Helical antenna
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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.
In the axial mode, the antenna produces true circular polarization. These antennas are best suited for space communication, where the orientation of the sender and receiver cannot be easily controlled, or where the polarization of the signal may change.
Helical antennas can have either a clockwise (right-handed) or counter-clockwise (left-handed) polarization. Helical antennas can receive signals with any type of polarization, such as horizontal or vertical polarization, but clockwise polarized antennas suffer a severe gain loss when receiving counter-clockwise signals, and vice versa.
Helical antennas are made of a single driven element which is coiled in a spiral, or helix. The direction of the coil determines its polarization, 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 impedence in axial mode ranges between 100 and 200 Ohms. The resistive part is approximated by:
- <math> R \simeq 140 \left ( \frac{C}{\lambda} \right )<math>
Where R is resistance in Ohms, C is the circumference of the helix, and λ is the wavelength.
The maximum directive gain is approximately:
- <math> D_o \simeq 15 N \frac{C^2 S}{\lambda^3} <math>
Where N is the number of turns and S is the spacing between turns.
See also
- Telstar (article includes image of a helical antenna)
References
- Constantine Balanis, "Antenna Theory, Analysis and Design", 1982, John Wiley and Sonsja:ヘリカルアンテナ