Spiral Antennas

<jats:title>Abstract</jats:title><jats:p>Spiral antennas were introduced in the mid‐1950s. They were the first antennas to exhibit uniform impedance and pattern performance over multioctave bandwidths. Spirals were the cornerstone and mathematical quintessence of a new concept in antenna theory termed<jats:italic>frequency-independent antennas</jats:italic>. In this article, various aspects related to the analysis and design of spiral antennas are discussed. The theory of spiral operation, often referred to as<jats:italic>band theory</jats:italic>, is discussed. Also included are characteristic geometries and shapes including equiangular and Archimedian planar and conical spirals. Theoretical and practical aspects needed for successful design and understanding of spiral antennas are reviewed. Special attention is given to understanding the modal performance of multiarm spiral antennas and its importance to overall antenna characteristics, analysis theory, mode‐forming networks, multimode excitations, and similar. Various feeding methods including infinite and tapered baluns, as well as coaxial bundles are described. Dual‐polarized spiral designs including inside/outside‐fed spirals and modulated armwidth (MAW ) spirals, and bandwidths of various spiral geometries are discussed. The article also includes some aspects of spiral miniaturization; discussions of challenges of nondissipative cavity backing, utilization of spirals as reflector feeds, the modal decomposition technique; and an overview of numerical methods utilized for successful design of spiral antennas. The article is organized in several sections, each devoted to a separate topic, with illustrations and tables utilized for further clarification and demonstration of discussed concepts.</jats:p>