Helical antenna

Helical antenna for WLAN communication, working frequency app. 2.4 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 end-fire) 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.

Broadside helical

Radiating at 90 degrees from the vertical or horizontal plane this design is efficient as a practical reduced-length radiator when compared with the operation of other types such as base-loaded, top-loaded or center-loaded whips.

These are usually wound in a linear spiroidal pattern (constant parallel spaced turns) providing consistent uniform radiation as a reduced sized equivalent in respect to the standard 1/4 wave antenna. They are typically used for mobile communications applications where reduced size is a critical operational factor.

An effect of this type of 'reduced size 1/4 wave' is that the matching impedance is reduced from the nominal 50 ohms to between 25 to 35 ohms base impedance. This does not seem to be adverse to operation and matching with a normal 50 ohm transmission line, provided the connecting feed is the electrical equivalent of a 1/2 wave at the frequency of operation.

Another example of the type as used in mobile communications is "spaced constant turn" in which two or more different linear windings are wound on a single former and spaced so as to provide an efficient balance between capacitance and inductance for the radiating element at a particular resonant frequency.

Many examples of this type have been used extensively for 27 MHz CB radio with a wide variety of designs originating in the US and Australia in the late 1960s.

End-fire helical

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 a true and consistent circular polarization.

This design is 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. Large antenna size would make it unwieldy for low frequency operation, so the design is 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 polarized 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 polarization, while the space between the coils (approximately 0.25 x wavelength) and the diameter of the coils (approximately 1/pi of the wavelength) determine its wavelength. The length of the coil determines how directional the antenna will be as well as 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 ohms. The resistive part is approximated by:

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:

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

The half-power beamwidth by:

The beamwidth between nulls by:

See also

• Telstar

References

• 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

External links

• Helical Antennas Antenna-Theory