Grid Array Antenna Radiating a Circularly Polarized Wave # Y. Iitsuka 1 , P. H. Phuong 2 , J. Yamauchi 1 , and H. Nakano 1 1 Faculty of Science and Engineering, Hosei University Koganei, Tokyo, Japan, yasushi.iitsuka.o8@stu.hosei.ac.jp 2 Department of Telecommunications Engineering, HCMC University of Technology Ho Chi Minh City, Vietnam, phphuong@hcmut.edu.vn 1. Introduction Center-feed (or quasi-center feed) grid array antennas (GAAs) printed on a dielectric substrate (relative permittivity ε r > 1) have been investigated [1]-[6]. These GAAs are composed of microstrip lines. The investigation reveals that the radiation beam is in the direction normal to the grid array plane and the polarization of the radiation is linear, coinciding with the direction of the short side microstrip lines for the grid cells. The use of the dielectric substrate contributes to precise fabrication for the GAA. Note that, as the thickness of the dielectric substrate is increased, the radiation characteristics deteriorate due to effects of surface waves. For solving this issue, a new structure has been proposed [5][6], where the antenna pattern printed on a thin dielectric substrate is backed by an air layer. It is found that the frequency response of the gain and impedance characteristics is improved. In addition to linearly polarized (LP) GAAs, a circularly polarized (CP) GAA has been proposed [7] to apply it to CP communication systems, such as a satellite communication system. To radiate a CP wave, the microstrip radiation element for each grid cell in [5] has been replaced with a square loop having two perturbation elements. The height of this CP-GAA above the grand plane is chosen to be very small: approximately 0.08 wavelength at the operating frequency. Note that the CP-GAA in [7] has two feed points and excited in balanced mode with (+1, −1) volt. For this excitation, a circuit to realize the balanced mode is required, resulting in complicated design. In order to avoid this issue, the number of feed points for the CP-GAA has been reduced to one in [8], where an electromagnetic coupling feed has been adopted for impedance matching. This paper is a sequel to [8]. The input impedance (VSWR), axial ratio, and radiation pattern, are analysed and discussed. 2. Discussion 2.1 Basis for CP-GAA Analysis is performed using the finite integration technique (FIT by CST Microwave Studio [9]). The antenna is designed for a frequency band including 7 GHz. Fig. 1 shows a linearly polarized grid array antenna (LP-GAA), which is the basis for the CP antenna to be considered here, has a composite layer structure where the space between the grid array and the ground plane is filled with a dielectric layer (thickness B = 1 mm and relative permittivity ε r = 2.6) and an air layer. The feed point is located at point F (quasi-center feed point), to which the center conductor of a coaxial feed line is connected. The length of the x-directed microstrip line, L x , is chosen to be approximately 0.5λ g , where λ g is the guided wavelength of the current along the microstrip line on the dielectric layer. The length of the y-directed microstrip line, L y , is chosen to be 1λ g . The x-directed microstrip lines act as radiation elements. The number of radiation elements for the structure shown in Fig. 1 is chosen to be nine. Adjusting the widths of the x- and y-directed microstrip lines, W x and W y , and the height H (the thicknesses of a dielectric layer plus an air layer), the input impedance for the LP-GAA is matched 1D3-4 Proceedings of ISAP2012, Nagoya, Japan 130