MICROWAVES

INTRODUCTION

Microwaves are electromagnetic waves with wavelengths ranging from as

long as one meter to as short as one millimeter, or , with frequencies between 500

MHz and 100 GHz.[It can be even up to 300 G.Hz] .

Electromagnetic waves longer than microwaves ((lower frequency) are called

"Radio waves". Electromagnetic radiation with shorter wavelengths may be called

"millimeter waves".

MICROWAVES IN COMMUNICATIONS:

Microwave communication is the transmission of signals via radio using a series

of microwave towers. Microwave communication is known as a form of “line of sight”

communication, because there must be nothing obstructing the transmission of data

between these towers for signals to be properly sent and received.

The technology used for microwave communication was developed in the

early 1940’s by Western Union. The first microwave message was sent in 1945 from

towers located in New York and Philadelphia. After this successful attempt, microwave

communication became the most commonly used data transmission method for

telecommunications service providers. Microwave communication takes place both

analog and digital formats. While digital is the most advanced form of microwave

communication, both analog and digital methods gives certain benefits for the users.

Analog microwave communication may be most economical for use when compared to

digital communication. Digital microwave communication utilizes more advanced, more

reliable technology.

Typically, microwaves are used in television news to transmit a signal from a

remote location to a television station. Most satellite communication systems operate in

the C, X, Ka, or Ku bands of the microwave spectrum. These frequencies allow large

bandwidth while avoiding the crowded UHF frequencies and staying below the

atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for

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the traditional large dish fixed satellite service or Ku band for direct-broadcast satellite.

Military communications run primarily over X or Ku-band links.

Radar uses microwave radiation to detect the range, speed, and other

characteristics of remote objects. Most of the radio astronomy systems uses microwaves.

MICROWAVE FREQUENCY BANDS

The various bands of the Microwave region are shown in the following table.

S.No Type of Band Frequency Range

1 L band 1 to 2 GHz

2 S band 2 to 4 GHz

3 C band 4 to 8 GHz

4 X band 8 to 12 GHz

5 Ku band 12 to 18 GHz

6 K band 18 to 26.5 GHz

7 Ka band 26.5 to 40 GHz

8 Q band 33 to 50 GHz

9 U band 40 to 60 GHz

10 V band 50 to 75 GHz

11 E band 60 to 90 GHz

12 W band 7 5 to 110 GHz

13 F band 90 to 140 GHz

14 D band 110 to 170 GHz

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MICROWAVE PROPERTIES:

The Microwaves behaves similar to light rays. They exhibit the following properties.

(i)They can be focused with lenses made of wax or paraffin

(ii) They can be refracted with prisms of wax or paraffin materials.

(iii) They can be reflected from large, plane sheets of metal

(iv) Microwaves can be diffracted by slits in metal surfaces and interferometers can be

constructed for their use.

(v) Microwaves can pass through dry wood whereas the light waves cannot pass through.

(vi) Microwaves propagate in free space, in various materials, and in waveguides.

(vii) Microwaves undergo polarization with paraffin crystals.

(viii) Microwaves also exhibit total internal reflection.

(ix) Microwave radiation (at 2450 MHz) is non-ionizing

(x) Microwaves also cause heating

MICROWAVE GENERATION

A klystron tube is a special type of vacuum tube invented in 1937 by the Varian

brothers. A klystron tube is used to produce microwave energy. In this application, it

works similar to an organ pipe. When the air in the organ tube vibrates, the organ tube

emits sound energy of a specific frequency that we hear as a single note. When the

electrons in the klystron tube vibrate, the klystron tube emits high frequency microwave

energy that can be detected by a radar receiver.

There are two types of klystrons tubes in use: (i) The floating drift and (ii) The Reflex Klystron.

REFLEX KLYSTRON :

Reflex klystrons were developed in 1940 by the Soviet engineers N. D.

Deviatkov, E. N. Danil’tsev , and I. V. Piskunov, working as a group, and,

independently, by the Soviet engineer V. F. Kovalenko.

