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A helical antenna is a specialized antenna that is considered as a hybrid of the radiating elements — loop antennas and the dipole. In a helical antenna, the conducting wire is wound in the form of a helix. The antenna, in most cases, is mounted over a ground plane with a feed line connected between the ground plane and the bottom of the helix. As it is a traveling wave antenna, the current and phase vary continuously along the helical antenna. Because of their unique and special properties, helical antennas are widely used in simple and practical applications such as in radios and satellite communications.
Helical antennas have very complicated geometry when compared to loop or dipole antennas. They can operate in one of two modes: normal mode and axial mode. In normal mode, the diameter and pitch of the helix are small in comparison to the wavelength. As a result, simple analytical solutions are available to determine the properties of the helical antenna. The operation is similar to an electrical short monopole or dipole. The radiation would be linearly polarized parallel to the axis and the maximum radiation happens at right angles to the helix axis. As the dimensions are small, a helical antenna operating in normal mode has a narrow bandwidth and low efficiency. In axial mode, the diameter and pitch of the helix is comparable to the wavelength. It functions as a directional antenna. Unlike the normal mode, simple solutions are not available to determine the radiation properties in the case of axial mode. As a result, for axial mode experimentally determined numerical and analytical techniques are used to determine these factors.
A helical antenna can be constructed easily and can produce circularly polarized fields. It has a real input impedance and has wider bandwidth compared to other types of antennas.
A helical antenna is considered as an excellent feed for circular polarization. It can particularly serve as a good feed for small offset dishes in the case of satellite applications. Helical antennas operating in normal mode are used for mobile radios as well as in broadcasting antennas. Helical antennas operating in axial mode are mostly used for satellite communication.