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What are microwaves? Microwaves are those electromagnetic waves which have wavelengths ranging from as long as one meter to as short as one millimeter, or in other words with frequencies between 300 MHz (0.3 GHz) and 300 GHz. Such a definition includes both ultra high frequency (UHF) and Extremely High Frequency (EHF), and various sources use different boundaries. In all cases, microwave includes the entire Super High Frequency (SHF) band (3 to 30 GHz, or 10 to 1 cm) at minimum,
II .TYPES OF MICROWAVE
MICROWAVE communications are of various types:- A few are, Bluetooth, wi-fi, wi-max ,etc. Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use the microwave frequencies in the 2.4 GHz industrial, scientific and medical (ISM band, although 802.11a uses ISM band and
U-NII frequencies in the 5 GHz range Wireless Internet Access services for licensed long-range (up to about 25 km) have been used for almost a decade in many countries in the 3.5-4.0 GHz range. The Federal communication Commission (FCC) recently have designed spectrum for carriers that wish to offer services in such ranges in the United States, with 3.65 GHz.
a) Bluetooth is a device that uses microwave radio frequency to enables two devices, usually within a short range, to connect to each other without cables or wires, i.e. to establish a point to point connectivity. A point to point connectivity provides a dedicated connectivity between two devices. The entire capacity of the link is reserved between the two devices.
Bluetooth radios operate in the unlicensed ISM band at 2.4GHz by using 79channels between 2.402GHz. The Bluetooth communication is made within a range of 10 meters or so with a power consumption of 1mW. It supports two different types of links: -
1) Asynchronous Connectionless (ACL) for data transmission and
2) Synchronous Connection Oriented (SCO) for audio and video transmissions.
Bluetooth communication occurs between a master radio and a slave radio. They can be symmetric and the devices can operate as a master as well as a slave. Each radio has its own 48 bit unique device address that is fixed. If such devices are linked to each other more than two then such a network is said to be an Ad-HOC network or PICONETS. All such devices within one piconet share the same channel, with one master devices and more than one slave device. There may be up to seven slave devices in a piconet device.
Wireless Fidelity (Wi-Fi) is a device that provides communication between number of computers, thus enabling easy data sharing. Wi-fi uses both single-carrier direct-sequence spread spectrum radio-technology (part of the larger family of spread spectrum systems) and multi-carrier orthogonal frequency-division multiplexing (OFDM) radio technology.
Wi-Fi networks have limited range. A typical wireless router which is only using 802.11b or 802.11g with a stock antenna might have a range of 32 m (120 ft) indoors and 95 m (300 ft) outdoors. The new IEEE 802.11n however, can exceed that range by more than two times Range also varies with frequency band. Wi-Fi in the 2.4 GHz frequency block has slightly better range than Wi-Fi in the 5 GHz frequency block. Outdoor ranges can be improved through use of directional antennas located several kilometers or more from their base. In general, the maximum amount of power that a Wi-Fi device can transmit is limited by local regulations, such as FCC Part 15 in USA.
Wireless network bridges connect a wired network to a wireless network which differs from an access point: an access point connects wireless devices to a wired network at the data-link layer. Two wireless bridges may be used to connect two wired networks over a wireless link, useful in situations where a wired connection may be unavailable, such as between two separate homes.
`The atmosphere of the earth being the most important communication medium, the various layers of the earth play a role in propagating the radio wave. Among the four layers - troposphere, stratosphere ionosphere and exosphere interplanetary space, the ionosphere and the troposphere are useful for radio communication in certain frequency ranges. The line of sight path from the transmitter Tx to the receiver Rx.
III. ELECTROMAGNETIC WAVES
Electricity can be static, like what holds a balloon to the wall or makes your hair stand on end. Magnetism can also be static like a refrigerator magnet. But when they change or move together, they make waves - electromagnetic waves. Electromagnetic waves are formed when an electric field (shown as blue arrows) couples with a magnetic field (shown as red arrows). The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave. James Clerk Maxwell and Heinrich Hertz are two scientists who studied how electromagnetic waves are formed and how fast they travel.
IV. APPLICATION OF ELECTROMAGNETIC WAVES IN MICROWAVE COMMUNICATION
There are many applications having much importance in many fields. Most of it application are widely used in day today life.
a) ANALOG MICROWAVE COMMUNICATION: Analog microwave communication systems employ analog transmission techniques. Presently, major operational terrestrial and satellite microwave systems use analog FM modulation techniques and analog multiplexing techniques. However, more recently the microwave systems have gone digital employing digital modulation and multiplexing techniques.
The microwave frequency bands allocated for telecommunication include the S, C and low end K bands. However, satellite microwave systems are now being developed in high end K band. LOS systems usually operate at the relatively low transmitter ERP's over paths with a range of 15 to 85 km in length per relay for ground based communication systems. However with satellite based LOS systems it is possible to have, round the globe communication.
b) LOS MICROWAVE SYSTEMS: These systems have an inherit disadvantage in that the transmission distance is determined by the height of the antenna above the earth in view of the horizon limitation. Microwaves are normally bent or refracted beyond the optical horizon. However due to atmospheric refractive changes, the radio horizon could even be less than the optical horizon at times. LOS microwave systems also have some advantages like shorter installation time, high flexible channel capacity and better adaptation to difficult terrains and natural barriers. They are currently being employed in multichannel telephony and TV broadcasting both for national and international communication. Mostly they are fixed type although transportable systems are available as in case of military communication and outside TV broadcast. LOS microwave systems can operate in 14 Hz to 10 GHz frequency range.
