ANTENNAS презентация

junior research assistant, B.Eng, Tashkent Scientific Research Institute of Space Engineering

CSSTEAP
Fourth Postgraduate Course in Satellite Communication ISRO, SAC, Ahmedabad, INDIA, August 1, 2003 – April 31, 2004

IN SATELLITE COMMUNICATION

BASIC PARAMETERS

ANTENNA DESIGN

CONCLUSION

ANTENNAS APPLICATIONS

SATCOM-IV Fourth Postgraduate Course in Satellite Communication

is defined as a device for radiating and receiving radio waves. They are everywhere: at our homes and workplaces, on our cell phones, cars and aircrafts. In satellite communication antennas are the very important devices which allow us to transmit and receive a various types & amount of information on the long distances.

SATCOM-IV Fourth Postgraduate Course in Satellite Communication

ANTENNAS

The first radio antennas were built by Heinrich Hertz in Karlsruhe, Germany. In 1886, he assembled apparatus we would now describe as a complete radio system operating at meter wavelengths with a dipole as the transmitting antenna and a resonant square-loop as receiver. With mankind’s activities expanding into space, the need for antennas began to grow very fast. The future of antennas reaches reaches to the stars.

ANTENNAS

There are many types of antennas. Basically four main types of antenna can be defined in satellite communication:
- WIRE ANTENNAS (HELIX)
- HORN ANTENNAS
- ARRAY ANTENNAS
- REFLECTOR ANTENNAS
Different antennas perform different functions.

Wire antennas range from the simple dipole to more complex forms of helix and conical spiral. On this picture you can see the simple wire (helix) antenna.

Wire antennas, linear or curved, are some of the old-est, simplest and cheapest.

A horn antenna may be regarded as a open-out waveguide. The function of the horn is to produce a uniform phase front with a large aperture than that of the waveguide and hence greater directivity. Horn antennas can be used alone, usually for wide-angle coverage of the Earth, or as a feed to illuminate reflector antennas which typically provide narrower beam.

from a particular beam, usually a more complex shaped beam than would be easily attainable with a simpler antenna design. The array antenna can be made from a group of dipoles, helices and horns.

Array antennas is the 3rd basic type of antennas, consist of a number of radiating elements which can be electrically coupled

Reflector antennas take many geometrical configurations. Some of the most popular shapes are the plane, corner and curved reflectors (especially the parabolic). So I have pleasure to consider basically parabolic antennas, which are most applicable in satellite communication.

Fourth Postgraduate Course in Satellite Communication

Parabolic reflector antennas can be clas-sified into the two groups: - Center fed reflector antennas; - Offset fed reflector antennas;

Parabolic reflector antennas are the most desirable candidate for spacecraft antennas because of it’s lightweight, structural simplicity and design matu-rity.

REFLECTOR ANTENNAS:

Single reflector
(center fed)

c)

FED REFLECTOR ANTENNAS:

Single reflector
(offset fed)

a)

Single reflector
(offset fed)

b)

Single reflector
(offset fed)

c)

DIRECTIVITY

EFFICIENCY

POLARIZATION

EQUIV. NOISE TEMPERATURE

Communication

The GAIN is the useful measure describing performance of an antenna. Although the GAIN of antenna is closely related the directivity, it is a measure that takes into account the efficiency of the antenna as well as it’s directional capabilities. The gain can be calculated using the formula:

Where: - eff. app. of antenna

- wavelength

- directivity

- antenna efficiency

This parameter describes the ability of the antenna to concentrate the radiated energy in a proffered direction in the transmit made or the reject signals that are received off-axis to the normal or bore sight. Directivity is given by D:

Where: - effective aperture of antenna;

- wavelength

total antenna efficiency

is used to

take into account losses at the input terminals and within the structure of antenna. Reflection, conduction and dielectric losses may be due, referring to the picture:

Total efficiency is equal to:

of an antenna in a given direction is defined as “the polarization of the wave transmitted by the antenna.”Polarization is also very important parameter of an antenna. There are three kinds of polarizations:
- linear polarization - circular polarization - elliptical polarization

NOISE TEMPERATURE

Every object with a physical temperature above absolute zero (0K = - 273°C) radiate energy. And antenna noise temperature is equal to: