Characteristics of aperture-coupled coplanar microstrip subarrays T.M.Au K. F. lo n g K.M. Luk Indexing mm,y: Microstrip untennus Abstract: The characteristics of aperture-coupled microstrip antennas with parasitic elements are studied. Four configurations for increasing the bandwidth of aperture-coupled microstrip antennas are described. Additional patches are gap-coupled to the nonradiating edges of the rectangular fed patch. The spectral domain Green function approach and the reciprocity method are used for the analysis. The SWR bandwidth and the E- and H-plane beamwidths can be improved by changing the sizes and the locations of the parasitic patches. Low backlobe radiation can be achieved by locating a plane reflector slightly bellow the microstrip feedline. The bandwidth of the microstrip subarray can also be enhanced with the addition of the plane reflector. The theory is confirmed by experimental data. - 1 Introduction Microstrip antennas are inherently narrow bandwidth and low gain. The technique of utilising parasitic sub- arrays in the design of microstrip antenna arrays has the advantage of increasing the bandwidth [1] and of reducing the spurious radiation from the feed line. The aperture-coupled microstrip antenna [2] has the feature of isolating the feed networks by the ground plane from the radiation element and of providing size reduc- tion for active devices using a higher permittivity mate- rial. Experimental results for the return loss and the radial ion patterns of an aperture-coupled five-patch- cross coplanar microstrip subarray were demonstrated [3]. The lOdB return loss bandwidth of this microstrip subarray was found to be 10.5%~ This paper presents a reciprocity analysis of a rectan- gular microstrip antenna with coplanar parasitic ele- ments. Each of the configurations to be considered has additional patches which are gap-coupled to the nonra- diating edges of the fed rectangular patch. SWR band- @ IEE, 1997 IEE Procedings online no. 19971017 Paper first received 15th July and in revised form 14th November 1996 T.M. Piu is with the Center for Wireless Communications, National University of Singapore, 10 Kent Ridge Crescent, Engineering Block 4 #O 1-04, Singapore 1 19260 K.F. Tong and K.M. Luk are with the Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong width and far-field radiation patterns of the microstrip antenna are investigated. The spectral domain Green function approach and the reciprocity method are used. The formulation can be considered as a generalisation of the method by Pozar [4]. Numerical results for sev- eral aperture-coupled coplanar microstrip subarrays are obtained to compare with measurements. / "_" rectanguia I- stat a Y --+----- \ micmstrig line feed I~ b Fig. 1 putmitic ekemennls a Top view; h side view Geometry of un uperture-coupled microstrip antenna with two 2 Analysis The geometries of the configurations with two and with four parasitic elements are shown, respectively, in Figs. 1 and 2. Each configuration can be divided into two regions. In the feed region (z 5 0), an ;-directed infi- nitely long microstrip line is located at z = -d and y = 0. The dielectric constant and thickness of the feed sub- strate are and d. remectivelv. The centre of the aner- 'J . .. ~ . ture is located at the origin. In the patch region (z 2 U), the patches on z = 1 are supported by a dielectric layer gap-coupled to the nonradiating edges of the rectangu- 137 IEE Proc.-Mwow. Antennus I'ropag , Vol. 144, No 2, April 1997
Transcript
Characteristics of aperture-coupled coplanar microstrip subarrays
T.M.Au K. F. l o n g K.M. Luk
Indexing mm,y: Microstrip untennus
Abstract: The characteristics of aperture-coupled microstrip antennas with parasitic elements are studied. Four configurations for increasing the bandwidth of aperture-coupled microstrip antennas are described. Additional patches are gap-coupled to the nonradiating edges of the rectangular fed patch. The spectral domain Green function approach and the reciprocity method are used for the analysis. The SWR bandwidth and the E- and H-plane beamwidths can be improved by changing the sizes and the locations of the parasitic patches. Low backlobe radiation can be achieved by locating a plane reflector slightly bellow the microstrip feedline. The bandwidth of the microstrip subarray can also be enhanced with the addition of the plane reflector. The theory is confirmed by experimental data. -
1 Introduction
