book 3 (antenna)

8/12/2019 BOOK 3 (Antenna)

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Radartutorial (www.radartutorial.eu)

Radartutorial

Book 3: Antennae Techniques

Preamble:

The name antenna has its seeds in the work of the Italian inventor Guglielmo Marconi!uring his e"#eriments with the electromagnetic waves he used a woodenl$ tent #ole alongwhich was carried a radiating wire This tent #ole means in Italian language l'antennaThecommon use of this term in the descri#tion of the e"#eriments of Marconi led to the #o#ularname antenna for this com#onent #art of transmitter sites

An antenna transmits or receives electromagnetic waves It is a transducer to convertelectromagnetic waves into high%frequenc$ electrical currents and vice versa Themechanical si&es of the antenna are fractions of the used wavelength There are antennaswith more than hundred meters length using frequencies in long%wave range' and antennaswith a length of few millimeters for microwave ranges This (ook deals s#ecificall$ withantennae used in radar installations

Table of Contents :

)adartutorial *+ream(le: *,earning o(-ectives: *

.unctions of an Antenna /Antenna 0haracteristics /

Antenna Gain /Antenna +attern 3+olari&ation 1

2alf%wave Antenna +ara(olic Antennae Antennae with 0osecant 4quared +attern 5+hased Arra$ Antenna **

+rinci#le of 6#eration **,inear Arra$ */+lanar Arra$ */.requenc$ 4canning Arra$ */+hase 4hifter *3

Mono#ulse Antennae *7Mono#ulse 0once#t *70onical 4can *

Learning objectives:

The learning o(-ectives serve as a #review of the information $ou are e"#ected to learn in thecha#ter This cha#ter #rovides the (asis for understanding the s#ecific radar antennae 8#oncom#letion of this cha#ter' the student will (e a(le to:

descri(e antenna directivit$ and #ower gain characteristics9 descri(e the focusing action of a (asic #ara(olic antenna9 descri(e the (asic radiation #atterns of the most common #ara(olic reflectors9 descri(e the (asic characteristics of horn radiators9 descri(e the mono#ulse antennae conce#t

Author: 0hristian olff' graduated radar engineer' 4M4gt GA. ;)td< *


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.igure /: The di#ole antennas emission diagram is aslightl$ flattened torus

.igure *: An isotro#ic radiator has got a(all%sha#ed radiation


Functions of an AntennaThe antenna is one of the most critical #arts of a radar s$stem It #erforms the followingessential functions:

It transfers the transmitter energ$ to signals in s#ace with the required distri(utionand efficienc$ This #rocess is a##lied in an identical wa$ on rece#tion

It ensures that the signal has the required #attern in s#ace Generall$ this has to (esufficientl$ narrow in a&imuth to #rovide the required a&imuth resolution and

It has to #rovide the required frequenc$ of target #osition u#dates In the case of amechanicall$ scanned antenna this equates to the revolution rate A high revolutionrate can (e a significant mechanical #ro(lem given that a radar antenna in certainfrequenc$ (ands can have a reflector with immense dimensions and can weighseveral tons

It must measure the #ointing direction with a high degree of accurac$

The antenna structure must maintain the o#erating characteristics under all environmentalconditions )adom=s are generall$ used where relativel$ severe environmental conditions aree"#erienced

The (asic #erformance of radar can (e shown to (e #ro#ortional to the #roduct of theantenna area or a#erture and the mean transmitted #ower Investment in the antennatherefore (rings direct results in terms of s$stem #erformance

Taking into account these functions and the required efficienc$ of a radar antenna' twoarrangements are generall$ a##lied:

the #ara(olic dish antenna and

the arra$ antenna

Antenna Characteristics

Antenna Gain

Inde#endent of the use of a given antenna for transmittingor receiving' an im#ortant characteristic of this antenna isthe antenna gain

4ome antenna sources radiate energ$ equall$ in all directions )adiation of this t$#e isknown as isotropic radiation e all know the 4un radiates energ$ in all directions Theenerg$ radiated from the 4un measured at an$ fi"ed distance and from an$ angle will (ea##ro"imatel$ the same Assume that a measuring device is moved around the 4un andsto##ed at the #oints indicated in the figure to make a measurement of the amount ofradiation At an$ #oint around the circle' the distance from the measuring device to the 4un isthe same The measured radiation will also (e the same The 4un is therefore considered anisotro#ic radiator

All other antennae have a gain o##osite the isotro#ic radiator 4ome antennas are highl$directional9 that is' more energ$ is #ro#agated in certain directions than in others The ratio(etween the amounts of energ$ #ro#agated inthese directions com#ared to the energ$ thatwould (e #ro#agated if the antenna were notdirectional is known as its gain hen a

transmitting antenna with a certain gain is used asa receiving antenna' it will also have the samegain for receiving

