What is a Helical Antenna?

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- everything RF

Jul 26, 2017

A helical antenna is a specialized antenna that is considered to be a hybrid between a loop antenna and a dipole antenna. It consists of a conducting wire wound in the form of a helix. Helical antennas are usually mounted over a ground plane and the feedline is connected between the bottom of the helix and the ground plane.

Helix Antennas usually support wide bandwidths compared to other types of antennas. To the casual observer, they appear as one or more "springs" or helixes mounted against a flat reflecting screen. These antennas emit and respond to electromagnetic fields with circular polarization.

These antennas operate in one of two modes: normal mode and axial mode. In the normal mode, the diameter and pitch of the helix are small in comparison to the wavelength. As a result, the operation is similar to an electrical short monopole or dipole. The radiation would be linearly polarized parallel to the axis of the antenna with maximum radiation taking place at right angles to the helix axis. This mode/configuration has a narrow bandwidth and low efficiency. These are used for compact antennas for portable and mobile two-way radios, and for UHF television broadcasting antennas.

In axial mode, the diameter and pitch of the helix is comparable to the wavelength. In this case, 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. This mode/configuration is often used by earth-based stations in satellite communications systems.

Helical antennas are commonly connected together in so-called bays of two, four, or occasionally more elements with a common reflector. The entire assembly can be rotated in the horizontal (azimuth) and vertical (elevation) planes, so the system can be aimed toward a particular satellite. If the satellite is not in a geostationary orbit, the azimuth and elevation rotators can be programmed to follow the course of the satellite across the sky.

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