Helical antenna

Monofilar helical antennas for satellite communication

A helical antenna , also known as a helix antenna , is a directional antenna in the form of a helix for sending and receiving circularly polarized electromagnetic waves .

Helix antennas are available in different designs. What they all have in common is that they consist of one or more helically wound electrical conductors (tape or wire) whose dimensions and geometry depend, among other things, on the wavelength . The helix antenna was invented by John D. Kraus in 1947 .

Types

The different types of helix antennas can be roughly divided into the following main groups:

Monofilar helix antennas only consist of a helical conductor and the antenna needs a conductive surface at the rear end to counterbalance the feed to operate with maximum antenna gain . The circular polarization direction of the electromagnetic wave corresponds to the helical orientation of the electrical conductor.
Bifilar helical antennas consist of two conductors looped around each other that are fed in opposite directions by 180 °. After it is fed at the tip, it radiates against the direction of propagation on the waveguide structure. The bundling is less good than that of the monofilar helix antenna, but this is advantageous for some applications (parabolic antenna feed, satellite communication without tracking).
Quadrifilar helix antennas consist of four helical conductors, each offset by 90 °. Quadrifilar helix antennas do not necessarily need a reflector at their end, depending on the specific structure, and the circular polarization direction of the electromagnetic wave can, depending on the feed point, also be opposite to the direction of rotation of the four electrical conductors. Short quadrifilar helical antennas are resonant, therefore narrow-band and have no waveguide properties.

To increase the transmission bandwidth, helical antennas can also be conical or spherical . Such shapes combine the good directivity of helical antennas with the broadband nature of spiral antennas .

Monofilar helix antenna

The circumference of the helix of a monofilar helical antenna has the length of the insertion wavelength . So the diameter is : ${\ displaystyle \ lambda}$ ${\ displaystyle D}$

{\ displaystyle D = {\ frac {\ lambda} {\ pi}}}

Helical antennas are therefore primarily used in the decimeter wave range (0.3–3 GHz).

The pitch of the helix has an optimum at 0.2 to 0.3 times the wavelength. The gain increases with the number of turns and the directional characteristic of the antenna improves . ${\ displaystyle s}$ ${\ displaystyle N}$ ${\ displaystyle G}$

{\ displaystyle G = 15 \ cdot N \ cdot s \ cdot {\ frac {(\ pi \ cdot D) ^ {2}} {\ lambda ^ {3}}}}

or, if and are: ${\ displaystyle D = {\ frac {\ lambda} {\ pi}}}$ ${\ displaystyle s = {\ frac {\ lambda} {4}}}$

{\ displaystyle G \ approx 4 \ cdot N}

However, the maximum achievable profit approaches from a length of

{\ displaystyle 7 \ cdot \ lambda}

a limit of 15 dB. The higher the profit, the smaller the opening angle in degrees : ${\ displaystyle \ Theta}$

{\ displaystyle \ Theta = {\ frac {52 ^ {\ circ}} {\ pi \ cdot D}} \ cdot {\ sqrt {\ frac {\ lambda ^ {3}} {N \ cdot s}}}}

{\ displaystyle \ Theta \ approx {\ frac {200 ^ {\ circ}} {\ sqrt {G}}}}

The rear reflector surface serves as a counterweight for the coaxial feed. It is about one wavelength tall.

Monofilar helical antennas are relatively uncritical in terms of dimensioning and are therefore well suited for reproduction. This is indirectly related to their high relative bandwidth, which is already around 60% for a uniform helix.

The impedance (unit: Ohm , Ω) at the feed point or base point (start of the coil) is: ${\ displaystyle Z}$

{\ displaystyle Z \ approx 140 \ mathrm {\ \ Omega} \ cdot \ pi \ cdot {\ frac {D} {\ lambda}}}

or with : ${\ displaystyle D = {\ frac {\ lambda} {\ pi}}}$

{\ displaystyle Z \ approx 140 \ mathrm {\ \ Omega}}

A resonance transformer is required to adapt the base resistance of the antenna to that of the supply cable (coaxial cable, mostly 50… 75 Ω) , implemented for example by means of a correspondingly dimensioned sheet metal strip. ${\ displaystyle Z}$

Bifilar helix antenna

The bifilar design is less directional and works as a backfire antenna , i.e. it radiates against the propagation of the traveling waves on the conductors. Due to the traveling wave dampened by the radiation, it has a flat input impedance across the broad frequency band and the handling of the ends of the two conductors facing away from the beam direction is not critical. DE describes an antenna that is fed from a coaxial line leading to the tip, which at the same time forms one of the two coiled conductors. This means that the cable connection is at the rear. There the two helical ends are connected to each other so that sheath currents cancel each other out on the coaxial cable leading away. The antenna does not need a counterweight for the feed and no reflector.

