SATELLITE
SATELLITE-BASED WIRELESS SYSTEM
INTRODUCTION
Wired communication requires physical connection between sender and receiver. There are many situation in which it is difficult to run physical wires. To avoid some of the limitations of the wired systems, Communication companies have added communication satellites.
Satellites have now become an integral part of the world wide communications systems.
COMPONENTS OF SATELLITE
- The transponders
- Antenna system
- The power package
- Control information systems
- Rocket thruster systems
The transponders is a high-frequency radio receiver, a frequency down-converter and a power amplifier, which is used to transmit the down-link signal. The antenna systems contain the antennas to position them correctly. Once properly in place, they will generally function trouble-free for the life of the satellite. The power package is the power supply to the satellite, the control and information system and the racket thruster system are called the station keeping system and the function of the station keeping system is to keep the satellite in the correct orbit with the antennas pointed in the exact direction desired.
APPLICATIONS
- Weather forecasting
- Radio and TV broad casting satellites
- Military satellites
- Satellites for navigation
SATTELLITE ORBITS
Satellites Circulate in orbits around the earth, depending on the application. These orbits can be circular or elliptical. Satellites in circular orbits always keep same distance to the earth surface following a simple law.
The attractive force FG the earth due to gravity mg(R/r)2
The centrifugal force FC trying to pull the satellite away m.r.w2
To keep the satellite in a stable circular orbit the above forces should be equal. Any satellite orbiting must satisfied following equations.
4*1011
v= (d+6400)
Where
V=velocity in metre/second.
D=distance above earth’s surface in KM.
Four types of orbits can be identified as shown in the following figure
GEO
HEO
LEO
MEO
Antennas
Introduction
The antenna is the interface between the two media through which electromagnetic waves can propagate along transmission lines through space.
First, antennas are passive devices. Therefore, the power radiated by a transmitted antenna cannot be greater than the power radiated by a transmitter. In fact, it is always less because of loses. We will speak of antenna gain, but remember that gain in one direction results from a concentration of power and is accompanied by a loss in other directions. Antennas achieve gain the same way a flashlight reflector increases the brightness of the bulb: by concentrating energy.
The second concept to keep in mind is that antennas are reciprocal devices; that is, the same design works equally well as a transmitting or a receiving antenna and in fact has the same gain. In wireless communication, often the same antenna is used for both transmission and reception.
Essentially, the task of transmitting antenna is to convert the electrical energy traveling along a transmission line into electromagnetic waves in space. The energy in the transmission line is contained in the electric field between the conductors and in the magnetic field surrounding them. All that is needed is to launch these fields (and the energy they contain) into space.
At the receiving antenna, the electric and magnetic fields in space cause current to flow in the conductors that make up the antenna. Some of the energy is thereby transferred from these fields to the transmission line connected to the receiving antenna, and hence to the receiver.
Simple Antennas
The simplest antenna, in terms of its radiation pattern, is the isotropic radiator it has zero size, is perfectly efficient, and radiates power equally in all directions. Though merely a theoretical construct, the isotropic radiator makes a good reference with which to compare to gain and directionality of other antennas. That is because, even though this antenna cannot be built and tested, its characteristics are simple and easy to derive.
The half-wave dipole antenna, on the other hand, is a simple, practical antenna which is in common use.
Monopole antenna
Many wireless applications require antennas on vehicles. The directional effects of a horizontal dipole is possible but awkward to feed in the center and rather long at some frequencies. Similar results can be obtained by using a vertical quarter wave monopole antenna. It is mounted on a ground plane which can be the actual ground or an artificial ground such as the body of a vehicle. The monopole is fed at the lower and with coaxial cable. The ground conductor of the feedline is connected to the ground plane.
The Five-Eighths Wavelength Antenna
The antenna is often used vertically as either a mobile or base antenna in VHF and UHF systems. Like the quarter wave monopole it has omni-directional response in the horizontal plane. However the radiation is concentrated at a lower angle, resulting in gain in the horizontal direction, which is often useful.
It has a higher feedpoint impedance and therefore does not require as good a ground because the current at the feedpoint it less. The impedance is typically lowered to match that of a 50 ohms feedline by the use of an impedance matching section.
The Discone Antenna
It is characterized by very wide bandwidth, covering approximately a ten to one frequency range and an omni-directional pattern in the horizontal plane. The signal is vertically polarized and the gain is comparable to that of a dipole. The feedpoint impedance is approximately 50 ohms; the feedpoint is located at the intersection of the disk and the cone. The disk cone combination acts as a transformer to match the feedline impedance to the impedance of free space, which is 377 ohms. Typically the length measured along with the surface of the cone is about one-quarter wavelength at the lowest operating frequency.
The wide bandwidth of the discone makes it a very popular antenna for general reception in the VHF and UHF ranges. It is a favorite for use with scanners. These receivers can tune automatically to a large number of channels in succession and are often used for transmitting but seldom is. Most transmitting stations operate at the one frequency or over a narrow band of frequencies.
Helical Antennas
A helical antenna is a spiral, usually several wavelengths long. Such an antenna as shown below. Typically the circumference of each turn is about one wavelength and the turns are about a one-quarter wavelength apart.
Helical antennas produce circularly polarized waves whose sense is the same as that of the helix. A helical antenna can be used to received circularly polarized waves with the polarization in any direction. Helical antennas are often used with VHF and UHF satellite transmissions.
Slot Antenna
Slot antenna which is actually just a hole in a wave guide. The length of the slit is generally one-half wavelength. Its radiation pattern and gain are similar to those of a dipole with a plane reflector behind it. It therefore has much less gain than, for instance, a horn antenna. It is seldom used lone but is usually combined with many other slots to make a phased array(see the next section)
Horn Antenna
Horn antennas, like those shown in below can be viewed as impedance transformers that match waveguide impedances to that of free space. The examples in the figure represent the most common types. The E-andH-plane sectoral horns are named for the plane in which the horn flares; the pyramidal horn flares in both plane s. The conical horn is most appropriate with circular waveguide.