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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. 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 helix, 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.
In the axial mode or end-fire helix, 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.
Radiating at 90 degrees from the axis of the helix 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 centre-loaded whips. They are typically used for applications where reduced size is a critical operational factor.
These simple and practical "helicals" were primarily designed to replace very large antennas. Their reduced size is therefore most suitable for mobile and portable high-frequency (HF) communications in the 1 MHz to 30 MHz operating range.
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.
An effect of using a helical conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohms to between 25 to 35 ohms base impedance. This does not seem to be adverse to operation or 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. Multi-frequency versions with plug-in taps have become the mainstay for multi-band Single-sideband modulation (SSB) HF communications.
Most examples were wound with copper wire using a fibreglass rod as a former. This flexible radiator is then covered with heat-shrink tubing which provides a resilient and rugged waterproof covering for the finished mobile antenna.
These popular designs are still in common use today (2010) and have been universally adapted as standard FM receiving antennas for many factory produced motor vehicles as well as the existing basic style of aftermarket HF and VHF mobile helical. The most common use for broadside helixes is in the Rubber Ducky antenna found on most portable VHF and UHF radios.
In the axial mode, the helix dimensions are at or above the wavelength of operation. The antenna produces radio waves with circular polarisation. The main lobes of the radiation pattern are along the axis of the helix, off both ends. Since in a directional antenna only radiation in one direction is wanted, the other end of the helix is terminated in a flat metal sheet or screen reflector to reflect the waves forward.
In radio transmission, circular polarisation is often used where the relative orientation of the transmitting and receiving antennas cannot be easily controlled, such as in animal tracking and spacecraft communications, or where the polarisation of the signal may change, so end-fire helical antennas are frequently used for these applications. Since large helices are difficult to build and unwieldy to steer and aim, the design is commonly employed only at higher frequencies, ranging from VHF up to microwave.
The helix in the antenna can twist in two possible directions: right-handed or left-handed, as defined by the right hand rule. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the radio waves: a left-handed helix radiates left-circularly-polarised radio waves, a right-handed helix radiates right-circularly-polarised radio waves. Helical antennas can receive signals with any type of linear polarisation, such as horizontal or vertical polarisation, but when receiving circularly polarised signals the handedness of the receiving antenna must be the same as the transmitting antenna; left-hand polarised antennas suffer a severe loss of gain when receiving right-circularly-polarised signals, and vice versa.
The dimensions of the helix are determined by the wavelength λ of the radio waves used, which depends on the frequency. In axial-mode operation, the spacing between the coils should be approximately one-quarter of the wavelength (λ/4), and the diameter of the coils should be approximately the wavelength divided by pi (λ/π). 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.
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 standard 50 or 75 ohm coaxial cable is often done by a quarter wave stripline section acting as an impedance transformer 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 is:
The beamwidth between nulls is:
- 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
- Helical Antennas Antenna-Theory