satellite

TOPICS • • • • • • • • •

Introduction to Satellite Communications Orbital Aspects of Earth Satellites Types of Satellites Satellites Communication Systems S t llit Subsystems Satellite S b t Earth Stations Satellite Routing Satellite Handover Applications pp off Satellites

Introduction to Satellite Communications

1. Definition: f ™ Satellite is a pphysical y object j that orbits or revolves around some celestial body. g as either bus or ™ Satellite transmissions are categorized payload. The bus includes control mechanisms that support the payload operation. ™ The payload is the actual user information that is conveyed through the system. ™ In general Satellite is an artificial satellite stationed in space for the purposes of telecommunications, military, surveillance etc surveillance,

2 History: 2. ™ Th The first fir t artificial artifi ial satellite at llit was wa the th Soviet S vi t Sputnik-1, Sp t ik 1 launched la h d on October 4, 1957, and equipped with an on-board transmitter that worked on two frequencies, 20.005 and 40.002 MHz . ™ The first American satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages. ™ Telstar was the ffirst active,, direct relayy communications satellite. Belonging to AT &T in 1962. Telstar II was successfully launched in 1963. It was used for Telephone, television, fascimile and data transmissions. transmissions

Sputnik ‐ I • • • •

The first artificial satellite October 4, 1957 LEO frequencies 20 005 MHz • 20.005 MHz • 40.002 MHz. 

SCORE • First voice communication established via satellite (US) • 1958 • LEO • 35 days in orbit

ECHO I • First passive communication satellite launched into space • 1960

Telstar I First non nongovernment active communication satellite launched ´ 1962 ´ MEO ´

SYNCOM 2 First Geo Synchronous Satellite ´ Feb 1963 ´

INTELSAT I 1964: International Telecomm. Satellite Organization (INTELSAT) created. created 1965: First communications satellite launched into geostationary orbit for commercial use E l Bird Early Bi d

Ea ly satellites were Early we e both of the passive and active type. ‰ Passive satellite is one that reflects a signal back to earth: there are no ggain devices on board to amplify p fy or repeat p the signal p a signal g ‰ An active satellite is one that electronicallyy repeats back to earth.(i.e., receives, amplifies, and retransmits the signal)

• Syncom I, launched in February 1963, was the first attempt to pplace a ggeosynchronous y satellite into orbit. Syncom y I was lost during orbit injection. y II and Syncom y III were successfully f y launched in • Syncom February 1963 and August 1964, respectively. The Syncom III satellite was used to broadcast the 1964Olympic Games f from T k Tokyo. • In 1964 a commercial global satellite network known as I l Intelsat (I (International i l Telecommunications Tl i i S lli Satellite Organization) was established..

The first Intelsat satellite was Early Bird 1, which was l launched h d in i 1965 andd provided id d 480 voice i channels. h l From F 1966 to 1987, a series of satellites designated Intelsat II, Ill IV, Ill, IV V, V and VI were launched. launched 1ntelsat VI has a capacity of 80,000 voice channels. • Inn 1988 the fi first st satellite system fo for mobile phones and data communication INMARSAT-C was established. • During 1993 the first digital satellite telephone system was introduced and also in 1998 the global satellite systems for small mobile phones was launched. •

Types yp off Satellite Orbits The orbital elements off a particular p satellite depend p upon p its intended application. The satellite orbits can be classified on the basis of : 1. Orientation off the h orbital b l plane l 2. Eccentricity 3 Distance from Earth 3.

1. Orientation of the orbital plane • The orbital plane of the satellite can have various orientations ri t ti with ith respect r t to t the th equatorial q t ri l plane l off Earth. E rth The angle between the two planes is called the angle of inclination of the satellite. • On this basis, the orbits can be classified as equatorial orbits, ppolar orbits and inclined orbits. • In the case of an equatorial orbit, the angle of inclination is zero, i.e. the orbital plane of the satellite coincides with the Earth’s equatorial plane (Figure 1).

Figure 1. Equatorial orbit • A satellite in the equatorial orbit has a latitude of 0◦. For an angle of i li i equall to 90◦, inclination 90 the h satellite lli is i said id to be b in i the h polar l orbit bi (Figure 2). For an angle of inclination between 0◦ and 180◦, the orbit is said to be an inclined orbit.