Quadrifilar helix antenna

Resonant quadrifilar helical antenna with ½ turn for GPS reception on a radiosonde

The quadrifilar helix antenna is formed by four helically wound conductors that are parallel and at a constant distance from one another. The beginning of each conductor is rotated by 90 ° in the area of the reflector plane. In the non-resonant and broadband design of the quadrifilar helix antenna, the feed to the four conductors is rotated by 90 °. 7 to 8 turns are necessary for maximum gain. As with the monofilament version, a reflector wall is required for use as a directional antenna. The quadrifilar design has a higher antenna gain compared to the monofilar design with a higher manufacturing cost. Depending on the direction of rotation of the feed to the four individual conductors, the direction of rotation of the circular shaft is the same or opposite to the direction of rotation of the feed.

The resonant quadrifilar helix antenna, which is particularly narrow-banded and does not have a reflector wall and is therefore particularly light, is an essential design, especially for GPS receivers. It is characterized by four conductors which, depending on the type, are exactly ¼ or ½ turn long. The direction of rotation of the conductor is opposite to the direction of rotation of the circularly polarized wave.

application

Since the direction of rotation of the helix determines the direction of rotation of the radiated or received circularly polarized wave, the direction of rotation of the transmitting and receiving antenna must match. In contrast, every helical antenna is able to receive waves polarized linearly in any direction. They are therefore often used in cases where indefinitely linearly polarized waves are to be received - however, the gain when receiving linearly polarized waves is 3 dB less. Signals in the opposite circular polarization are, however, strongly suppressed, so that u. U. the independent use of the two polarizations in the same frequency range becomes possible.

Circular polarization has advantages in satellite and space communication because polarization fading does not occur if the direction of polarization of the waves is rotated in an unpredictable manner by the Faraday effect or space probes rotate in orientation when passing through the ionosphere .

Typical areas of application include:

Satellites and spacecraft: antennas are often tapered as a hybrid of helical and spiral antennas .
For terrestrial satellite communication such as connecting to weather satellites
As a directional antenna that is easy to set up for WLAN point-to-point connections and in the amateur radio sector .
Resonant, reflectorless quadrifilar helical antennas as very light GPS receiving antennas with high antenna gain, among other things on radiosondes for weather observation in meteorology.

Demarcation

The coil antennas, sometimes colloquially and vaguely known as "helical antennas", have nothing to do with the properties of a helical antenna. Coil antennas consist entirely or partially of a single-layer cylinder coil which, as an essential feature, is small compared to the wavelength. In principle, coil antennas are electrically shortened quarter-wave dipole antennas . The inductance of the coil increases the electrical length while reducing the overall length.

literature

John D. Kraus, Ronald J. Marhefka: Antennas - For All Applications . 3rd, International ed. McGraw-Hill Higher Education, 2002, ISBN 978-0-07-123201-2 .
Karl Rothammel , Alois Krischke : Rothammels Antennenbuch . DARC Verlag, Baunatal, ISBN 3-88692-033-X , Chapter 36.5 - Helical antennas.

Web links

Mathematical basics and a. the helical antenna

Individual evidence

^ John D. Kraus: Helical beam antenna . In: Electronics . No. 4109 , 1947, pp. 109 to 111 .
^ John D. Kraus: The helical antenna . tape 37 . Proceedings of the IRE , March 1949, pp. 263 to 273 ( online [PDF]).
↑ ^a ^b http://www.jhuapl.edu/techdigest/views/pdfs/V12_N1_1991/V12_N1_1991_Stilwell.pdf ROBERT K. STILWELL: SATELLITE APPLICATIONS OF THE BIFILAR HELIX ANTENNA in Johns Hopkins APL Technical Digest , vol. 12 (1991), no. 1, pages 75-80
↑ ^a ^b Bill Slade: The Basics of Quadrifilar Helix Antennas. Orban Microwave Inc., 2015, accessed March 17, 2017 .
↑ http://www.w1ghz.org/antbook/conf/Helical_feed_antennas.pdf Considerations on helical antennas for 2.4 GHz (English)

[kraus1-1] John D. Kraus: Helical beam antenna . In: Electronics . No. 4109 , 1947, pp. 109 to 111 .

[kraus2-2] John D. Kraus: The helical antenna . tape 37 . Proceedings of the IRE , March 1949, pp. 263 to 273 ( online [PDF]).

[„stilwell“-3] ttp://www.jhuapl.edu/techdigest/views/pdfs/V12_N1_1991/V12_N1_1991_Stilwell.pdf ROBERT K. STILWELL: SATELLITE APPLICATIONS OF THE BIFILAR HELIX ANTENNA in Johns Hopkins APL Technical Digest , vol. 12 (1991), no. 1, pages 75-80

[orban1-4] Bill Slade: The Basics of Quadrifilar Helix Antennas. Orban Microwave Inc., 2015, accessed March 17, 2017 .

[5] ttp://www.w1ghz.org/antbook/conf/Helical_feed_antennas.pdf Considerations on helical antennas for 2.4 GHz (English)