294 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 59, NO. 1, JANUARY 2011 Development of Shared Aperture Dual Polarized Microstrip Antenna at L-Band Satyajit Chakrabarti Abstract—A simple technique to develop dual feed dual linearly po- larized aperture coupled planar microstrip patch antenna at L-band frequency has been presented. A 4 2 array of aperture coupled square resonant patch antenna has been designed, fabricated and tested. The design optimization is carried out using commercial method of moment (MoM) based simulation software. The inter element spacing, inter-layer separation, isolation between the feeding ports, and other associated antenna parameters are optimized by successive simulations using the commercial software. Measured gain is better than 14 dBi. Measured results are also presented. Index Terms—Antenna arrays, aperture coupled antenna, microstrip antenna. I. INTRODUCTION Microstrip antennas with aperture coupled microstrip feed is a con- venient technique to achieve broad impedance bandwidth [1]–[3]. The technique has been established to be a reliable and robust feeding tech- nique for microstrip antenna. The radiating patches and the feed lines are situated at the opposite sides of ground plane in which coupling apertures are created, as shown in Fig. 1. Power from the feed lines gets coupled to the radiating patch through the electrically small aper- ture in the ground plane. The ground plane prevents feed line radiation to corrupt the desired patch radiation. Basic aperture coupling makes the pattern more accurate and also prevents polarization and side lobe level degradation. The lack of galvanic contact makes it relatively in- sensitive to small mechanical misalignments. Careful optimization of the coupling aperture dimensions makes it broad band with impedance bandwidth ranging from 5%–60%. The structure is suitable for dual feed dual polarized design. As with other types of microstrip antennas, dual polarized aperture coupled antenna can be realized using two orthogonal feeds [4]. Shared aperture dual polarized antennas are popularly used in many applications to reduce cost and size. The issue of achieving dual polarized antenna using single patch has been studied by several investigators [4]–[6]. Two popular way of realizing dual polarized microstrip antennas are: (i) two off-centered orthogonal coupling slots [4], [7], [8] and (ii) crossed slot located at the center of the patch [9], [10]. The latter technique requires complicated feed arrangement. The problem associated with the first approach is that the asymmetry of the slot positions with respect to the center of the radiating patch constrains the slot length and hence the cou- pling level. The orthogonal off-centered slots limit the inter-port isola- tion and degrade the polarization purity. For realizing shared aperture dual polarized dual feed aperture coupled antenna, the basic radiating patch element is preferred to be square. The disadvantage associated with the square patch is the poor cross polarization. Requirement of high gain compact dual polarized microstrip antenna at L-band for air borne application is the prime consideration of this work. In the present communication, a simple technique to realize dual Manuscript received October 21, 2009; revised September 15, 2010; accepted September 20, 2010. Date of publication November 09, 2010; date of current version January 04, 2011. This work was supported by MCIT, Govt. of India. The author is with the SAMEER Kolkata Centre, Kolkata 700091, India (e-mail: satyajit2.chak@gmail.com). Color versions of one or more of the figures in this communication are avail- able online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2010.2091249 Fig. 1. Basic aperture coupled antenna geometry. polarized aperture coupled antenna with square patch as the basic ra- diating element with high cross polarization discrimination has been outlined. Simultaneously high inter-port isolation, high gain, with suf- ficient power coupling has been achieved. In this present work, de- sign optimization has been carried out by successive simulations using method of moments (MoM) based simulation software, IE3D. The an- tenna is fabricated and tested. Measured gain is better than 14 dBi with isolation better than 30 dB and cross-polarization is better than dB. II. DESIGN OF THE ANTENNA Two off-centered mutually orthogonal coupling slots beneath a square radiating patch element are used to realize the shared aperture dual polarized antenna, as shown in Fig. 2. The length of the patch an- tenna determines the resonant frequency and the width determines the resonance impedance. Hence, for a dual feed antenna, the dimensions of the square patch are to be decided judiciously to achieve the desired resonance frequency along with the desired impedance matching. The thickness of the dielectric substrate determines the impedance bandwidth. The antenna is initially simulated for two orthogonal feed ports in- dependently. One of the off-centered slots is linear slot, while the other is C-type bent slot. The latter is compact in size. The length of the coupling slot determines the coupling level. Hence the slot length is not selected longer than that required for desired impedance matching of the overall structure. In case of linear slot, as the slot-offset from the patch center is increased, the resonance frequency of the overall structure shifts in downward direction. The variation is relatively in- significant at the lower slot offsets, but has significant impact on the resonant frequency and return loss at the higher slot offsets. The vari- ation of resonance frequency and impedance bandwidth with normal- ized slot offset is given in Table I. However, as expected, the slot width has negligible dependence on the resonance frequency and impedance bandwidth. When the slot offset is more than , the impedance bandwidth increases abruptly with minor change in return loss. In case of C-type bent slot, as the slot-offset is increased along X-di- rection, the resonance frequency of the overall structure shifts in down- ward direction. Unlike linear slot, the resonance frequency variation of C-type bent slot is relatively insensitive to the slot-offset along X-direc- tion. The variation of resonance frequency and impedance bandwidth with normalized slot offset is given in Table II. The slot offset has al- most no effect on impedance bandwidth. The antenna is next simulated for dual feed port simultaneously. As the bent slot is moved away from the central line of the square radi- ating patch (along Y-direction), the isolation between the ports de- grades drastically. The variation of the inter-port isolation with nor- malized slot offsets is plotted in Fig. 3. 0018-926X/$26.00 © 2010 IEEE Authorized licensed use limited to: SOCIETY FOR APPLIED MICROWAVE ELEC ENG AND RES. Downloaded on July 18,2025 at 03:40:18 UTC from IEEE Xplore. Restrictions apply.