The Reflex Klystron is a single cavity variable frequency microwave

generator oscillator. It has low power and low efficiency. The principle of the Reflex

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klystron is that , the electron beam, having passed through the resonator gap, arrives at

the decelerating field of the reflector, to be repelled by the field and pass through the

resonator gap in the opposite direction .During the first transit through the gap, the

ultrahigh frequency electric field of the gap modulates the electron velocities. The second

time, moving in the opposite direction, the electrons arrive at the gap grouped in bunches.

The ultrahigh frequency field in the gap retards these bunches and converts some of their

kinetic energy to the energy of ultrahigh-frequency oscillations. This is nothing but the

Microwave energy.

Construction: The Reflex Klystron consists of electron gun, filament surrounded by a

cathode and a focusing electrode at cathode potential. The electron beam emitted from

the cathode is accelerated by the Grid and passed through the anode cavity to the repeller

space between the anode cavity and repeller electrode as shown in figure.1.

Working: The electron beam from the cathode is velocity modulated by the cavity gap

voltage.Due to this some of the electrons accelerates and enters the repeller space with a

greater velocity than the velocity electrons with unchanged velocity.Some of the

electrons decelerates and enters the repeller space with less velocity.In the repeller region

all the electrons are bunched together and pass through the cavity gap for every one cycle

as shown in figure 2. During the returning path the bunched electrons pass the gap during

the negative cycle and deliver the kinetic energy to the electromagnetic energy of the

field in the anode cavity.The output is taken from the anode cavity.

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Reflex klystrons are the most widely used ultrahigh-frequency device. They are

manufactured for operation in the decimeter, centimeter, and millimeter wave bands.

Their output power ranges from 5 mW to 5 W. The efficiency of the Reflex Klystron

ranges from 20% to 30%. Reflex klystrons are used as heterodynes in superheterodyne

radio receivers, as driving oscillators in radio transmitters, as low-power oscillators in

radar, in radio navigation.

APPLICATIONS OF MICROWAVES :

Microwaves find applications in various fields . They are

(1) Microwaves are used in RADAR communications.

(2) Microwave ovens are used for cooking the food at a very faster rate.(2.45G.Hz,600W)

(3 ) Microwave heating is used in rubber, plastic, paper industries for drying and curing

Products and food processing industries.

(4) Microwaves can be used to transmit power over long distances

(5) Microwave radiation is used in electron paramagnetic resonance (EPR or ESR)

Spectroscopy

(6) Used in long distance communications like, Telephone networks, T.V Networks,

Telemetry etc...

(7) Microwaves are used in Microstrip and disk filters, delay lines, and phase shifters.

(8) Microwaves are used in Mining industries ,for tunneling and breaking rocks etc..

(9) Used in Bio-medical applications (Diathermy for localized superficial heating)

(10) Microwaves are used in tumor detection based medical applications.

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(11) In Microwave tomography

(12) In Microwave acoustic imaging.

(13) In identifying the objects by non-contact method

(14) Microwave radiometers are used to map atmospheric temperatures , moisture

conditions.

(15).Satellite and terrestrial communication links with very high capacities are possible.

(16).Various molecular, atomic, and nuclear resonances occur at microwave frequencies,

so, there are unique applications in the areas of basic science, remote-sensing, medical

diagnostics and treatment.

INTRODUCTION TO SATELLITE COMMUNICATION

INTRODUCTION :

The first artificial satellite was placed in orbit by the Russians in 1957. That

satellite was called Sputnik and it is the beginning of an era. During the early 1960s,

the Navy used the moon as a medium for passing messages between ships at sea and

shore stations. This method of communications proved reliable when other methods

failed. Communications via satellite is a natural outgrowth of modern technology and of

the continuing demand for greater capacity and higher quality in communications.

A Satellite is defined as a body that revolves around another larger body in a

path called orbit. For example the moon is the natural satellite to the earth. Similarly

Earth is the satellite to the Sun. A communication satellite is a microwave repeater station

that is used for tele-communication, radio and television signals. There are nearly 750

satellites in space which are mostly used for communication applications.

A satellite communications system uses satellites to relay radio transmissions

between earth terminals. There are two types of communications satellites .One is

ACTIVE and the other is PASSIVE. A passive satellite only reflects received radio

signals back to earth.whereas an active satellite acts as a REPEATER ; it amplifies

signals received and then retransmits them back to earth. This increases signal strength at

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the receiving terminal to a higher level than would be available from a passive satellite. A

typical operational link involves an active satellite and two or more earth terminals. One

station transmits to the satellite on a frequency called the UP-LINK frequency. The

satellite then amplifies the signal, converts it to the DOWN-LINK frequency, and

transmits it back to earth. The signal is next picked up by the receiving terminal.

