Helical antenna

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Helical antenna for WLAN communication, working frequency app. 2.5 GHz

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.

B: Central Support,
C: Coaxial Cable,
E: Spacers/Supports for the Helix,
R: Reflector/Base,
S: Helical Aerial Element

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 polarization. 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 polarization 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) polarization. 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 S which is coiled in a helix. In axial-mode operation, the winding sense of the coil determines its polarization, while the space between the coils (app. 0.25 x wavelength) and the diameter of the coils (app. 1/3 of the wavelength) 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 R 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:

$R \simeq 140 \left ( \frac{C}{\lambda} \right )$

where R is resistance in ohms, C is the circumference of the helix, and λ is the wavelength. Impedance matching to the cable C is often done by a short stripline section between the helix and the cable termination.

The maximum directive gain is approximately:

$D_o \simeq 15 N \frac{C^2 S}{\lambda^3}$