Helical antenna

A helical antenna, also known as helical antenna is a helical antenna for transmitting and receiving circularly polarized electromagnetic waves. It consists in unbalanced ( coaxial ) power supply from one or a symmetrical supplying of two helically wound conductors ( wire or tape ). It was invented in 1947 by John D. Kraus.

The coil antennas sometimes also referred to as helical antennas have nothing to do with the properties of a "real" helical antenna. They are entirely or partially made ​​of a single solenoid coil, but which has dimensions that are small compared to the wavelength. These antennas are in principle shortened quarter-wave dipoles. The inductance of the coil increases the electrical length with shortened overall length.

Execution

The periphery of a coil of a helical antenna, the length of the use wavelength. Thus, the diameter:

Therefore primarily in the decimeter range ( 0.3-3 GHz ) are used helical antennas.

The pitch ( lead) of the coil has an optimum at 0.2 - to 0.3 - times the wavelength. With the number of turns of the profit increases and improves the directivity of the antenna.

Or, if and are:

However, the maximum achievable gain approaches from a length of

A threshold of 15 dB. The higher the gain, the smaller the opening angle in degrees:

On the back is a metal surface as a counterweight ( reflector ) for the coaxial feed. It is approximately one wavelength in size. Helical antennas are relatively uncritical in terms of sizing and therefore well suited for reproduction. Which is indirectly connected with their high relative bandwidth is about 60% already at a uniform helix.

The impedance (unit: ohm, Ω ) at the feed point, or base point ( the beginning of the helix ) is:

Or with:

Often, therefore, is an adaptation of the antenna to the Fußpunktwiderstandes of the supply cable (coaxial cable, usually 50 ... 75 Ohm ) is required, for example by a sheet metal strip of appropriate size (resonance transformer).

To increase the transmission bandwidth helical antennas can also be formed as conical or spherical cap. Such forms combine the good directivity of helical antennas with the broadband spiral antennas.

Function

Since the direction of rotation of the helix determines the direction of the emitted or received circularly polarized wave, the direction of rotation of the transmitting and receiving antennas must match. However, each of the helical antenna capable of receiving linearly polarized waves in an arbitrary direction. Therefore, they are employed often in cases that should be used in which indeterminate linearly polarized waves received - but the payoff when receiving linearly polarized waves is lower by 3 dB. Signals in the respective opposite circular polarization are, however, strongly suppressed, so may the independent use of the two polarizations in the same frequency range is possible.

The circular polarization has advantages in satellite and space communications because no polarization fading occurs when the passage through the ionosphere (possibly even linear ) polarization direction of the waves is rotated in an unpredictable manner (see Faraday effect ) or the spacecraft ( with possibly linearly polarizing antenna) rotates.

Applications

Helical antennas you will find:

• On satellites and space vehicles (often tapered as a mixed form between spiral and spiral antenna)
• The ground receiving images from weather satellites
• In wireless point-to -point connections
• As antennas in amateur radio.