Hexagonal-Triangular Combinatorial Structure Based Dual-Band Circularly Polarized Patch Antenna for GAGAN Receivers

A multilayer gap-coupled dual-band hexagonal microstrip patch antenna is presented for GPS Aided GEO Augmented Navigation (GAGAN) receivers. GAGAN is developed by AAI (Airport Authority of India) and ISRO (Indian Space Research Organization) exclusively for aviation purposes, with GPS frequencies of L5 (1176.45 MHz) and L1 (1575.42 MHz). The proposed GAGAN antenna has three trapezoidal and four equilateral triangular patch resonators configured to form a gap-coupled hexagonal patch on the top layer and seven equilateral triangular patches with slots in between to form the middle layer. Both layers are fed by a single coax feed to achieve a dual-band compact microstrip antenna with dimensions of 55 mm x 55 mm x 3.2 mm. Instead of designing the hexagonal antenna conventionally from a circular patch antenna, the sides of the equilateral triangular patch are truncated to obtain the proposed compact hexagonal antenna, as the triangular antenna occupies less area than circular or square patches for a fixed frequency. To bring novelty with improved performance, the combinatorial structured configuration is explored through step-by-step optimization. GAGAN requirements, such as circular polarization with a single input, dual-band operation, and high gain with gain flatness, are achieved with this compact structured antenna when compared to square and circular patch antennas. Combinatorial structures, such as perturbation and truncation, introduced in the patches are responsible for exciting two near-degenerate orthogonal modes that produce circular polarization (CP). A cross-slot etched in the center patch results in gain enhancement with a gain above 3.2 dBi over a bandwidth of 20 MHz in both bands. Five shorting pins are added to achieve high impedance matching and good CP, with an axial ratio (AR) of less than 2 dB. The simulated and measured results are found to be in good agreement with greater than 20 MHz operational bandwidth exhibiting CP with an omni-directional radiation pattern having high cross-polar levels of -22 dB. The size reduction achieved in the proposed design is 56.79% compared with the conventional hexagonal patch antenna. The measured 10 dB impedance bandwidth is 2.7% and 2.1% for L5 and L1 bands respectively with a 3 dB AR bandwidth of 2.3% for L5 and 2.0% for L1, satisfying the GAGAN requirements.