Microstrip antennas are inherently narrow bandwidth and low gain. The technique of utilising parasitic sub- arrays in the design of microstrip antenna arrays has the advantage of increasing the bandwidth [1] and of reducing the spurious radiation from the feed line. The aperture-coupled microstrip antenna [2] has the feature of isolating the feed networks by the ground plane from the radiation element and of providing size reduc- tion for active devices using a higher permittivity mate- rial. Experimental results for the return loss and the radial ion patterns of an aperture-coupled five-patch- cross coplanar microstrip subarray were demonstrated [3]. The lOdB return loss bandwidth of this microstrip subarray was found to be 10.5%~
This paper presents a reciprocity analysis of a rectan- gular microstrip antenna with coplanar parasitic ele- ments. Each of the configurations to be considered has additional patches which are gap-coupled to the nonra- diating edges of the fed rectangular patch. SWR band-
@ IEE, 1997 IEE Procedings online no. 19971017 Paper first received 15th July and in revised form 14th November 1996 T.M. Piu is with the Center for Wireless Communications, National University of Singapore, 10 Kent Ridge Crescent, Engineering Block 4 #O 1-04, Singapore 1 19260 K.F. Tong and K.M. Luk are with the Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong
width and far-field radiation patterns of the microstrip antenna are investigated. The spectral domain Green function approach and the reciprocity method are used. The formulation can be considered as a generalisation of the method by Pozar [4]. Numerical results for sev- eral aperture-coupled coplanar microstrip subarrays are obtained to compare with measurements.
/ "_" rectanguia I- stat
a
Y
--+----- \
micmstrig line feed
I~
b Fig. 1 putmitic ekemennls a Top view; h side view
Geometry of un uperture-coupled microstrip antenna with two
2 Analysis
The geometries of the configurations with two and with four parasitic elements are shown, respectively, in Figs. 1 and 2. Each configuration can be divided into two regions. In the feed region ( z 5 0), an ;-directed infi- nitely long microstrip line is located at z = -d and y = 0. The dielectric constant and thickness of the feed sub- strate are and d. remectivelv. The centre of the aner-
' J . . . ~ . ture is located at the origin. In the patch region ( z 2 U), the patches on z = 1 are supported by a dielectric layer
gap-coupled to the nonradiating edges of the rectangu-
137 IEE Proc.-Mwow. Antennus I'ropag , Vol. 144, No 2, April 1997
lar patch. The resonant lengths of parasitic patches are equal to those of the fed patch, i.e. al = a2. The dimen- sions of the fed patch and parasitic patches are 2al x 2w2 and 2al x 2w2, respectively. The displacement in the x-direction between the centres of the fed patch and the parasitic elements for the four parasitic elements case (Fig. 2) is xd. The centre of the fed patch is located at (xl, 0, I> and the centres of the parasitic patches are located at (xl*xdr 2y2, I), with xd = 0 for the two parasitic element case
dielectric substrate i
i
a
COppQr i
b Fig. 2 parasitic elements a Top view; h side view
Geometry of an aperture-coupled microstrip antenna with four
The formulation of the problem requires the use of the equivalence principle. The coupling aperture in the ground plane is replaced by an equivalent surface mag- netic current density on the ground plane. The electro- magnetic fields produced by individual current elements are obtained by considering Fourier trans- forms of the field components in different regions and then matching the tangential field components at all interfaces with appropriate boundary conditions. Rig- orous spectral domain Green functions are then derived [4]. The reflection coefficient in the microstrip feedline is introduced by using the reciprocity theorem.
Table 1: Dimensions of the subarrays
The moment method, together with the Galerkin procedure, are employed to determine the unknown surface current densities on the patches and the aper- ture. The electric and magnetic field integral equations are formulated by enforcing appropriate boundary con- ditions at the interfaces. One of the joundary condi- tions is zero tangential electric field E on each metal patch. Another boundary2ondition is the continuity of the tangential magnetic H across the aperture. Piece- wise sinusoidal functions are used as basis and testing functions. The paths of integration are selected to avoid the surface wave poles. A Gaussian quadrature procedure is applied to evaluate the integrals. The tech- nique for evaluating each submatrix element can be referred to in [4, 51. The matrix equations are solved by a Gaussian method with complete pivoting.
