A Monofilar Spiral Antenna and Its Array Above a Ground Plane—Formation of a Circularly Polarized Tilted Fan Beam

A monofilar spiral antenna is analyzed in the presence of a conducting plane reflector, using the method of moments. The circumference of the spiral antenna is chosen to be 2.3 wavelengths. A tilted beam of circular polarization is realized by superposing the fields from two active regions. The gain of the tilted beam is approximately 8 dB. The frequency bandwidths for 1-dB gain drop and 3-dB axial ratio criterions are 12% and 23%, respectively. An array consisting of the four monofilar spiral antennas is also analyzed, where the array element spacing is chosen to be 0.8 wavelength at a design frequency f0. The input impedances of the four spirals are almost the same as the impedance of the single monofilar spiral antenna at f0. The array radiates a tilted fan beam with a gain increase of approximately 6 dB from the gain of the single spiral antenna at f0. The frequency bandwidth for a 3-dB axial ratio criterion is almost the same as that of the single spiral antenna.