Antennas Intro

8/6/2019 Antennas Intro

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Transm ission lines &antennasJavier Leonardo Araque Quijano

[email protected] 453, Oficina 2043165000 Ext. 14083

Introduction to antennas -som e antenna typesWire antennas Printed antennas~J~ 1 ~

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WG apertureantennas

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mailto:[email protected]:[email protected]


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Antenna examples

Reflect arrays forcommunications

Direct radiationarray for RADARapplications

Large reflector forradio-astronomyapplications

Antenna parametersradiation pattern

Half-power bc:amwidtll(HPBW}FirSl mllll)~~LlllWidlIHFNBW)Plgure 2.1 Coord inate sys tem for antcu

- Antenna principal planes- Antenna lobes- Beamwidth

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Antenna Pattern types. Isotropic: equal radiation in all directions (not physicallyrealizable). Directional: power is radiated more effectively in some directionstha n others.

- Omnidirectional: non-directional in one plane anddirectional in any perpendicular plane.- pencil beam: considerable radiation only in a smallneigborhood of a unique direction.- contoured beam: considerable radiation only in ashaped portion of solid angle.Omnidirectional pattern Pencilbeampattern

Contoured beam pattern

Field regionsReactive near-field:R< 0.62-/ D3 I A

_..., Fields are mostly reactive./ \T 1 / \ Radiating near-field

I I 'I i Far-Fi";J~ 0.62-/ D3 /"A ? ; ; ; ~ ~ ; ~ S : ~ i ~ ~ ~ ~ ~ dnL : - - - - '- - - - - - ' - - - ---'d"'-i,_"",,~tance,ields are

transverse. 6/16


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F ar fieldIn the far field, dependence on r and spherical angles theta,phi are separable, the wavefront is spherical and fieldsare transverse:................... Antenna-specificte - jkof\.... pattern, vector withE(P) =( ~(e , c p ) ) . . . . . . . . . . . . . . . .egligible radial

. .~ 4 7 C r .T . . . . . . . . . . . . . . . . . . . . . . . . . . . . componentUniversal \


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Antenna DirectivityRatio between the radiation intensity in a given directionand the average radiation intensity. If direction is notspecified, the maximum value is implied.

U 4JrUD=-=--u; P r a mDirectivity can be specified separately for individual fieldcomponents: . 4nUeDJ,=-----(Pradh + (Prnd).p

4nU .po,= (Prad)g + ( P r n d ) . p Max directivity isalways >=1

Antenna parametersimpedance

b

b

TX RX

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A n ten n a E fficien cyNot all the power delivered in input to theantenna is radiated: reflections, conduction anddielectric losses reduce antenna efficiency, i.e.the ratio between total radiated power and inputpower.

. : : , . . . : : : ; .eo = erecd = ecd(1 - ; t r I r : : : =: : : : . ~ 0 1 ~ In this course we will not include the impedance mismatchfactor in the total efficiency, i.e. eO = ecd

A n ten n a G ainGain is defined as the ratio of the intensity, in a given direction, tothe radiation intensity that would be obtained if the poweraccepted by the antenna were radiated isotropically.

~, . .. radiation intensity . U (0, )GaIn=n =n ---. total input (accepted] power PinWith this definition, gain is simply the antenna efficiency timesantenna directivity G(O, t /J ) - e , c d D , ( O , t / J )

Per-component Gain definitions are possible as with Directivity12/16

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A ntenna polarization andpo larizatio n m ism a tchAntenna polarization is defined as the polarization of the wave itradiates in a given direction. When direction is not specified, themaximum radiation direction is implied. It can be linear, circularor elliptical. It is a unit vector.

By reciprocity, it is possible to compute the fraction of the incidentenergy that is captured by a an antenna from an incident wavewhen it has a different polarization:

For ellyptical polarization, the "axial ratio" is defined as the ratio ofthe largest to the smallest ellypse semiaxis, and is thus >=1,with 1 corresponding to circular polarization.

A ntenna vector effectivelengthThe vector effective length is a complex vector relating anincident field with the open-circuit voltage at the antennaterminals. It relates also the current in the antennaterminals with the field radiated in a given direction (in farfield).

". Ej~ IiVoc :=,'. ~- t , ! ! : "

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Antenna effective areaThe effective area (effective aperture) is the ratiobetween the incident power density and the poweravailable at the antenna terminals consideringpolarization matching:

. PI Ih~2Rr/2A,;=-. =--....;...._W " W iOne may define other equivalent areas considering thescattered and lost power.It can be shown that the maximum effective area is relatedto the maximum antenna directivitv:

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Antenna BandwidthThe range of frequencies over which antenna performanceconforms to a given standard. It can be stated in terms ofpolarization purity, directivity, input impedance, etc.

-20l__-----~------j0.5 1frequency

-100L__-----~------j0.5 1frequency

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dS(A/m)

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Antennas Intro

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