3 Results
Four aperture-coupled microstrip subarrays are investi- gated first. The sizes of the parasitic patches are identi- cal for each configuration. The dielectric constant and thickness of the antenna substrate are E, = 2.32 and 1 = 3.2mm, respectively. The dimensions of the fed patch are 19.3 (2aJ x 28.5" (214,). The dielectric constant of the feed substrate is Erf = 2.32 and its thickness is d = 1.6".
For fixed sizes of aperture and fed patch, the maxi- mum SWR bandwidth is attained by varying x,, w2 and xd (xd = 0, for the two parasitic element case). The dimensions of each configuration are shown in Table 1. Subarrays 1L and IS have two parasitic elements, while subarrays 2L and 2s have four parasitic elements. In all cases, the gap between the two adjacent metal patches is 1.0" in the y-direction. The width of the open-circuited microstrip line is wf and the length of the stub from x = 0 to the open end of the microstrip feedline is +.
Figs. 3-6 show the input SWR against frequency and far-field radiation patterns for subarrays. 1s and 2L. For the E-plane radiation pattern it is observed that the strongest radiation direction shifts slightly away from the broadside direction at higher operating frequencies. The front to back ratio and the crosspo- larisation are increased with operating frequency. The beamwidth is slightly reduced at higher operating frequencies. The crosspolarisation levels are always < -35dB for all cases. The calculated 3dB beamwidth, SWR bandwidth, percentage bandwidth (%BW), mid- band frequency A., lower and upper cutoff frequencies CfL, f H ) for all the configurations as mentioned above are shown in Table 2. Good agreement between theory and experiment is observed. With a smaller aperture,
Configuration
Subarray 1L
Subarray 2L
Subarray I S
Subarray 2 s
Subarray L Subarray S
z r o z r o
W f ' f a0 WO XI w2 xd Y2
4.6 16.0 0.75 15.0 8.9 7.125 0.0 22.375
4.6 16.0 0.75 15.0 8.9 7.125 11.65 22.375
4.8 14.0 0.25 1.25 7.5 8.550 0.0 23.800
4.8 14.0 0.25 12.5 7.5 8.550 12.65 23.80
4.6 16.0 0.75 15.0 8.9 - - -
4.8 14.0 0.25 12.5 7.5 - - -
138
Values in millimetres L denotes a large aperture size, while S denotes a small one
Proc . -Muow. Antennas Puopag., Vol. 144, No. 2, Apvil i Y Y 7
Table 2: Calculated parameters of the subarrays
Calculated (measured) Configuration
fL, GHz fH, GHz f,, GHz 3dB beamwidth E x H BW, GHz %BW
Subarray 1L 3.896 4.400 4.148 118" x 62" 0.504 12.15
Subarray 2L 3.980 (3.810) 4.540 (4.620) 4.260 (4.215) 94" x 52" (94" x 52") 0.560 (0.81) 13.15 (19.21)
Subarray I S 3.812 (3.810) 4.530 (4.480) 4.171 (4.145) 116" x 52" (116" x 52") 0.718 (0.67) 17.21 (16.16)
Subarray 2 s 3.935 4.534 4.235 92" x 50" 0.599 14.14
Single-patch L 3.943 4.247 4.095 120" x 78" 0.304 7.42
Single-patch S 4.063 4.335 4.199 118" x 78" 0.272 6.48
the SWR bandwidth of subarray. 1s is slightly greater than that of subarray 1L. It is found that subarray 2L attains the largest bandwidth. For brevity, the results of the input SWR and far-field radiation patterns for subarrays 1L and 1s are not shown.
