Spiral Antenna Theory

A fractional bandwidth of up to 30:1 characterizes spiral antennas, making them highly effective across a very wide frequency range, often from 1 GHz to 30 GHz. The resource details two primary types: the Log-Periodic Spiral Antenna and the Archimedean Spiral Antenna, defining each with specific polar functions and illustrating their planar configurations. It explains that spiral antennas are typically circularly polarized, with a Half-Power Beamwidth (HPBW) of approximately 70-90 degrees, and a peak radiation direction perpendicular to the spiral plane.

The content elaborates on critical design parameters affecting radiation, including the total length (outer radius) for lowest frequency, the flare rate ('a' constant) for optimal radiation versus capacitive behavior, the feed structure (often an infinite balun) for high-frequency operation, and the number of turns (typically 1.5 to 3 turns). It also discusses the theoretical impedance of 188 Ohms for Log-Periodic spirals, derived from Babinet's Principle, noting actual impedances are often 100-150 Ohms. The article presents a simple construction method for an Archimedean spiral, demonstrating VSWR and efficiency measurements.

Measurements from a constructed spiral antenna show a VSWR that is fairly constant across the band, albeit with a mismatch loss of about 3 dB. The antenna efficiency remains around -5 dB (31.6%) across its operating range, indicating a decent wideband radiator despite opportunities for optimization.