SATELLITE COMMUNICATIONS

Introduction. For many years, the notion of satellite communications was a fantasy produced by fertile mind of Arthur C Clarke; a brilliant idea but rather impractical. As with so many creative ideas, technology eventually caught up and now satellites are commonplace. For the communication engineer, however, they represent challenging and stimulating field of work. Within satellite communication system we find the whole gamut of technologies operating in a strange and demanding as little energy to run as possible, the system must be capable of withstanding the arduous journey from earth to orbit. Consequently a careful balance must be struck between the mechanical, structural, electronic, electrical and electromagnetic engineering requirements of the system. Satellite communications provide opportunities, and pose problems, in communication methods. Their large area of access (footprint) allows a single transmission to cover an enormous number of receivers, thus allowing broadcast signals to be transmitted simultaneously to large number of people. However, this feature itself creates difficulties; partly political and partly economic. National boundaries are no barrier whatsoever, and the charging mechanism required to allow the satellite operator to recover the cost of development and provide continuous support requires a novel solution. Satellites can travel in variety orbits, basically elliptical in shape. A geostationary satellite system which orbit is fixed and essentially circular. It maintains a fixed position relative to points on earth, and this allows a cheap receiving antenna to be set up once and then fixed position.  

The disc will be as small as possible and the amplifier as cheap as possible, consistent with obtaining an adequate signal for most of the year. Outage will be high, compare with that for a better quality receiver, but must still be at reasonable level. The preoccupation in satellite link design is the power budget; how much power can be obtained from transmitter, how much power can be directed towards the receiver, how much power is lost over the link, and at the transmitter and receiver, and how much is left at the detector? What margin is necessary above the minimum detectable signal for the detector being used? Essentially the problem is to ensure that the signal level at the detector is large enough to produce a satisfactory output for a large part of the year. Since the satellite distance is fixed, the attenuation (except for affect of rain) is also fixed, and it is high.

In satellite communications the direction of transmission is indicated by the term used: the downlink is transmission from satellite to earth station and the uplink is transmission in the opposite direction.

As we have mentioned before, communications by satellite impose enormous problems on systems designer. In the uplink we have availability of high intensity transmission beam; because the transmitter is ground based and therefore power is not a problem, coupled with a sensitive receiver on the satellite. While in the downlink we have a low power source in the satellite and a highly sensitive receiver in the earth station. However, this presupposes that the earth station cost is not a constraint. For public utility applications such as telephony, cost will not be a limiting factor; the high cost of an expensive receiving station can be spread over a very large number users. In direct broadcast television however, the story is different. A single user has to pay for receiver and therefore its cost must be low. This means that the earth station will be ‘basic’.

****Source : Telecommunication Engineering ; J Dunlop and D G Smith; Chapman &Hall; London

THE OPTICAL FIBER COMMUNICATION

The optical components of fiber communication are, in simple term, a light emitter, which initiates the optical signal, a fiber transmits it, and a detector which receives it and converts it into an electrical equivalent. If several fibers need to be joined, end to end, the couplers must ensure that the fibers are correctly aligned and butted, to reduce any joining losses to a minimum. Each of these components has essential ancillary parts; the detector and emitter are driven by stabilized voltages, and mounted in such a way that maximum transfer of light between them and the fiber is achieved. The fiber itself must be clad in short protective coating and made up into a cable that withstand the rigours of installation over long distance.

Optical fiber communications present the most exciting, and probably the most challenging, aspect of modern systems. Fibers are exciting because they seem to offer so many benefits ---low cost, enormous bandwidth, very small attenuation, low weight and size, and very good security again external interference. Physically, fibers occupy very little space, and they are so flexible that they can be used in places that would not be accessible to conventional cable. An optical fiber is, in essence, a dielectric wave guide. It has been known for a long time that high-frequency electromagnetic energy can be transmitted along a glass or plastic rod and, indeed, observation shows that short rods are translucent to light. However, two factors prevented that knowledge from being used to product useful light guides: (1) energy leaked from outside of the dielectric to the surrounding air (2) the attenuation was so large that worthwhile lengths could not be achieved. The first difficulty though virtually insurmountable at microwave frequencies, can be overcame in the optical and infrared parts of spectrum by enclosing the guide in cladding of similar material, but which has slightly smaller refractive index. The boundary between the cladding and the core acts as a reflecting surface to transmitted light. The second problem that of high attenuation, could be reduced only by refining the methods of producing and drawing the glass so that the impurities and irregularities were reduced to a minimum. The attenuation now achievable in the laboratory is almost as low as possible, at about 0.2 db/Km. Fiber of varying quality are used for communications, but when distance are significant, care is taken to ensure that lowest attenuation possible is achieved. This involves choosing the best operating frequency for particular fiber material, and ensuring that any contaminating elements are remove from the glass during manufacture. Before considering the loss mechanism inherent in any fiber, we will look at the different fibers used, and examine, with the help of ray theory, the way in which light propagates along an optical waveguide.

