Parabolic antenna

A parabolic antenna, known colloquially as dish antenna or satellite dish, combines microwave radiation in the focal point of a parabolic mirror metallic. Where the radiation is detected by a detector, usually a horn, detected and forwarded.

Areas of application

To communicate with LEO satellites using reflectors with diameters of 10 m and more. Large parabolic antennas with diameters of up to 100 meters can be found in the ground stations for monitoring and control of interplanetary spacecraft, with radio telescopes and radars for extraterrestrial application. ESTRACK is the name of the " Deep Space " network ESA system that performs these tasks.

The antenna of the Arecibo Observatory with 304.8 m in diameter is not parabolic, but spherical.

Specifications

The gain G of a parabolic antenna increases with the antenna area A and decreases with the wavelength λ is a dimensionless parameter and is the aperture efficiency. Typical values ​​for satellite dishes are from 0.55 to 0.6:

For example, an antenna diameter of 70 cm for a frequency of 10 GHz has a gain of about 35 dBi.

With increasing gain, the radiation concentration, the opening angle α increases decreases. From the ratio of the wavelength λ and the aperture diameter D approximately follows the angle, if one limits the drop in intensity of the beam to within ± 3 dB:

Or, as a rough approximation:

The 70 - cm parabolic antenna from the example above therefore has a beam focusing by about 4 °. Satellites that are close together can not be registered independently.

A horn antenna has an efficiency of more than 80 %. For its maximum gain is limited to 25 dB. The efficiency of a parabolic antenna is at 50-70 %. It should therefore are only used when high gain and small opening angle is required. However, they will only bring good profits if their diameter is large compared to the transmitted wavelength. The lower limit is at about eight times the wavelength.

For example, send geostationary satellites with horns without a reflector, if they want to illuminate as much of the earth's surface. The earth from the satellite appears at an angle of approximately 17 °. A parabolic antenna could achieve this great opening to (2) only at a diameter of 4λ, but at much lower efficiency and ' ugly ' radiation pattern.

Designs

With decreasing wavelength microwaves to approach the properties of light. The roughness or structural dimensions of the surfaces that reflect the radiation must, below about a quarter of the wavelength lie in microwave thus in the millimeter to centimeter range. Therefore often also satisfy metal grid to reflect the radiation. This reduces the wind load and the ground.

We distinguish between different types of parabolic antennas (that is approximately parabolic antennas):

Parabolic antenna

The normal form of the parabolic antenna consists of a rotationally symmetric structure ( paraboloid ) whose sectional image is a parabola. The exciter ( receive or transmit member) is located in the focus of the paraboloid to the axis of rotation (no offset) or outside of the rotational axis ( offset). The design with no offset is applied, especially with large antennas.

Offset antenna

Offset antennas are used, because in the mold without the offset problem in that the primary radiator between the reflector and the incident wave is. He and the arm to which it is attached, so cast a shadow on the reflector. This area of the reflector is therefore ineffective. When exposed to sunlight, this phenomenon can be visually understand, since the shading on the light is similar to that of the received waves. With decreasing antenna faces the disturbing shadowing grows. This problem is solved in that only a small section of the paraboloid of revolution lying laterally is realized, so that the excitation (focus point ) comes to lie next to the incoming wave front. This also changed the angle of incidence. Due to the changed angle of incidence of the parabolic mirror of the incoming wavefront is inclined towards and it is thereby in our latitudes, a nearly vertical placement of the mirror distance. This has the further advantage that there is no snow on the mirror can be deposited, which would otherwise lead to a damping of the waves.

Cassegrain antenna

In Cassegrain antennas [ kasgʀɛn ] is located on the site of the exciter, a subreflector. This sub-reflector is hyperbolic convex and has two foci. A focal point of the hyperboloid of this coincides with the focal point of the parabolic mirror and is situated downstream of the sub-reflector, the second focal point is located at the position at which the exciter is located, usually in a hole in the center of the parabolic surface. The Cassegrain principle and also the subsequently described Gregory principle can also be found in reflecting telescopes, see Cassegrain telescope and Gregory telescope.

Gregory antenna

Gregory antennas have similar Cassegrain antennas, a subreflector. This sub-reflector is ellipsoidal concave and has two foci. A focal point of this ellipsoid coincides with the focus of the paraboloid and is located between two reflectors, the second focal point is located at the position at which the exciter is located, usually in a hole in the center of the parabolic surface.

Bundling

The larger a parabolic antenna is, the more energy is collected at the focal point and the more accurate it must on the other party, such as a satellite, to be aligned.

Surface treatment

The surface of the parabolic mirror can be treated to protect against corrosion or to adapt to the environment. These paints or powder coatings are used by the manufacturers. Adhesive films and synthetic resin paints are not suitable because they introduce an additional attenuation in the receive path. In addition, the paint should be matte, otherwise the solar radiation is reflected on the horn when it is behind the target object ( Sun Outage ). The concentrated solar radiation can the horn otherwise overheat and destroy.