Spiral And Helical Antennas

broad-band traveling-wave antennas that radiate or receive circularly or elliptically polarized electromagnetic waves. Such antennas are used primarily in the decimeter and centimeter wavelength ranges either as

Figure 1. Spiral antennas: (a) spiral of Archimedes, (b) equiangular spiral

independent units or as radiators in reflector and lens antennas—for example, in space communications systems.

Spiral antennas are usually made from two conductors, each of which has the shape of a spiral of Archimedes (Figure 1 ,a) or an equiangular (logarithmic) spiral (Figure l,b). The transmitter or receiver is connected to the spiral arms in the center of the antenna by a coaxial line or a two-conductor open-wire line. The ratio of the maximum operating frequency to the minimum operating frequency can be as high as 20. The front-to-rear ratio is usually equal to several units.

Figure 2. Helical antennas: (a) cylindrical, (b) conical; (1) metal helix, (2) metal screen, (3) coaxial line

Helical antennas have a cylindrical (Figure 2,a) or conical (Figure 2,b) shape and consist of a metal conductor that is connected to the inner conductor of a coaxial line; the outer conductor of the line is connected to a flat metal screen. Such antennas are generally used in frequency ranges where the ratio of the maximum to the minimum frequency is 2–3. The front-to-rear ratio can reach 100 or more.