Figure 2. Polar orbit

• For inclinations between 0◦ and 90◦, the satellite travels in the same direction as the direction of rotation of the Earth. The orbit in this case is referred to as a direct or prograde orbit ((Figure 3). For inclinations between 90◦ and 180◦, the satellite orbits in a direction opposite to the direction of rotation of the Earth and the orbit in this case is called a retrograde d orbit bi (Figure (Fi 4) 4). Figure 3. Prograde orbit

Figure 4. Retrograde orbit

2. Eccentricity of the Orbit : On the basis of eccentricity, eccentricity the orbits are classified as elliptical (Figure (a)) and circular (Figure (b)) orbits. Needless to say, N y, w when the orbit eccentricityy lies between w 0 and 1, the orbit is elliptical with the centre of the Earth lying at one of the foci of the ellipse. When the eccentricity is zero, the orbit becomes circular.

Molniya Orbit  ƒU Usedd by b Russia i for f decades. d d ƒ Molniya Orbit is an elliptical orbit. The satellite remains in a nearly l fixed fi d position iti relative l ti to t earth th for f eight i ht hours. h ƒ A series of three Molniya satellites can act like a GEO satellite. ƒ Useful U f l in i near polar l regions. i

One of the 910re interesting orbital satellite systems is the Soviet Molniya system. system This is also spelled Molnya and Molnia, which means "lightning" in Russian (in colloquial Russian it means "news flash"). f ) • The Molniya satellites are used for television broadcasting and are presently the only nonsynchronous-orbit commercial satellite system in use. Molniya uses a highly elliptical orbit with apogee at about 40,000 km and perigee at about 1000 km •

GEO

36,000 km

MEO

5,000 – 15,000 km

LEO

500 -1000 km

Satellite communication • Satellite is a p physical y object j that orbits a celestial body. • Communication satellite containing electronics equipment q p acts as a repeater or relay station between two earth station. ´

Transponder is the basic component of a communication satellite. satellite

Satellite communication •

Satellites are located by earth coordinates expressed in terms of latitude and longitude.

•

The two angles used to point a ground station antennas are azimuth and elevation angles . The main p power supplies pp of a satellite are Solar Panels. During eclipse, the satellite is powered by Batteries. The main used of satellite is for communications, Consumers uses satellite for TV reception. reception

Why do satellites stay moving and in orbit?

F2 F1

• F1 ‐ gravitational force  • F2 ‐ centripetal force 

Geocenter - the center of gravity of the earth Period - the time of one orbit

Satellite Orbit • Satellite orbits the earth from the height of 100 to 22,300 mi and travel at speeds p of 6,800 , to 17,500 , mi/hr.

Geostationary satellite A satellite that orbits directly over the equator 22,300 mi from earth. SYNCOM1 in February (comms. failed).

1963:

SYNCOM2 in February 1963 SYNCOM3 in August 1964 At the Geostationary orbit the satellite covers 42.2% of the earth’s surface.

Satellite Orbit

P i Perigee

Circular orbit

A Apogee

Elliptical orbit

Orbits • LEO:  500 to 1 500 km 500 to 1,500 km Low Earth Orbit.

MEO: 8,000 km - 18,000 km M Medium Earth Orbit di E th O bit ´

GEO:  35,863 km Geostationary Geostationary Earth Orbit Earth Orbit ´

GE O

LE O

MEO

Frequency Bands • Three common bands: Band

Up‐Link  (Ghz)

Down‐link

ISSUES

(Ghz) 

C

4

6

Interference  Interference with ground  links.

Ku

11

14

Attenuation  due to rain

30

High  Hi h Equipment  cost

Ka

20

Lunching Satellites •Many satellites are put in orbit by launching them form NASA’ss space shuttle. NASA shuttle

How Does a Satellite Work? • One Earth Station sends a transmission to the satellite. This is called a Uplink. • The satellite Transponder converts the signal and sends it down to the second earth station. station This is called a Downlink.

End of life option of de-orbiting the satellite l leaving i the th satellite t llit in i its it currentt orbit moving g the satellite to a graveyard g y orbit.

Kepler’s first law states that the path followed byy a satellite around the primaryy will be an ellipse. An ellipse has two focal points shown as F1 and F2

The center of mass of the two two-body body system system, termed the barycenter, barycenter is always centered on one of the foci

• In In our specific our specific case, because of the enormous difference  case because of the enormous difference between the masses of the earth and the satellite, the center  of mass coincides with the center of the earth, which is  therefore always at one of the foci. • The The semimajor axis of the ellipse is denoted by a, and the  semimajor axis of the ellipse is denoted by a and the semiminor axis, by b. The eccentricity e is given by e=

a2 − b2 a

For an elliptical orbit, 0 < e < 1. When e = 0, the orbit becomes circular.

