Wiley Periodicals, Inc.
APERTURE-COUPLED C-SHAPE SLOT
CUT SQUARE MICROSTRIP ANTENNA
FOR CIRCULAR POLARIZATION
Nasimuddin Z. N. Chen and Xianming Qing
Institute for Infocomm Research, 20 Science Park Road, 02-21/25
TeleTech Park, Singapore 117674, Singapore; Corresponding author:
nasimuddin@i2r.a-star.edu.sg
Received 30 March 2008
Figure 8 Measurement and full-wave simulation results of a third-order
filter prototype similar to the one shown in Figure 6. In this case, the filter
has a fractional bandwidth of 9%
DOI 10.1002/mop
ABSTRACT: A new C-shape slot cut square microstrip antenna is proposed for circular polarization (CP) using an aperture-coupled feed.
The C-shape slot cut square microstrip antenna is fed at the center using
an aperture coupled to get CP operation. About 4.3% measured 3-dB axial
ratio (AR) bandwidth with 13% 10-dB return loss bandwidth is achieved.
The overall antenna size is 65.0 mm ⫻ 65.0 mm ⫻ 11.5 mm. The proposed
technology can be used to design low-cost, compact, broadband circularly
polarized microstrip antennas and arrays. © 2008 Wiley Periodicals, Inc.
Microwave Opt Technol Lett 50: 3175–3178, 2008; Published online in
Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.
23913
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 12, December 2008
3175
Key words: circular polarization; aperture coupled; slot cut microstrip
patch; axial ratio
1. INTRODUCTION
Circularly polarized microstrip antennas are widely used in many
wireless communication applications. Circular polarization (CP) is
beneficial because commercial and military applications require
additional design freedom for alignment of the polarization at the
receiving and transmitting locations. Single-feed wideband circularly polarized microstrip antennas are currently receiving much
attention. Single feed allows a reduction in the complexity, weight,
and loss of any array feed, which is desirable in situations where
it is difficult to accommodate dual orthogonal feeds with a power
divider network. On the other hand, single-feed circularly polarized microstrip antennas can be arrayed and easily fed like any
linearly polarized patch antenna. However, single-feed microstrip
antennas usually have very narrow 3-dB axial ratio (AR) bandwidth, which is not useful for many wireless communication
applications [1, 2]. Several kinds of patch radiating elements like
square, circular, triangular, and ring shapes have been used to
obtain a CP using a single feed [3]. Iwasaki [4] has reported a
circularly polarized circular patch antenna with a centrally located
cross-slot on the patch conductor using a proximity feed. The
cross-rectangular slot provides necessary perturbation to excite
dual orthogonal modes to generate CP. However, the 2-dB AR
bandwidth of asymmetric cross-slot in the circular patch is 0.65%.
In [5], another design was proposed, in which CP operation was
accomplished by using an U-slot of unequal lengths embedded in
the square microstrip using single coaxial feed. The asymmetrical
U-slot structure can generate two orthogonal modes for CP radiation; therefore, no extra stubs, notches, or chamfering at corners
of the square patch are necessary. The overall antenna size was 102
mm ⫻ 102 mm ⫻ 14 mm. The resulting 3-dB AR bandwidth was
4%. However, the coaxial feed in this antenna makes it unsuitable
for design of the low-cost antenna array. The aperture-coupled
feed methods have been attracting much attention, because their
geometries are suitable for monolithic integration with microwave
or millimeter devices. Also the aperture-coupled feed is suitable
for the array design.
In this article, we propose a new C-shape slot cut square
microstrip antenna for CP. The C-shape slot is embedded in the
square microstrip patch and fed by an aperture coupling. The
C-slot dimensions are optimized to radiate a wide circularly polarized wave. The antenna can achieve CP without a hybrid coupler to excite two orthogonal modes with equal magnitude and 90°
relative phase shift. The proposed antenna design and optimization
is conducted with the help of a commercial EM software, IE3D.
The designed antenna is fabricated and tested. The measured
results are compared with the results obtained by IE3D.
2. Proposed Antenna Geometry
The cross-section of the aperture-coupled circularly polarized Cshape slot cut square microstrip antenna is shown in Figure 1(a). A
C-slot with width, S, is embedded in the square patch as shown in
Figure 1(b). The r is the radius of the arc, and the C-shape slot has
four arcs. The two arcs are combined on one side and the other two
arcs are separated by length, Sl2 as shown in Figure 1(b). The
aperture-coupled feed is located at the center of the C-shape slot
cut square microstrip antenna. The single mode (x or y) could be
selected by placing the C-shape slot, which resulted in a perturbation segment, parallel to the x or y direction in order to shift the
resonant frequency of one mode below or above that of the other
mode so as to radiate circularly polarized waves. The frequency
3176
Figure 1 Proposed aperture coupled circularly polarized microstrip antenna: (a) cross-section, (b) C-shape cut square microstrip patch
shift and two orthogonal modes with equal magnitude and 90°
relative phase shift could radiate a good circularly polarized wave
over a wide frequency range. This can be easily obtained by
properly adjusting the lengths (Sl1 and Sl2) of the C-shape slot. The
50-⍀ microstrip feed line and aperture printed on a substrate with
a thickness of 1.524 mm (h1) has a relative permittivity of 3.38.
