Effects of a dielectric substrate on the radiation characteristics of a spiral antenna with slots

The arms of a spiral antenna are printed on the bottom surface of a dielectric substrate and are placed in contact with an absorbing strip installed in a shallow cavity. The antenna characteristics when a slot is cut into each of the arms are investigated with the permittivity of the dielectric substrate as a parameter. The investigation shows that, as the permittivity is increased, the center frequency of a stopband, f-stop, in the frequency response becomes lower. Subsequently, the change in f-stop is investigated with the thickness of the substrate as a parameter. It is found that the effect of the thickness on f-stop is not remarkable. It is also revealed that the inherent radiation pattern and gain of the spiral antenna are maintained outside the stopband.

I. Introduction

A circularly-polarized (CP) antenna has the advantage that polarization alignment between transmission and reception antennas is not needed, unlike linearly-polarized antennas. A patch antenna and a loop antenna are representative resonant antennas with linear polarization and can be transformed into CP antennas using perturbation elements, as shown in Figs. 1(a) and (b). However, the bandwidth as a CP antenna is not wide because the perturbation elements are also resonant elements that only work across a narrow frequency range. Fig. 1.

Resonant CP antennas. (a) Patch antenna. (b) Loop antenna.