Kepler`s Kepler s Second Law Kepler’s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital plane, focused at the barycenter.

Thus the farther the satellite from earth, the longer it takes to travel a given distance

Kepler’s Third Law •

According to the Kepler Kepler’ss third law, law also known as the law of periods, periods the square of the time period of any satellite is proportional to the cube of the semi-major semi major axis of its elliptical orbit. orbit

•

The expression for the time period can be derived as follows.

•

A circular orbit with radius r is assumed. assumed Remember that a circular orbit is only a special case of an elliptical orbit with both the semi-major axis and semi-minor semi minor axis equal to the radius.

•

Equating the gravitational force with the centrifugal force gives

Solar day y and Sidereal day y • A day is defined as the time that it takes the Earth to rotate on i axis. its i • However, there is more than one way to define a day: – A sidereal day is the time that it takes for the Earth to rotate with respect to the distant stars. – A solar dayy is the time that it takes to rotate with respect p to the Sun. • A solar day is measured using the passage of the Sun across th sky—it the k it lasts l t 24 hours h • A sidereal day (from the Latin word meaning star) is measured with respect to fixed stars stars—it it lasts a little less than 24 hours.

Orbital Aspects of Earth Satellites

Here we deal with the following concepts: 1. 2 2. 3. 4 4. 5. 6.

Orbit Fundamentals Geosynchronous Satellites Station Keeping A i d Control Attitude C l Satellite Position Satellite ll Launching h

1. Orbit Fundamentals: ™ Satellite keeps moving around the Earth in some orbital pattern . ™ Orbit Fundamentals is based on a Orbit Shape a. b. Direction of satellite’s revolution c. Satellite S t llit Speed S d andd Period P i d d. Satellite Angles e. Satellite lli Repeaters

a. Orbit Shape ™ Satellite keeps moving around the Earth in some orbital pattern called “Orbit Orbit Shape Shape”. ™ Orbit Shape can be either a. Circular Ci l Orbit O bi b Elli b. Elliptical i l Orbit O bi

b. Direction of satellite satellite’ss revolution 1 Posigrade Orbit 1. i.e. satellites revolution=direction of Earth’s rotation 2 Elliptical Orbit 2. i.e. satellites revolution=against the direction of Earth’ss rotation Earth

c. Satellite Speed and Period ™ The speed of the satellite is measured in miles per hour, kilometer per hour , or knots. knots ™ Speed varies depending upon the distance of the satellite from Earth. ™ Two types of Periods ----- 1. Sideral Period 2. Synodic Period

d. Satellite Angles 1 Angle 1. A l off Inclination I li ti Is the angle formed between the equatorial plane and the satellite satellite’ss orbital plane as the satellite enters the northern hemisphere.

2. Angle of Elevation Is the angle that appears between the line from the Earth station’s antenna to the satellite and the line between the Earth station’s antenna and the Earth’s horizon.

3. Polar Orbit 4. Equatorial Orbit

e. Satellite S t llit Repeaters R t ™ To use a satellite ffor communications relayy or repeater p purposes ground station antenna must track or follow the satellite as it passes overhead. ™ Height and speed only determines how long the satellite can stay connected with the ground station. ™ Some time the satellite may disappear around the other side id off the th Earth. E th p orbit. ™ To solve this its be launched in a veryy longg elliptical

2. Geosynchronous Satellites: ™ A geostationary satellite revolves around the earth at a constant speed once per day over the equator. ™ It appears to be in a fixed position to an earth-based observer. ™ Usually geosynchronous satellites are placed at a distance of 22,300 miles or 35,860 km above the Equator. ™ The Th satellite lli at that h distance di revolves l aroundd the h Earthh in i exact 24 hours. ™ Speed of the satellite=7000 miles/hour

Advantages of Geosynchronous Satellites: ™ Since the satellite remains apparently fixed, no special earth station tracking antennas are needed ™ The antenna can simply be pointed at the satellite and remain fixed. ™ Continuous communications are possible. ™ Most communication satellites used today are geosynchronous h satellites. lli

