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A helical antenna is an antenna consisting of one or more conducting wires wound in the form of a helix. A helical antenna made of one helical wire, the most common type, is called monofilar, while antennas with two or four wires in a helix are called bifilar, or quadrifilar, respectively.
In most cases, directional helical antennas are mounted over a ground plane, while omnidirectional designs may not be. 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 or axial.
In the normal mode or broadside helical antenna, the diameter and the pitch of the aerial are small compared with the wavelength. The antenna acts similarly to an electrically short dipole antenna or monopole antenna, equivalent to a wave vertical and the radiation pattern, similar to these antennas is omnidirectional, with maximum radiation at right angles to the helix axis. For monofilar designs the radiation is linearly polarized parallel to the helix axis. These are used for compact antennas for portable hand held as well as mobile vehicle mount , and in larger scale for UHF television broadcasting antennas. In bifilar or quadrifilar implementations, broadside circularly polarized radiation can be realized.
In the axial mode or end-fire helical antenna, the diameter and pitch 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 polarized radio waves. These are used for satellite communication. Axial mode operation was discovered by physicist John D. Kraus
The loading provided by the helix allows the antenna to be physically shorter than its electrical length of a quarter-wavelength. This means that for example a wave antenna at 27 MHz is long and is physically quite unsuitable for mobile applications. The reduced size of a helical provides the same radiation pattern in a much more compact physical size with only a slight reduction in signal performance.
An effect of using a helical conductor rather than a straight one is that the matching impedance is changed from the nominal 50 ohm to between 25 and 35 Ω base impedance. This does not seem to be adverse to operation or matching with a normal 50 Ω transmission line, provided the connecting feed is the electrical equivalent of a wavelength at the frequency of operation.
Multi-frequency versions with manual plug-in taps have become the mainstay for multi-band single-sideband modulation (SSB) HF communications with frequency coverage over the whole HF spectrum from 1 MHz to 30 MHz with from 2 to 6 dedicated frequency tap points tuned at dedicated and allocated frequencies in the land mobile, marine, and aircraft bands. Recently these antennas have been superseded by electronically tuned antenna matching devices. Most examples were wound with copper wire using a fiberglass rod as a former. The usually flexible or ridged radiator is then covered with a PVC or polyolefin heat-shrink tubing which provides a resilient and rugged waterproofing covering for the finished mobile antenna. The fibreglass rod was then usually glued and/or crimped to a brass fitting and screw mounted onto an insulated base affixed to a vehicle roof, guard or bull-bar mount. This mounting provided a ground plane or reflector (provided by the vehicle) for an effective vertical radiation pattern.
These popular designs are still in common use and the constant turn design originating in Australia 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. Another common use for broadside helixes is in the so-called rubber ducky antenna found on most portable VHF and UHF radios using a steel or copper conductor as the radiating element and usually terminated to a BNC/TNC style or screw on connector for quick removal.
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 of the antenna can twist in two possible directions: right-handed or left-handed, the former having the same form as that of a common corkscrew. The 4-helix array in the first illustration uses left-handed helices, while all other illustrations show right-handed helices. In an axial-mode helical antenna the direction of twist of the helix determines the polarisation of the emitted wave. Two mutually incompatible conventions are in use for describing waves with circular polarisation, so the relationship between the handedness (left or right) of a helical antenna, and the type of circularly-polarized radiation it emits is often described in ways that appear to be ambiguous. However, J.D. Kraus (the inventor of the helical antenna) states "The left-handed helix responds to left-circular polarisation, and the right handed helix to right-circular polarisation (IEEE definition)". The IEEE defines the sense of polarisation as:
Helical antennas can receive signals with any type of linear polarisation, such as horizontal or vertical polarisation, but when receiving circularly polarized signals the handedness of the receiving antenna must be the same as the transmitting antenna; left-hand polarized antennas suffer a severe loss of gain when receiving right-circularly-polarized signals, and vice versa.
The dimensions of the helix are determined by the wavelength () of the radio waves used, which depends on the frequency. In order to operate in axial-mode, the circumference should be equal to the wavelength. The pitch angle should be 13°, which is a pitch distance (distance between each turn) of 0.23 times the circumference, which means the spacing between the coils should be approximately one-quarter of the wavelength (). The number of turns in the helix determines how directional the antenna is: more turns improves the gain in the direction of its axis at both ends (or at one end, when a ground plate is used), at a cost of gain in the other directions. When it operates more in normal mode where the gain direction is a donut shape to the sides instead of out the ends.
Terminal impedance in axial mode ranges between 100 and 200 Ω, approximatelyJohn D. Kraus (2025). 007123201X, The McGraw-Hill Companies Inc.. 007123201X
where is the circumference of the helix, and is the wavelength. Impedance matching (when to standard 50 or 75 Ω coaxial cable is often done by a quarter wave stripline section acting as an impedance transformer between the helix and the ground plate.
The maximum directive gain is approximately:Wayne Tomasi (2025). 981247093X, Pearson Education SE Asia. 981247093X
where is the number of turns and S is the spacing between turns. Most designs use and so the gain is typically In , the gain is
The beamwidth between nulls is:
The gain of the helical antenna strongly depends on the reflector. The above classical formulas assume that the reflector has the form of a circular resonator (a circular plate with a rim) and the pitch angle is optimal for this type of reflector. Nevertheless, these formulas overestimate the gain by several decibel. The optimal pitch that maximizes the gain for a flat ground plane is in the range 3–10° and it depends on the wire radius and antenna length.
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