A line-of-sight (LOS) microwave system comprises of one or more point-to-point hops. Each hop is designed so that it can be made to satisfy a worldwide communications network system .LOS systems have many characteristics. In these systems, propagation is only affected by changes in troposphere. The distance between microwave system hop points ranges from 50 to 150 miles). These systems are capable of handling up to 600 4-KHz voice channels and can also transmit television. These signals can usually be transmitted with less than 10 watts of power. Both the transmitter and receiver antennas are horn-driven paraboloids that provide high gain and narrow beam widths. In some applications, as shown in figure 5-8, plane reflectors are used with the paraboloids. These systems are very reliable. They are designed to operate over 99 percent of the time. These systems are well adapted to multichannel communications and closed circuit television.
OVER THE HORIZON MICROWAVE SYSTEMS: The most common method of constructing an OTH radar is the use of ionosphere reflection. Given certain conditions in the atmosphere, radio signals broadcast up towards the ionosphere will be reflected back towards the ground. After reflection off the atmosphere, a small amount of the signal will reflect off the ground back towards the sky, and a small proportion of that back towards the broadcaster. Only one range of frequencies regularly exhibits this behavior: the high frequency (HF) or shortwave part of the spectrum from 3 - 30 MHz
C ) DIGITAL MICROWAVE SYSTEM: These are also known as digital radio system, PCM radio system or digital line of sight microwave systems. Digital source block may have many PCM converted digitized voice channels, digital computer or other data channels. The transmitting block modulates the radio frequency carrier digitally by bit streams from digital source. After amplification, the modulated signal is radiated by a suitable antenna. Duplexer is employed to have the same antenna while transmitting or receiving. The receiver block demodulates the modulated signal and gives the digital message to the digital sink block which in turn gives the original message. For a digital microwave system two factors are important:
Source bit rate , and
Digital signal code.
Transmission rate for low capacity microwave systems ≤ 1Mb/sec. Transmission rate for high capacity microwave syatem≥300Mb/sec. TDM-PCM channel banks are main sources of digital radio systems. Hence digital radio systems are also known as PCM radio systems.
FDM (frequency division multiplexing): It turns out that many wires have a much higher bandwidth than is needed for the signals that they are currently carrying. Analog Telephone transmissions, for instance, require only 3 000 Hz of bandwidth to transmit human voice signals. Over short distances, however, twisted-pair telephone wire has an available bandwidth of nearly 100 000 Hz.
There are several terrestrial radio based communications systems deployed today. They include:
Digital microwave radio
Mobile radio service was first introduced in the St. Louis in 1946. This system was essentially a radio dispatching system with an operator who was able to patch the caller to the PSTN via a switchboard. Later, an improved mobile telephone system, IMTS, allowed customers to dial their own calls without the need for an operator. This in turn developed into the cellular radio networks we see today.
The long haul PSTNs and packet data networks use a wide variety of transmission media including
Frequency Division Multiplexing (FDM) allows engineers to utilize the extra space in each wire to carry more than one signal. By frequency-shifting some signals by a certain amount, engineers can shift the spectrum of that signal up into the unused band on that wire. In this way, multiple signals can be carried on the same wire, without having to divvy up time-slices as in Time-Division Multiplexing schemes. In analog transmission, signals are commonly multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into sub channels of different frequency widths, each carrying a signal at the same time in parallel
d ) MICROWAVE ANTENNAS: Microwaves are used for communication and not for broadcasting purposes. Hence, Omni directional antennas are not required. As they are for communication, they need to be highly directional. Also higher antenna gains are required to take care of induced and shot noise effects during microwave generation and reception. Moreover microwave output decreases as frequency is increased on account of lower efficiencies of generation. Higher antenna gains result in higher effective radiated power (ERP). Since wavelength is small at microwaves, a small amount of antenna is adequate to obtain a good (l/λ) ratio and in fact microwave antennas have heights several times the wavelength.
All antennas operated under microwave
Frequencies of operation are above 30 MHz, straight line (LOS) propagation will be preferred. Planer wave fronts are preferred. Planer wave fronts are preferred for straight line communication in order to increase the directivity and to receive more power with least distortion. When the field components measured along a circle with source at the centre is in phase, the transmission is in circular wave fronts. When the field components measured along a plane is in phase, then the transmission is said to be in planer wave fronts.
The various antennas used in microwave communication are:
Micro strip antenna.
From this term paper it can be concluded that there is much use of microwave in our daily life as well as in the other various things. From this term paper one comes to know about the methods of propagation of microwaves .This microwave which is the kind of electromagnetic wave thus have various uses in the various fields.