Disadvantages of Geosynchronous Satellites: ™ During an eclipse the Earth or moon gets between the satellite t llit andd the th Sun, S is i causes the th sunlight li ht to t be b blocked bl k d from the solar panel. ™ So an eclipse shuts off all power to the satellite. ™ To avoid this backup batteries are used. used

3. SStation Keeping: K p g ™ E Even with ith a very goodd launch l h the th satellite t llit can drift d ift somewhat from its orbit. This is called “Orbital Drift”. ™ It is caused by a variety of forces like sun’s, moon’s gravitational pull, etc. ™ The process of firing the rockets under ground control to maintain or adjust j the orbit is referred f to as “Station Keeping”

4. A Altitude C Control: ™ Satellites have to be placed in some altitude for optimal performances. f Thi is This i called ll d as Altitude Altit d Control. C t l ™ Stabilizingg the satellite is also called as Altitude Control. ™ Two types stabilization are there: ™ ™

Spin Stabilization Three axis Stabilization

™ M Mostt common is i the th Spin S i Stabilization, St bili ti where h r the th satellite spins around using the thrusters attached to it on its primary axis.

5. Satellite Positioning: ™ In order to use a satellite, it has to be positioned in space properly, usually it a predetermined by design of the satellite and is achieved during launch. launch ™ Once the position is known, the earth station antennas have to pointed at the satellite for optimal transmission and reception. ™ The location of a satellite is generally specified in terms of latitudes and longitudes. g

6. Satellite Launching: ™ SSatellites t llit are placed l d into i t their th i orbits bit by b mounting ti them th on top t off rockets which literally shoot them into space. ™ Occasionally, i ll the h rocket k will ill contain i more than h one satellite. lli Here H the h main satellite is called as “Initial Payload” and others as “Secondary p y payload”. ™ The satellite is first put into what is called a “transfer orbit”, a highly p orbit that ppermits adjustments j to the satellite to be made elliptical prior to its being placed into final position.

Types T ypes off S ll Satellites

Satellites

Natural Satellites

E.g.: Moon

Natural Satellites

Based on Orbiting the Earth

Geostationary Sate tes Satellites Medium Earth Orbitingg Satellites Low Earth L E th Orbitingg Satellites Highly Elliptical Orbiting g Satellites Polar Satellites

Based on Application Remote Sensing Satellites Meteorological Satellites Communication Satellites Navigation Satellites Scientific and Military y Satellites

A. Natural Satellites: ™ A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. primary Technically, Technically the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy. ™ E.g.: Moon

Fig : Jupiter Jupiter'ss Moon

B Based on Orbiting the Earth: B. ™ There are five types. 1. Geostationary Satellites - Satellites S lli are placed l d above b the h equator at a distance di off about b 36000 km. - A Almost todayy all satellite orbitingg the Earth are off this type yp 2. Medium Earth Orbiting Satellites - Operate at a distance of about 5,000-12,000 km. - Up to now there has not been many satellites in this class.

3. Low Earth Orbiting Satellites -

Are placed at an altitude of 5,00-1,500 km. Typical duration of them are 95-120 minutes. They try to ensure a high elevation for every spot on Earth to provide high quality communication link. U advanced Uses d d compression i schemes, h t f rate transfer t off 2,400 2 400 bits/sec bit / can be enough for voice communication.

4 Highly 4. Hi hl Elliptical Elli ti l Orbiting O biti Satellites S t llit -

Comprises of all satellites with a relatively low-altitude perigee and an extremely t l high-altitude hi h ltit d apogee. It has the advantage of long dwell times at a point in the sky during pp to and descent ffrom apogee. p g the approach E.g.: US’s Sirius Satellite

5. Polar Satellites -

These satellites Th lli orbit bi from f N h Northern H i h Hemisphere to Southern h hemisphere. E.g.: PSLV, Polar Wind(USA) They follow highly elliptical orbit, inclined about 86 o with an orbital period of 18 hours It gathers multi-wavelength imaging of the aurora, and measures the entry off plasma l into the h polar l magnetosphere, h etc..