For covering the majority portion of the earth a minimum of three satellites are

required.

KEPLER’S lAWS :

In the early 1600s, Johannes Kepler proposed three laws of planetary motion.

These Kepler’s laws are found to be very useful in understanding not only the planetary

motion but the satellite motion also.The satellites also obey the Kepler’s laws.

Kepler's three laws can be described as follows :

(i)The Law of Ellipses

The path of the planets about the sun is elliptical in shape, with the center of the sun

being located at one of its foci.

(ii) The Law of Equal Areas

An imaginary line drawn from the center of the sun to the center of the planet will sweep

out equal areas in equal intervals of time.

(iii)The Law of Harmonies

The ratio of the squares of the periods of any two planets is equal to the ratio of the

cubes of their average distances from the sun.

GEO-STATIONARY ORBIT :

A geostationary orbit or Geostationary Earth Orbit (GEO) is a circular

geosynchronous orbit directly above the Earth's equator (0° latitude), with a period equal

to the Earth's rotational period and an orbital eccentricity of approximately zero. An

object in a geostationary orbit appears motionless, at a fixed position in the sky, to

ground observers.

So,the relative velocity between the Earth and the Geostationary orbit is zero.

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Communications satellites and weather satellites are placed in geostationary

orbits, so that the satellite antennas that communicate with them do not have to move to

track them, but can be pointed permanently at the position in the sky where they stay.

Due to the constant 0° latitude and circularity of geostationary orbits, satellites in GEO

differ in location by longitude only.

Geostationary orbits are useful because they cause a satellite to appear stationary

with respect to a fixed point on the rotating Earth, allowing a fixed antenna to maintain a

link with the satellite.

The height of a Geostationary satellite from the surface of the earth is 35,786

kilometres or nearly 36,000 km.

TRANSPONDERS

A transponder is an automatic electronic control device that receives, cross-

examines, amplifies and retransmits the received signalon a different frequency. It is

mainly used in wireless communication. The word ‘Transponder’ is a combination of

two words; transmitter and responder.A communications satellite’s channels are also

called transponders, because each is a separate transceiver or repeater.

A transponder works by receiving a signal on a component called “interrogator”

since it effectively inquires for information, then automatically transmitting a radio wave

signal at a predestined frequency. In order to broadcast a signal on a dissimilar frequency

than the one received, a special component called the “frequency converter” is provided.

By receiving and transmitting on dissimilar frequencies, the interrogator and transponder

signals can be sensed concurrently.

Transponders are basically of two types; active transponders and passive

transponders. An active transponder includes its very own power supply and constantly

emit radio signals which are tracked and monitored. These can also be automatic devices

which strengthen the received signals and relay them to another location.

A passive transponder does not include its own power source. The passive

transponder collects power from a close by electric or magnetic field offered by a reader.

The reader cross-examines the neighboring field for transponders that may be in its

proximity and stimulates enough power into the transponder’s electronic circuitry that the

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transponder becomes active and retransmits to the reader its identification ID as well as

any added information required.

Block Diagram of the Transponder :

A transponder is not a single unit. It consists of a Diplexor,band pass

filter,wide-band receiver, power amplifiers, Input De-Mux and output Mux etc.A

Diplexor is used to allow simultaneous transmission and reception.The Diplexor is a two

way microwave gate that permits the received carrier signals from the antenna and

transmitted carrier signals to the antenna. A basic band width of 500 M.Hz is available at

C – band frequencies with an input link frequency range of 5.925 to 6.425 G.Hz .These

frequencies are passed through a wide-band ,Band-pass filter(BPF) to limit the noise and

interference.After this passed on to a wide band receiver which provides a frequency

down conversion common to all channels. The wide band receiver also provides low

noise amplification needed at the input to maintain a satisfactory signal to noise

ratio.The output frequency range is 3.7 to 4.2 G.Hz which is the down link frequency

band.

