Unbalanced-Mode Spiral Antenna Backed by an Extremely Shallow Cavity

概要

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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 fLd = 3 GHz to fHd = 9 GHz (1:3 bandwidth), where the antenna height is selected to be extremely small (7 mm = 0.07 wavelength at fLd) 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 fLd to fHd. 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.