(1) Microwaves are electromagnetic waves with wavelengths between 1 mm and 1 m. They are used for radar, wireless communications, microwave ovens, and more. (2) Microwave communication uses a series of microwave towers to transmit signals via radio in a line-of-sight manner. It was developed in the 1940s and became a major method for telecommunications. (3) Microwaves are generated using klystron tubes, which modulate an electron beam to produce microwave oscillations. Reflex klystrons are commonly used to generate microwaves for applications like radar and communication systems.

1. 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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2. 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
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
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3. (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
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.
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4. 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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5. 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.
(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.
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6. (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
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.
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7. 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.
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.
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8. 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
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
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9. 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.
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.
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10. 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
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
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11. 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
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
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12. 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
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
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13. 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)
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 R4max λ2
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14. So, R4max = [Pt SA02 /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
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.
14

15. (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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