Unbalanced-Mode Spiral Antenna Backed by an Extremely Shallow Cavity

This paper describes a two-arm Archimedean spiral antenna backed by a conducting cavity, where only one arm is directly excited, with the other arm being parasitically excited; in other words, the spiral arms are excited in an unbalanced mode. A balun circuit required for a conventional two-arm spiral is not used for this unbalanced-mode spiral. The design of the unbalanced-mode spiral is performed over a frequency range of f_Ld = 3 GHz to f_Hd = 9 GHz (1:3 bandwidth), where the antenna height is selected to be extremely small (7 mm = 0.07 wavelength at f_Ld) to realize a low-profile antenna. For reference, a corresponding spiral antenna excited in balanced mode is also analyzed. It is found that the unbalanced-mode spiral shows an acceptably small VSWR over the design frequency range of f_Ld to f_Hd. The radiation is circularly polarized around the antenna axis normal to the spiral plane. The gain shows behavior similar to that of the balanced-mode spiral. Results for other antenna heights (5 mm, 10.5 mm, and 14 mm) are also presented and briefly discussed. It can be said that the unbalanced-mode spiral is a circularly polarized wideband antenna with a simple feed system.