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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. Helical antennas can operate in one of two principal modes: normal (broadside) mode or axial (or endfire) mode.

In the normal mode, the dimensions of the helix are small compared with the wavelength. The far field radiation pattern is similar to an electrically short dipole or monopole. These antennas tend to be inefficient radiators and are typically used for mobile communications where reduced size is a critical factor. A Tesla coil secondary coil is also an example.

In the axial mode, the helix dimensions are at or above the wavelength of operation. The antenna then falls under the class of waveguide antennas, and produces true circular polarisation. These antennas are best suited for animal tracking and space communication, where the orientation of the sender and receiver cannot be easily controlled, or where the polarisation of the signal may change. Antenna size makes them unwieldy for low frequency operation, so they are commonly employed only at frequencies ranging from VHF up to microwave.

Axial-mode helical antennas can have either a clockwise (right-handed) or counter-clockwise (left-handed) polarisation. Helical antennas can receive signals with any type of linear polarization, such as horizontal or vertical polarization, but clockwise polarised antennas suffer a severe gain loss when receiving counter-clockwise signals, and vice versa.

Helical antennas are composed of a single driven element which is coiled in a helix. In axial-mode operation, the winding sense of the coil determines its polarisation, while the space between the coils and the diameter of the coils determine its wavelength. The length of the coil determines how directional the antenna will be and its gain; longer antennas will be more sensitive in the direction in which they point. A reflector is almost always used to increase the sensitivity, or gain, in one direction (away from the reflector).

Terminal impedance in axial mode ranges between 100 and 200 Ω. The resistive part is approximated by:

where R is resistance in ohms, C is the circumference of the helix, and λ is the wavelength.

The maximum directive gain is approximately:

where N is the number of turns and S is the spacing between turns.

The half-power beamwidth by:

The beamwidth between nulls by:

Yagi-Uda antenna signal-gathering action compared to other end-fire, backfire and traveling-wave types.

• Telstar (article includes image of a helical antenna)

• John D. Kraus and Ronald J. Marhefka, "Antennas: For All Applications, Third Edition", 2002, McGraw-Hill Higher Education

• Constantine Balanis, "Antenna Theory, Analysis and Design", 1982, John Wiley and Sons

• Warren Stutzman and Gary Thiele, "Antenna Theory and Design, 2nd. Ed.", 1998, John Wiley and Sons