Characteristics of modified GPS antennas

A hybrid-spiral antenna, or a GPS-oriented antenna fabricated on an experimental basis employs a spiral antenna that radiates circularly polarized waves as its main element, in the neighborhood of which half-wave cross dipoles and quarter-wave radial monopoles are placed as parasitic elements, has a wider directivity. The design frequency is set to 1575 MHz, which is the center frequency of the higher frequency band used in GPS. The length of the helical antenna element is a 1/2 of the wavelength of the design frequency. A quadrifilar fraction-turn wound conical helical antenna is constructed by placing at right angles four antenna elements that are bent by 90° along the surface of circular truncated cone, in the neighborhood of which parasitic elements are placed. The variation of its characteristics due to the change of these constituent elements are analyzed by means of moment method, and its application to GPS antennas are described