Hemispherical coverage using a double-folded monopole

A traveling-wave current distribution can be produced on a monopole over a ground plane by inserting a resistance of suitable magnitude one-quarter wave length from its end. It has been shown that the resistor can be replaced with a modified folded dipole which has a radiation resistance approximately equal to that of the resistor. Furthermore, it was found that when the folded element was about 0.1λ above the ground plane, near hemispherical coverage was obtained in the plane of the element. In this paper, the resistor is replaced with orthogonal folded elements. The main objective is to obtain an antenna which is circularly polarized and which has near hemispherical coverage. The input impedance, current distribution, and radiation patterns of this antenna were computed using the numerical electromagnetics code (NEC); the radiation patterns and input impedance were also measured. To produce a circularly polarized wave it is necessary to tilt the folded elements. After experimenting with a number of different configurations an antenna design having directional and polarization properties approaching those that were desired was obtained. All computations and measurements were conducted near 1.6 GHz, a frequency close to that of the proposed IRIDIUM communications system and the L1 band of the Global Positioning System (GPS). The double-folded monopole may prove to be a very low cost option as a ground antenna for these systems