A helical antenna is a compact, spiral aerial used primarily for mobile high frequency communications as an alternative to longer aerial types. Helical antennas consist of aerial conductors which are evenly wound around support elements. The compact nature of their construction means that standard antenna lengths may be greatly reduced for use on portable communications equipment. The ground plane of a helical antenna is typically formed by an integral reflective element or the item on which the antenna is mounted such as an automobile body. Helical antennas can be applied in either conventional broadside or end-on, axial radiation configurations.
To construct a helical antenna, a standard length aerial conductor is wound around a suitable support in a spiroidal pattern. This means that the coil or helix consists of a series of evenly spaced parallel turns. Constructing the antenna in this fashion produces an end result which features the same conductor length in a far shorter package than the extended conductor. The supports used may be of various cross sections and made of any insulating material. The most common support is tubular and made of a flexible material such as fiberglass.
Helical antennas are generally presented in two basic configurations. The first is the broadside configuration which radiates its signal at 90 degrees to the antenna axis. These are the more common of the two types of helical antenna and are popular alternatives to standard length whip style antennas. Broadside helical antennas are commonly used on motor vehicle, citizen band (CB) radios, and hand held two-way radios. They are also widely used as standard equipment for motor vehicle FM radios.
The second helical antenna configuration is the axial or end-fire type. In broadside antennas, the coil diameter is a lot smaller than the designated frequency wavelength. In an end-fire configuration, the helix diameter is at or above the intended wavelength. This type of antenna consists of a single driven conductor typically coiled inside a protective tube. A parabolic reflector is often included behind the antenna to increase its directional sensitivity.
This type of aerial features reliable circular polarization and is generally classed as a waveguide antenna. This means that the antenna may be rotated to polarize radiation in a more focused manner. This is then also true of the receiving antenna which may be rotated to improve reception. End-fire helical antenna designs lend themselves well to applications such as tracking animals fitted with radio collars. This type of antenna is often used in space communication.