3 5 3 7 3 9 L 1 4 3 L 5 1 7 L 9 -180 -120-60 0 60 120180 frequency, GHz 9, deg
a b Fig. 3 (tuhairay IS) a Input SWR against frequency 0 0 0 mcdsurcd ~ calculated h Fdr-field rddidtion patterns for operating frequency of 3 RGHz
Calculaicd Measured
Character irtics of apei ture-coupled coplanai miuosti ip subai ra j
~~~~ Lopolarised E-plane ** copolarised H-planc 0 an
copolarised H-planc 0 C 3 ........ . . crosspolarised E-plane
crosspolarised H-plane
As observed in Figs. 3-6, the backlobe level may be too high for practical applications. Conceptually, the backlobe radiation can be eliminated by using an infi- nite plane reflector sheet one-quarter of a free-space wavelength below the feed substrate. We are going to investigate this approach. The reflector is assumed to be located at z = -d-t. Since the dominant radiation is contributed by the metal patches, the spacing t between the feed substrate and the reflector may be permitted to be much smaller that one-quarter of a free-space wave- length. This could preserve the low profile characteris- tic of a microstrip antenna. The SWR bandwidths and the far-field radiation patterns of subarrays with plane refled ors are studied.
a b Fig. 5 (subarray 2L) U Input SWR against frequency: 0 0 0 measured ~ calculated h Far-field radiation patterns for operating frequency of 4.0GHz:
Calculated
Characteristics of aperture-coupled coplanar microstrip subarray
Me as u r e d ~~~~ copolarised E-plane * *
copolarised H-plane 0 no
...... - crosspolarised H-plane
........... crosspolarised E-plane
-180-120 -60 0 60 120 180 0 , deQ 8, deQ 0 b
Fig. 6 u f = 4.2GHz; b f = 4.5GHz
Calculated Measured ~~~~ copolarised E-plane **
. . . copolarised H-plane 0 n o
. . . . . . crosspolariscd H-planc
Far-field radiation patterns as Fig. j f o r other frequencies
crosspolarised E-planc
In the analysis, a boundary condition for the feed region has to be modified, i.e. the tangential electric field components are zero at z = -d-t.
Figs. 7 and 8 show the SWR behaviour of subarray 2L with a plane reflector for different values of t . The E-plane and M-plane beamwidths are changed slightly across the passband, with the addition of the plane reflector. The agreement between theory and experi- ment is reasonably good. The small discrepancy between theory and experiment may be caused by the tolerance in geometrical and physical parameters. The calculated results for subarrays IS and 2L with plane reflectors are listed in Table 3. The maximum band- width for the subarray is attained as t = 1.5". It is found that the bandwidth of the microstrip antenna can be increased significantly with the addition of a plane reflector. For brevity, the results of input SWR and far-field radiation patterns for subarray 1s with a plane reflector are not presented.
In this paper, the method of moments has been employed to evaluate the characteristics of an aperture- coupled rectangular microstrip antenna with coplanar parasitic elements. The parasitic elements are gap-cou- pled to the nonradiating edges of the fed patch. The SWR bandwidth and far-field radiation patterns have been studied. It is found that the parasitic elements can improve the antenna bandwidth and directivity but that high backlobe level has been observed. To reduce the backlobe radiation, microstrip subarrays with plane reflectors have also been examined. In addition to the reduction of backlobe level, the bandwidth can be enhanced significantly with a suitable distance between the plane reflector and the feed substrate.
5 Acknowledgment
This project is supported by the City University of Hong Kong Strategic Research Grant and the UGC Earmarked Grant of Hong Kong.
6 References
1 CHEN, W., LEE, K.F., and LEE, R.Q.: 'Special-domain moment-method analysis of coplanar microstrip parasitic subar- rays', Microw. Opt. Technol. Lett., 1993, 6, pp. 157-163
2 POZAR, D.M.: 'Microstrip antenna aperture-coupled to a micro- strip line', Electron. Lett., 1985, 21, pp. 49-50
3 MACKINCHAN, J.C., MILLER, P.A., STAKER, M.R., and DAHELE, J.S.: 'A wide bandwidth microstrip subarray for array antenna applications fed using aperture coupling'. IEEE AP-S Int. Symp. Digest, 1989, pp. 878-881 POZAR, D.M.: 'A reciprocity method of analysis for printed slot and slot-coupled microstrip antennas', IEEE Trans., 1986, AP-34, pp. 1439-1446 AU. T.M., and LUK, K.M.: 'Effect of parasitic element on the characteristics of microstrip antenna', ZEEE Trans., 1991, AP-39, pp. 1247-1251
