Helical antenna

A helical antenna is an antenna consisting of a conducting wire wound in the form of a helix. In most cases, helical antennas are mounted over a ground plane. The feed line is connected between the bottom of the helix and the ground plane. Helical antennas can operate in one of two principal modes — normal mode or axial mode.

In the normal mode or broadside helical antenna, the dimensions of the helix (the diameter and the pitch) are small compared with the wavelength. The antenna acts similarly to an electrically short dipole or monopole, and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. The radiation is linearly polarised parallel to the helix axis. These are used for compact antennas for portable and mobile two-way radios, and for UHF television broadcasting antennas.

In the axial mode or end-fire helical antenna, the dimensions of the helix are comparable to a wavelength. The antenna functions as a directional antenna radiating a beam off the ends of the helix, along the antenna's axis. It radiates circularly polarised radio waves. These are used for satellite communication.

If the circumference of the helix is significantly less than a wavelength and its pitch (axial distance between successive turns) is significantly less than a quarter wavelength, the antenna is called a normal-mode helix. The antenna acts similar to a monopole antenna, with an omnidirectional radiation pattern, radiating equal power in all directions perpendicular to the antenna. However, because of the inductance added by the helical shape, the antenna acts like a inductively loaded monopole; at its resonant frequency it is shorter than a quarter-wavelength long. Therefore normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big. As with other electrically short antennas, the gain, and thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes "helicals" useful as antennas for mobile and portable communications equipment on the HF, VHF, and UHF bands.

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