Broadband circularly polarized antennas for UHF SATCOM

Novel circularly polarized (CP) antenna configurations derived from Moxon type antenna (bent dipole element over a ground plane) for broadband UHF SATCOM applications. A sequence of topologies starting from a single vertical element to two vertical elements of the Moxon arms, then widened strip arm elements were studied. Further, arms were widened to bowtie structures with bents at 90°. for achieving broadband operation. Bowtie elements were further split and optimized at a certain angle to achieve wider bandwidth. The logic in this evolution was to obtain highest possible gain based on Fano-Chu limits, which suggests that higher gain can be achieved in an electrically small antenna with maximized metallization in the structure that fill the volume. Circular polarization is obtained by two Moxon based cross elements that are fed through a hybrid 90° quadrature coupler. For the antennas that are prototyped, return loss S11 measurements were performed, and gains are simulated using HFSS. For the band of 225-400 MHz, antenna gain varies between 8-12 dB, and S11 is measured to be below 10 dB. Fabricated antennas coupled to a hybrid coupler yielded excellent bandwidths, low cross-polarization and low back lobes on the finite ground planes.