A monopole loaded with a loop antenna

A traveling-wave distribution of current can be produced on a monopole over a ground plane by inserting a resistance of suitable magnitude one-quarter wavelength 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. In this paper, the resistor is replaced with a loop antenna. The main objective is to obtain an antenna having hemispherical coverage and is circularly polarized. Input impedance, current distribution, and radiation patterns of this antenna are computed using the numerical electromagnetics code (NEC); radiation patterns are also measured. Monopoles which are loaded with both series- and parallel-fed circular and square loops are investigated. It was shown that circular polarization could be obtained over most of the hemisphere with both types of series-fed loops. The parallel-fed loop insertions produced more directional circularly polarized patterns. Most computations and measurements were made at a frequency of 1.6 GHz. The frequency dependence was examined and it was found that these antennas can operate over the band from 1.4-2.0 GHz. These very simple, low-cost antennas have potential application for systems such as IRIDIUM and Global Positioning System.