C. Based on Applications : ™ There Th are five fi types. 1. Remote Sensing Satellites - Are a series of Earth Observation satellites, which observes weather, landscapes, atmosphere, oceanic surface, climate changes, urban planning, etc.. - Two types of remote sensing --- 1. Active 2. Passive

2. Meteorological Satellites - a type of satellite that is primarily used to monitor the weather and climate of the Earth. - Satellites can be either polar orbiting, orbiting or geostationary, geostationary etc.. etc - It sees clouds and cloud systems, City lights, fires, effects of pollution, auroras, sand and dust storms, snow cover, ice mapping, boundaries of ocean currents, energy flows, etc., are other types of environmental information collected using weather satellites.

3 Communication Satellites 3. - They aid telecommunications, as by reflecting or y g a radio. relaying -

have been a significant part of domestic and global communications since the 1970s.

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Uses --- Telephony, Satellite TVs, Satellite Internet, Satellite Radio, Aircraft communications, etc..

4. Navigation Satellites -

Global Navigation Satellite Systems (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo geo-spatial spatial positioning with global coverage. coverage

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allows small electronic receivers to determine their location (longitude (longitude, latitude latitude, and altitude) to within a few meters using time signals transmitted along a line-of-sight by radio from satellites.

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Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.

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As of 2009, the United States NAVSTAR Global Positioning y (GPS).) (G ) is the only fully operational GNSS. System

A handheld GPS Receiver

5 Military and Scientific Satellites 5. -

A militaryy satellite is an artificial f satellite used ffor a militaryy purpose, often for gathering intelligence, as a communications satellite used for military purposes, or as a military weapon.

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Many cryptographic algorithms are used to encode the signals, use special frequency ranges, advanced transmitting and receiving equipments .

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Scientific satellites gather data for scientific analysis. This includes observations of the atmosphere of our planet, planet the stars, stars the sun and other parts of space.

Military Satellite

Satellite Communication Systems

™ Communication Satellites are originators of i f information. ti ™ They instead relay stations for other sources. Here we deal with the ffollowingg concepts: p 1. 2 2. 3. 4 4.

Transponders Satellite Frequency Allocations Satellite Bandwidth I Increasing i Channel Ch l Capacity C it

1. Transponders -

Satellite contains a receiver which picks up the transmitted signal, amplifies it, and translates it into another frequency. The transmitter-receiver combination in the satellite is known as “Transponder”. Uplink – Upto 6GHz Downlink – Upto 4GHz T i l transponder Typical d has h a wide id bandwidth. b d id h But B use only l a single i l signal to minimize interference and to improve communication reliability.

2. Satellite Frequency Allocations -

Most satellites operate in microwave frequency spectrum.

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It is divided up into frequency bands which have been allocated into satellite as well as other communications services such as radar. radar

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The most widely used satellite communications band is the C band.

Fig: Frequency bands used in satellite communications FREQUENCY

BAND

225-390 MHz

P

350-530 MHz

J

1530-2700 MHz

L

2500 2700 MHz 2500-2700

S

3400-6425 MHz

C

7250-8400 MHz

X

10.95-14.5 GHz

Ku

17.7-21.2 GHz

Kc

27.5-31 GHz

K

36-46 GHz

Q

46-56 46 56 GHz

V

56-100 GHz

W

3. Increasing Channel Capacity -

-

Although the transponders are quite capable, they nevertheless rapidly become overloaded with traffic. For these reasons, numerous techniques have been developed to effectively ff l increase the h band-width b d d h andd signall carrying capacity of the satellite. Two of these techniques are: 1. Frequency Reuse 2. Spatial p Isolation

Satellite Subsystems

Regulators, protection and conditioning

DC/DC Converters, DC/AC Inverters

DC to all subsystem

DC and AC to special subsystem

Charger and Batteries

Solar Panel

Communication Subsystem Receiver

Frequency Translator

Power Sub System

Altitude Control Subsystem

Transmitter

Transponder Other Transponders

Antenna Subsystem

Communications Antennas

I/Ps from onboard sensors

Telemetry, Tracking, and Control S b Subsystem

Telemetry Antenna

Propulsion Subsystem

Ctrl Sgls to all subsystems

AKM

Jet Thrusters

™ Generally ll satellites lli have h many subsystems b which hi h join j i together for the fully operation of the satellite. ™ The Th various i subsystems b t in i a generall communication i ti satellites t llit are: 1 Power Subsystem 1. 2. Communication Subsystem 3 Antenna 3. A t S b t Subsystem 4. Telemetry, Tracking, and Control Subsystem 5. Propulsion l Subsystem b 6. Altitude Control Subsystem