The aperture size is length (La ⫽ 34.0 mm) and width (Wa ⫽ 3.0
mm). The aperture-coupled feed of the antenna is shown in Figure
2. The ground plane size is G ⫻ G. The square microstrip patch
with embedded C-slot has a side length L, and is fabricated with a
0.5-mm copper sheet and mounted on a foam of thickness, h2 and
␧r ⫽ 1.07. The C-shape slot dimensions are optimized for good
CP. From the optimization, the slot dimensions of the proposed
circularly polarized antenna should be Sl1 ⫽ Sl2 ⱖ 2r. The antenna’s designed and optimized dimensions are given in Table 1.
3. RESULTS AND DISCUSSIONS
The designed antenna is fabricated and tested. The fabricated
antenna performances are measured by an Agilent vector network
analyzer N5230A and Orbit MiDAS antenna test system. Figure
3(a) shows the measured and simulated return loss of the antenna
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 12, December 2008
DOI 10.1002/mop
Figure 2 Aperture-coupled feed of the antenna
with frequency. The measured 10-dB return loss bandwidth is 13%
(2.18 –2.485 GHz) and the simulated one is 13.5% (2.17–2.49
GHz). The measured return loss with frequency is in good agreement with simulated results. The AR is measured using a spinning
linear method, where a rotating linearly polarized transmit horn
antenna is used to measure the CP performance of the antenna. The
measured and simulated AR at boresight of the antenna is shown
in Figure 3(b). The measured and simulated 3-dB AR bandwidth is
within the 10-dB return loss bandwidth frequency range. The
measured 3-dB AR bandwidth is 4.3% (2.30 –2.40GHz) and follows simulated AR curve with frequency. Figure 3(c) shows the
measured and simulated gain at boresight with frequency of the
antenna. The measured maximum gain is around 6.8 dBic at 2.35
GHz. The gain at boresight is more than 6.0 dBic over the 3-dB
AR bandwidth. The measured gain values are in good agreement
with the simulated gain results.
Figure 4 shows the measured spinning linear radiation pattern
at 2.32 and 2.34 GHz, respectively, on the xz- and yz-plane of the
antenna. A rotating linearly polarized transmit horn antenna is used
to measure the CP performance of the antenna. On both principle
planes (xz and yz), the AR is found to be less than 3 dB across a
TABLE 1
Designed and Optimized Antenna Dimensions
Parameters
(mm)
L
Ground plane (G ⫻ G)
S
Sl1
Sl2
r
Wf
La
Wa
Sf
h1
h2
DOI 10.1002/mop
44.7
65.0 ⫻ 65.0
2.0
14.0
14.0
6.85
3.6
34.0
3.0
4.5
1.524
10.0
Figure 3 (a) Measured and simulated return loss with frequency. (b)
Measured and simulated AR at boresight with frequency. (c) Measured and
simulated gain at boresight with frequency
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 12, December 2008
3177
Figure 4 Measured spinning linear radiation patterns at (a) 2.32 GHz and (b) 2.34 GHz for both xz- and yz-plane
90° beamwidth over the 3-dB AR bandwidth frequency range. The
slight discrepancy between the planes (xz and yz) AR patterns is
due to the antenna mounting error.
4. CONCLUSION
A new aperture coupled C-shape slot cut square microstrip antenna
has presented for circular polarization. The antenna has a return
loss bandwidth of 13%, 3-dB AR bandwidth of 4.3%, and a gain
of more than 6 dBic over the 3-dB AR frequency range. The 3-dB
AR beamwidth is around 90° over the CP bandwidth. The proposed C-shape slot cut microstrip patch is useful to design lowcost compact circularly polarized antennas and arrays.
Benalla (Eds.), Microstrip antenna design, Artech House, Norwood,
MA, 1988, pp. 313–321.
3. K.L. Wong, Compact circularly polarized microstrip antennas, In: Compact and broad band microstrip antenna, Wiley, 2002, New York, NY,
Chapter 5, pp. 162–220.
4. H. Iwasaki, A circularly polarized small size microstrip antennas with
cross slot, IEEE Trans Antenna Propag 44 (1996), 1399 –1401.
5. K.-F. Tong and T.-P. Wong, Circularly polarized U-slot antenna, IEEE
Trans Antenna Propag 55 (2007), 2382–2385.
© 2008 Wiley Periodicals, Inc.
REFERENCES
1. J.R. James, P.S. Hall, and C. Wood, Microstrip antenna theory and
design, Peter Peregrinus Ltd., London, UK, 1981.
2. M. Haneishi and S. Yoshida, A design method of circularly polarized
rectangular microstrip antenna by one-point feed, In: K.C. Gupta and A.
3178
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 12, December 2008
DOI 10.1002/mop