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An input demultiplexer following the wideband receiver is an arrangement of

Microwave circulators and filters that separates the 500 M.Hz band into the separate

transponder channel bandwidth cahnnels. Following the demultiplexer ,power amplifiers

are provided for the individual transponder channels which the power levels up to those

required for retransmission on the downlink.

INTRODUCTION TO RADAR SYSTEMS

RADAR FUNDAMENTALS :

The term RADAR is an acronym for, Radio Detection, And Ranging.

It refers to electronic equipment that detects the presence, direction, height,

and distance of objects or targets by using reflected electromagnetic energy. The

RADAR works on the simple principle that “ Radio waves are sent towards an object

( target)and the reflected wave (Echo) is received and analysed to get the information

about the target. The frequency of electromagnetic energy used for radar is

unaffected by darkness and weather. This permits radar systems to determine the

position of ships, planes, and land masses that are invisible to the naked eye

because of distance, dark-ness, or weather. Most of the present day radars use

wavelengths between 1 mm to 1m. Broadly speaking there are two types of Radar

systems.(i) Pulsed Radar System and (ii) CW Doppler Radar system.

Any radar system has several subsystems that perform standard functions. A

typical radar system consists of

(i) SYNCHRONIZER

(ii) TRANSMITTER,

(iii) DUPLEXER,

(iv) RECEIVER each connected to a directional antenna.

The synchronizer is also known as s the "heart" of the radar system because it controls

and provides timing for the operation of the entire system. The specific function of the

synchronizer is to produce TRIGGER PULSES that start the transmitter, indicator sweep

circuits, and ranging circuits.

The TRANSMITTER produces the short duration high-power RF pulses of energy that

are radiated into space by the antenna towards Target.

DUPLEXER

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Whenever a single antenna is used for both transmitting and receiving, problems

arise. Switching the antenna between the transmit and receive modes gives problems.

The simplest solution is to use a switch to transfer the antenna connection from the

receiver to the transmitter during the transmitted pulse and back to the receiver during the

return (echo) pulse. No practical mechanical switches are available that can open and

close in a few microseconds. Therefore, ELECTRONIC SWITCHES must be used.

Switching systems of this type are called DUPLEXERS.

RECEIVER.

The energy reflected from a target to the antenna in a radar system is a very

small fraction of the original transmitted energy. The echoes return as pulses of RF

energy of the same nature as those sent out by the transmitter. However, the power of a

return pulse is measured in fractions of microwatts instead of in kilowatts, and the

voltage arriving at the antenna is in the range of microvolts instead of kilovolts. The radar

receiver collects those pulses and after analyzing the data gives the information like

range,direction and velocity etc.. of the target. Very often the receiving antenna is same

as that of transmitting antenna.

Block diagram of the RADAR

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FREQUENCIES USED IN RADAR :

The frequencies lying above UHF and the microwave ranges are used in

RADAR systems.The various frequency ranges and the maximum available peak power

and the frequency band name are given in the table 1.below.From the table it is clear that

the frequencies ranging from 300M.Hz to 170G.Hz are used in RADAR systems.For the

various ranges of frequencies different band names are given.

S.No Band Name Frequency- Range G.Hz Maximum peak power MW

1 UHF 0.3-1.0 5.0

2 L 1.0 - 1.5 30.0

3 S 1.5-3.9 25.0

4 C 3.9-8.0 15.0

5 X 8.0-12.5 10.0

6 Ku 12.5-18.0 2.0

7 K 18.0-26.5 0.6

8 Ka 26.5-40.0 0.25

9 V 40.0-80.0 0.12

10 N 80.0-170 0.01

Each frequency band has its own particular characteristics that make it better for certain applications than for others.

With a suitably large antenna, UHF is a good frequency for reliable long range

surveillance radar, especially for extraterrestrial targets such as spacecraft and ballistic

missiles. L band is the preferred frequency band for land based long-range air

surveillance radars. S band is the preferred frequency band for long-range weather

radars that must make accurate estimates of rainfall rate. It is also a good frequency for

medium-range air surveillance applications such as the airport surveillance radar.

C-band frequency has been used for multifunction phased array air defense radars and

for medium-range weather radars.