****Source : Telecommunication Engineering ; J Dunlop and D G Smith; Chapman &Hall; London

FLUORESCENT LAMP

Fluorescent lamp is a special of the arc lamp. In a fluorescent lamp the glass case of lamp tube is coated with a fluorescent powder consisting primarily of phosphors. Fluorescent is the property of material to reemit radiation in the visible range of wavelengths when irradiated by energy at wavelength outside the visible range. In a fluorescent lamp an arc is created in a mercury vapor. The mercury vapor emits photons at visible and ultraviolet wavelengths. These ultraviolet (253.7 nm) photons cause the lamp coating to fluoresce. The fluorescence caused by the interaction of the ultraviolet photons and the lamp coating results in a concentration of the emitted energy in visible region. For a given watt of input electrical power, a fluorescent lamp will have more emitted energy in the visible spectra than a incandescent lamp. The input power not radiated as visible light is given off as infrared radiation and conducted heat.

**** Sumber : Optoelectric Devices and Principles; William J Mooney; Prentice Hall; New Jersey;1991

LASER TECHNOLOGY

Laser as family devices have found widespread use throughout the industrial and scientific environments. Laser devices are used in the operating room to repair eye tissue and in the machine shop to cut, drill, and weld. The information carried by optical fiber communication systems is usually put into the cable by a semiconductor laser. Continuous wave gas lasers are used in the optical laboratory to align, test, and characterize optical elements and devices. The word laser is an acronym for light amplification by stimulated emission of radiation. The central element gas laser is a gas-filled tube much like a gas lamp. It is within this tube that the stimulated emission and light amplification takes place. The low pressure gas filled tube is excited into emission by a high value DC voltage or by a radio frequency (RF) voltage source.

The DC excitation of the gas takes place through electrode extending into the gas just like electrode of gas lamp. A typical laboratory low power helium-neon laser will use  DC voltage of 7000 V to start the arc in the gas and have an operating voltage of 1800 V. Current flow will be about 5 mA. This type of laser will deliver 3 mW of radiated power for an electrical power input of 9 W. Radiation will emitted from the gas when electrons make energy transitions.

In RF laser, voltage sources cause excitation by capacitive coupling or inductive. In these laser types the atoms are excited by the rapidly changing RF electric or magnetic fields. RF excited lasers do not have electrodes embedded in the glass; rather energy is transferred through the glass by capacitive coupling or magnetic induction. Coils of wire about the glass tube or capacitive plates on provide the means of energy transfer. RF-excited laser typically use a power supply frequency in the range of 20 to 30 KHz. In general, the RF voltages used are much lower than the DC voltage used, but the RF current will be much higher than typical DC current.

The next key elements in a laser are the end mirrors. Reflecting mirrors are placed at each end of the excited gas tube. Gas laser can contain two or more gases. The applied electric field causes the atoms of one of these gases to become excited. Collisions of the excited gas with the other gas force it into excitation and emission. The emitted photons reflecting back and ford between the mirrors cause the amplification process of stimulated emission to take place. Stimulated emission takes place when a photon falls on an atom that is in an excited state. When this occurs, the atom immediately makes energy level transition. The transition causes the atom to emit a photon of the same wavelength and phase as the incident photon. The gases used in the laser tube, the excitation level achieved, the transmission and  the reflection  characteristic of the end mirror all affect the laser’s output.

THE USED OF INFORMATION

What is the used of terrabytes worth of stores information? Data have value only if they can be used. “It’s like having a bank account with millions of dollars in it but No ATM card. If you can’t get it out and can’t make it work for you, then it is not really used. ”

**** Sumber : Buku “Marketing Management 12e”;Philip Kotler/Kevin Keller;Pearson Prentice Hall;