Helical antenna

Normal-mode helical UHF TV broadcasting antenna 1954

If the circumference of the helix is significantly less than a wavelength and its pitch (axial distance between successive turns) is significantly less than a quarter wavelength, the antenna is called a normal-mode helix. The antenna acts similar to a monopole antenna, with an omnidirectional radiation pattern, radiating equal power in all directions perpendicular to the antenna's axis. However, because of the inductance added by the helical shape, the antenna acts like a inductively loaded monopole; at its resonant frequency it is shorter than a quarter-wavelength long. Therefore, normal-mode helices can be used as electrically short monopoles, an alternative to center- or base-loaded whip antennas, in applications where a full sized quarter-wave monopole would be too big. As with other electrically short antennas, the gain, and thus the communication range, of the helix will be less than that of a full sized antenna. Their compact size makes "helicals" useful as antennas for mobile and portable communications equipment on the HF, VHF, and UHF bands.

A common form of normal-mode helical antenna is the "rubber ducky antenna" used in portable radios.

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 1/4 wave antenna at 27MHz is 2.7 m (108”) 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 ohms to between 25 and 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 wavelength at the frequency of operation.

End fire helical satellite communications antenna, Scott Air Force base, Illinois, USA. Satellite communication systems often use circularly polarized radio waves, because the satellite antenna may be oriented at any angle in space without affecting the transmission, and axial mode (end fire) helical antennas are often used as the ground antenna.

Helical antenna for WLAN communication, working frequency app. 2.4 GHz

When the helix circumference is near the wavelength of operation, the antenna operates in axial mode. This is a nonresonant traveling wave mode, in which instead of standing waves, the waves of current and voltage travel in one direction, up the helix. Instead of radiating linearly polarized waves normal to the antenna's axis, it radiates a beam of radio waves with circular polarisation along the axis, off the ends of the antenna. 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 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, 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)".[2] The IEEE defines the sense of polarisation as "the sense of polarization, or handedness ... is called right handed (left handed) if the direction of rotation is clockwise (anti-clockwise) for an observer looking in the direction of propagation" [3] Thus a right-handed helix radiates a wave which is right-handed, the electric field vector rotating clockwise looking in the direction of propagation.

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 λ 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.[4] The pitch angle should be 13 degrees, 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 (λ/4). 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 1 end when a ground plate is used), at a cost of gain in the other directions. When C<λ 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 ohms, approximately

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

where C is the circumference of the helix, and λ is the wavelength. Impedance matching (when C=λ) 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 ground plate.

The maximum directive gain is approximately:

${\displaystyle Gain\simeq 15\left({\frac {C}{\lambda }}\right)^{2}\left({\frac {NS}{\lambda }}\right)}$[5]

where N is the number of turns and S is the spacing between turns. Most designs use C=λ and S=0.23*C, so the gain is typically G=3.45*N. In decibels, the gain is ${\displaystyle G_{dBi}=10\cdot \log _{10}\left(0.8N\right)}$.

The half-power beamwidth is:

${\displaystyle {\text{HPBW}}\simeq {\frac {52}{{\frac {C}{\lambda }}{\sqrt {\frac {NS}{\lambda }}}}}\,{\text{degrees}}}$[5]

The beamwidth between nulls is:

${\displaystyle {\text{FNBW}}\simeq {\frac {115\lambda ^{3/2}}{C{\sqrt {NS}}}}\,{\text{degrees}}}$

The gain of the helical antenna strongly depends on the reflector.[6] 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 for several dB.[7] The optimal pitch that maximizes the gain for a flat ground plane is in the range from 3° to 10° and it depends on the wire radius and antenna length.[7]

• Telstar

General

• 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
• The Basics of Quadrifilar Helix Antennas by Bill Slade, Orban Microwave