Miniaturization of a Planar Array Using Mode Mixing

Abstract

This paper presents a broadside radiating wideband array antenna at 26.5 GHz. Both the TM 11 and the next higher order mode, TM 21 are excited on a single circular patch sitting on a large ground plane. The combination of these two modes provides elliptical beam in the broadside direction. Then, this patch is used as the basic radiating element for an array antenna. Because of the shaped radiation pattern, a 1 × 2 half wavelength array of the proposed antenna provides similar pattern like a 2 × 2 array of circular patches operating in only TM 11 mode. With same inter element distance, a 1 × 2 array gives better performance in terms of gain bandwidth and size requirement as compared to conventional 2 × 2 array. Individual element size is slightly larger than the conventional TM 11 mode antenna but the overall size of the array antenna is reduced to 50% without degrading the gain value. Further, the mode mixing technique provides a wide bandwidth and lower side lobe level. A prototype array element is fabricated in 1.52 mm thick RO4003C substrate. It provides an average broadside gain of 7.1 dBi with gain variation less than 0.8 dBi over 25.3527.65 GHz. The side lobe level is less than -14 dB from the broadside gain. Average total efficiency of the antenna is 86.6%.