E th Earth St ti Stations

LO

Base Ba and O/P

BPF

Demodulator

BPF

DEMUX

LNA

Power Divvider

Down Converter

Receive Subsystem GCE-Receive

Antenna Subsystem

Driver BPF HPA

Power Divider

UP Converter Demodulator

LO GCE-Transmit

Transmit Subsystem

Fig: General Block Diagram of an Earth Station

Power Subsystem

DEMUX

Carrier Oscillator

Base Band I/P

Di l Diplexer

™ The earth station on the ground is the terrestrial base of the system. system ™ Th The earth rth station t ti communicates i t with ith the th satellite t llit to t carry rr out designated mission. ™ It may be located at the end user’s facilities or may be located with ground-based intercommunication links between the earth station and the end user. ™ Many earth stations are now located on top of tall buildings or in other urban areas directlyy w where the end user resides.

™ The various subsystems in an earth station are: 1. Antenna Subsystem 2. Receive Subsystem 3. Transmit Subsystem 4. Ground Communication Equipment (GCE) Subsystem 1. 2.

GCE Transmit Subsystem GCE Receive Subsystem

5. Power Subsystem

S Satellite Routing outing

Satellite Routing Inter Satellite Link (ISL)

Spot Beam

Base Station Or Gateway

ISDN

PSTN User Data

GSM

S Satellite Handover andove

™ There are ffour types yp off satellite handovers. Theyy are: 1. Intra Intra-satellite satellite Handover 2 Inter-satellite handover 2. 3 Gateway Handover 3. 4 Inter-system 4. I t t Handover H d

Applications Of Satellite

Various applications of satellites are: 1. Remote Sensing Satellites 2. Meteorological / Weather Satellites 3. Communication Satellites 4. Navigation Satellites 5. Military Satellites 6. Space Exploration Satellites

1. Remote Sensing Satellites

Indian Remote Sensing satellite’s image

2. (a) Meteorological Satellites

2. (b) Weather Satellites

Image g off a Weather Satellite Report p

Various weather satellites orbiting the Earth

3. Communication Satellites

4. Navigation Satellites

5. Military Satellites

6. Space Exploration Satellites

E.g. Martian Communication

SPACE CRAFT ANTENNAS • The function of the spacecraft antennas is to receive and transmit two distinct classes of signals: g broadband microwave frequencies for communication service, such as television; and narrow-band VHF for beacon, command, and telemetry. • The Th requirements i t on these th antennas t resulted lt d from f a thorough th h systems analysis which led to many factors involving not only the spacecraft, but the ground station as well. • These factors include such vital questions as: the modulation method, the location of frequencies, the choice of polarizations the degree of attitude stabilization, polarizations, stabilization etc,. etc

Spin stabilization Spin stabilization • Stabilization accomplished p byy rotatingg the spacecraft p mass,, thus using gyroscopic action as the stabilizing mechanism. Thrusters are fired to make desired changes in the spinstabilized attitude. attitude • Spin-stabilized spacecraft provide a continuous sweeping desirable for fields and particle instruments, but they may require complicated systems to de-spin antennas or optical instruments which must be pointed at targets.

• • • •

Wire antennas : monopoles and dipoles Wire antennas : monopoles and dipoles Horn antennas Reflector antennas fl Array antennas

• Wire antenna are used pprimarilyy at VHF and UHF to pprovide communications for the TT&C systems. They are positioned with great care on the body of the spacecraft in an attempt to provide omni-directional omni directional coverage. coverage • Most spacecraft measure only a few wavelengths at VHF frequencies which makes it difficult to get the required antenna patterns. • An antenna pattern is a plot of the field strength in the far field of the antenna when the antenna is driven by a transmitter. transmitter It is usually measured in decibels below the maximum field strength.

• Horn antennas are used at microwave frequencies when relatively l i l wide id beams b are required, i d as for f global l b l coverage. A horn is a flared section of waveguide that provides an aperture g wide and a ggood match between the several wavelengths waveguide impedance and free space. • Reflector antennas are usually illuminated by one or more horns and provide a larger aperture than can be achieved with a horn alone. • For maximum g gain,, it is necessaryy to ggenerate a pplane wave in the aperture of the reflector. This is achieved by choosing a reflector profile that has equal path lengths from the feed to the aperture so that all the energy radiated by the feed and aperture, reflected by the reflector reaches the aperture with the same phase angle and created a uniform phase front.