Spiral antennas belong to the class of “frequency independent” antennas; these antennas are characterized as having a very large bandwidth. Spiral antennas are travelling wave structures and are well-known for their wideband performance. A bandwidth of 5:1 or 10:1 is easily obtained and a stable input impedance is achieved through a self-complementary geometry. This wideband characteristic of the spiral antenna makes it an attractive choice where a single antenna is required to send / receive over multiple channels. Spiral antennas are usually circularly polarized. The spiral antenna’s radiation pattern typically has a peak radiation direction perpendicular to the plane of the spiral (broadside radiation). The Half-Power Beamwidth (HPBW) is approximately 70-90 degrees.
Spiral antennas are widely used in the defense industry for sensing applications, where very wideband antennas that do not take up much space are needed. Spiral antenna arrays are used in military aircraft in the 1-18 GHz range. Other applications of spiral antennas include GPS, where it is advantageous to have RHCP (right hand circularly polarized) antennas.
There are several kind of spiral antennas .Spiral antennas are classified into different types; Archimedean spiral, square spiral and star spiral etc. Archimedean is the most popular amongst them because of their ability to maintain consistent gain and input impedance and excellent performance in circular polarization. An Archimedean spiral made of two equal lengths of coaxial cable seems to be the easiest circularly polarized antenna to make that’ll cover a broad range. Below are some spiral antenna designed using EM simulation