Spiral antenna

Lore & Background

The spiral antenna was first described in 1956. It operates using three modes: traveling wave, fast wave, and leaky wave. The traveling wave formed on the spiral arms enables broadband performance; fast wave arises from mutual coupling between the arms; and leaky wave leaks energy during propagation to produce radiation. The working principle is explained by ring theory (band theory), which states that the antenna radiates from an active region where the circumference of the spiral equals the wavelength.

Spiral antennas are classified into different configurations, including Archimedean spiral, logarithmic spiral, and square spiral. Archimedean spirals satisfy the equation r = aφ + b, where a corresponds to growth factor and b to multiplication factor, resulting in equal spacing between successive turns. Logarithmic spirals satisfy r = ae^(bφ), producing more widely spaced outer turns that can accommodate widening arms. Design parameters include spacing between turns, width of arm, inner radius, outer radius, number of turns, number of arms, and the surrounding materials.

The antenna usually has two conductive spiral arms extending from the center outward, with the direction of rotation defining the polarization. Additional spirals may form a multi-spiral structure. The output of a two-arm or four-arm spiral antenna is a balanced line; a balun or transformer is added if a single input or output line is desired. The spiral is often cavity-backed, with a cavity of air, non-conductive material, or vacuum surrounded by conductive walls, to produce a unidirectional pattern.

Reader's Guide

Spiral antennas are significant for their ability to transmit circularly polarized radio waves and receive linearly polarized waves in any orientation, while drastically attenuating circularly polarized signals of opposite rotation. This polarization selectivity makes them useful for wide-band communications and monitoring of the frequency spectrum, where one antenna can receive over a wide bandwidth, such as a 5:1 ratio between maximum and minimum frequency. In such applications, a pair of spiral antennas with opposite polarization (one right-hand, one left-hand) is often used. Spiral antennas are also employed for microwave direction-finding. Their compact size, achieved through the winding structure and the use of dielectric substrates like Rogers RT Duroid, allows for reduced physical dimensions, though such materials can be less available and more costly. The ability to alter frequency by choosing a dielectric medium with specific permittivity adds to their design flexibility. Lossy cavities placed at the back eliminate back lobes, as a unidirectional pattern is usually preferred.

Did You Know?

• Spiral antennas were first described in 1956.
• The direction of rotation of the spiral defines the direction of antenna polarization.
• Ring theory (band theory) explains that the antenna radiates from an active region where the spiral circumference equals the wavelength.
• A pair of spiral antennas with opposite polarization is often used for wide-band spectrum monitoring.