Author: 0hristian olff' graduated radar engineer' 4M4gt GA. ;)td< /


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.igure 3: Antenna #attern in a #olar%coordinate gra#h

.igure 7: The same antenna #attern in a rectangular%coordinate gra#h


Antenna Pattern

Most radiators emit ;radiate< stronger radiation inone direction than in another A radiator such asthis is referred to as anisotro#ic 2owever' astandard method allows the #ositions around a

source to (e marked so that one radiation #atterncan easil$ (e com#ared with another

The energ$ radiated from an antenna forms a fieldhaving a definite radiation #attern A radiation#attern is a wa$ of #lotting the radiated energ$ froman antenna This energ$ is measured at variousangles at a constant distance from the antennaThe sha#e of this #attern de#ends on the t$#e ofantenna used

To #lot this #attern' two different t$#es of gra#hs'rectangular%and #olar%coordinate gra#hs are used

The #olar%coordinated gra#h has #roved to (e ofgreat use in stud$ing radiation #atterns In the#olar%coordinate gra#h' #oints are located ($ #ro-ection along a rotating a"is ;radius< to anintersection with one of several concentric' equall$%s#aced circles The #olar%coordinategra#h of the measured radiation is shown in .igure 3

The main beam ;or main lobe< is the region around the direction of ma"imumradiation ;usuall$ the region that is within 3 dB of the #eak of the main (eam degrees: 37 !eci(elsdegrees in other radars

The radiation #attern of a #ara(olic antenna contains a ma-or lo(e' which is directed alongthe a"is of #ro#agation' and several small minor lo(es Der$ narrow (eams are #ossi(le withthis t$#e of reflector as shown in .igure 3 and 7 in a #revious cha#ter

The gain #of an antenna with #ara(olic reflector can (e determined as follows:

2160

Az El

G

here: A$@ (eamwidth in a&imuth angle

El @ (eamwidth in elevation angle ;/F ;the hori&on line< to #rovide constantdetection for targets a##roaching on sea surface

)tac$ed Beam Cosecant )*uared Antenna

A cosecant squared #attern can (e achieved($ two or more horns feeding a #ara(olicreflector

Hver$ feed horn alread$ emits directionall$ Ifone distri(utes the transmit #ower unevenl$on the single radiating elements' then theantenna #attern a##roaches a cosecant

squared #atternAt use of several receiving channels a heightallocation also can (e carried out The targetscan (e assigned to (eams with definedelevation there

The cosecant squared #attern isnKt restricted to #ara(olic reflectors This can (e reali&ed alsowith other kind of antennae At an antenna arra$ with agi% antennae the #attern is achieved($ interference of the direct wave with this at the earthKs surface reflected quotas

Author: 0hristian olff' graduated radar engineer' 4M4gt GA. ;)td< E


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Proof

The term 0osecant sounds ver$ much like a mathematical triangular function This is rightN2owever' what does this term have to do with our antennaO

The height 2 and the range ) define the elevationangle %

e remem(er what is written there on the #revious#age:

&'he cosecant s(uared pattern is a means of achievin) auniform si)nal stren)th at the input of the receiver as a tar)etmoves with a constant hei)ht within the beam.*

If we convert the formula according to the range andusing the trigonometric relation mentioned a(ovealread$ a##ears the term 0osecant P furthermore

hat there still was with the &...uniform si)nalstren)th...*

e can derive this connection from the radarequation:

If the echo has a uniform signal strength at the in#utof the receiver than the range is de#endent on thesquare of the antenna gain in the fourth #owerlinearl$

e can shorten the #owers arithmeticall$

e re#lace the range ($ the formula with the u##er0osecant%formula now According to definitionmentioned a(ove the height also shall (e constant

That means' we can shorten the height too withoutchanging the de#endence

4o we get the mathematical descri(ing of anantenna with a cosecant squared #atternN

Author: 0hristian olff' graduated radar engineer' 4M4gt GA. ;)td< *>


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.igure />: electronic (eam%deflection'u##er case: Boresight' lower case: !eflected

.igure /*: electronic (eam%deflection ($ a #hased arra$' left: Boresight' right: !eflection


Phased Arra, Antenna

Princi!le of (!eration

A #hased arra$ antenna is com#osed of lots of radiatingelements each with a #hase shifter Beams are formed ($

shifting the #hase of the signal emitted from each radiatingelement' to #rovide constructivedestructive interference soas to steer the (eams in the desired direction

In .igure /> ;u##er case degrees earlierthan of the u##er radiating element Because of this themain direction of the emitted sum%signal is moved

u#wards

;Lote: )adiating elements have (een used without reflector inthe figure Therefore the (ack lo(e of the shown antenna diagrams is -ust as large as the main lo(e

book 3 (antenna)

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