RADAR –RANGE EQUATION

The Radar range equation is used to calculate the maximum range at which a

Radar can detect a target.. To determine the maximum range of a Radar ,it is necessary

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to determine the power of the received echoes, and to compare it with the minimum

power that the receiver can handle satisfactorily. If the peak value of transmitted pulse

power is Pt ,the power density at a distance r from the antenna is given by

P = Pt / 4πr2 --------------------------(1)

If Ap is the maximum power gain of the antenna usedfor transmission,the power density

at the target is given by

P = Ap .Pt / 4πr2 (2)

The power intercepted by the target depends on its Radar cross section or effective

area..If this area is S ,the power hitting the target will be

P = PS = Ap .Pt S / 4πr2 (3)

Since the direction of the antenna id omnidirectional, the power density of its radiation

at the receiving antenna will be P1 = P / 4πr2

or P1 = Ap .Pt S / ( 4πr2 )2 (4)

Similar to target, the receiving antenna also intercepts a part of the radiated power,which

is proportional to the cross-sectional area of the receiving antenna..But here we consider

the capture area of the receiving antenna..So,the received power is

Pr = P1 A0 = Ap .Pt S A0 / ( 4πr2 )2 (5)

Here the A0 is the capture area of the receiving antenna.

Suppose the same antenna is used for both reception and transmission ,the maximum

power gain is given by

Ap = 4π A0 / λ2 (6)

Substituting (6) in the above equation (5) we get

Pr = [4π A0

/ λ2 ] Pt S A0 / ( 4πr2 )2

Pr = [4π A0

/ λ2 ] Pt SA0 /16π2 r4

Pr = Pt SA02 /4π r4 λ2 (7)

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The maximum range r max is obtained when the received power is equal to the minimum

receivable power of the receiver, Pmin .Substituting this value in equation (7) and making

r as the Rmax ,we get

Pmin = Pt SA02 /4π R4

max λ2

So, R4max = [Pt SA0

2 /4π Pmin λ2 ]

Or Rmax = [Pt SA02 /4π Pmin

λ2 ]1/4 (8)

Substituting the value A0 = Apλ2 /4π in the above equation, we get

Rmax = [Pt S λ2A2p /( 4π)3 .Pmin

]1/4 (9)

Equations (8) and ( 9) are the two forms of the Radar-Range equations.As we have

considered all the ideal conditions in the above derivation ,the actual value will be less

than the value given by the Radar –range equation.

FACTORS INFLUENCING THE MAXIMUM RANGE

Radar performance is affected by many factors. These conclusions can be made

form Radar-range equation.

1. The maximum range of the Radar is proportional to the fourth root of the peak

transmitted pulse power. i.e for doubling the maximum range ,peak power must be

increased sixteen fold.

2.A decrease in the minimum receivable power will increase the maximum range.

3.Maximum range is proportional to the square root of the capture area of the antenna or

directly proportional to its diameter if the wavelength is kept constant

4. Atmospheric conditions also affect the performance of the Radar. For example,

temperature inversion, moisture lapse, water droplets, and dust particles decrease the

accuracy of the Radar.

5.The maximum range depends on the curvature of the earth.

6.Noise also affects the performance of the RADAR. With increase of Noise in the

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medium ,there is a possibility of decrease in the maximum range of the Radar.

APPLICATIONS OF RADAR

Radar find wide spread applications in the different fields like Navigation, Over

the sea, on the ground and in space also. The applications can be classified into three

groups.

(i) General applications

(ii) Defence or military applications

(iii) Scientific applications

General Applications

1. Navigational aids using RADAR

2.Weather forecasting

3.Tracking the space crafts

Military and defence applications

4. Aiming at the enemy targets

5.Detecting and obstructing the selected objects during nights

6.Searching and aiming the submarines

7. Assisting the fighter aircrafts

8.In providing the proper guidance to missilies

Scientific applications

9. Study of planets and terrestrial space

10. Applications in microwave spectroscopy.

11.Tracking and guiding the space probes.

LIMITATIONS :

1. The CW Doppler Radar has a limitation in the maximum transmitted power .So it has a

limitation on the maximum range.

2. The presence of large number of Targets affects the performance of the CW Radar

3. The Doppler Radar is incapable of indicating the range of the Target,it can only show

only its velocity,

ELECTROMAGNETIC SPECTRUM- MICROWAVE BANDS

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