An antenna is a transducer of electrical and electromagnetic energy. electromagnetic. electrical

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1 Basic Atea Cocepts A atea is a trasducer of electrical ad electromagetic eergy electromagetic electrical electrical Whe We Desig A Atea, We Care About Operatig frequecy ad badwidth Sometimes frequecies ad badwidths Iput impedace (varies with frequecy) Radiatio patter (Gai) Polarizatio fficiecy Power hadlig capacity Size ad weight Fits ito compoet packagig (aesthetics) Vulerability to weather ad physical abuse Cost 1

2 Cosider Oe Way To Lauch A Wave Start with a twi-lead or parallel-plate trasmissio lie (they both look the same from the side) H Source TM wave i the guide By bedig ad termiatig the guide the fields are forced to leap ito free space Oe Way To Lauch A Wave () By bedig the trasmissio lie so that it forms a 90º agle, you get the classic dipole atea H Source If the total legth of the dipole is /, the iput of the dipole (the gap betwee the two legs ) will be 73+j4

3 Dipole Impedace versus Legth Half-wave dipole Network Aalyzer Smith Chart Display 3

4 Atea Patters Atea patters idicate how the radiatio itesity (-field, H- field, or power) from a atea varies i space (usually specified i spherical coordiates. The patter is usually plotted agaist oe spherical agle ( or ) at a time. xample: The radiatio patter for a half-wave dipole Physical atea The agle(s) at which imum radiatio occurs is called boresight Normalized radiatio patter Atea Patters () Patters usually represet far-field radiatio Far eough away from the atea so that the 1/R & 1/R 3 field terms have dropped out Rule of thumb: the far field begis at d /, where d is the imum dimesio of the atea (or atea array) Ateas are geerally categorized as isotropic (equal radiatio i all directios) or directioal There is o such thig as a isotropic atea (i.e., oe that is isotropic i ad I some cases we are oly iterested i particular compoets (e.g., (,), H (,), etc.) 4

5 Atea Patter xample (hor atea) Side lobes Mai lobe Atea Patter xample (4-elemet Yagi) levatio Patter Azimuth Patter Defiitio: Frot-to-back ratio is the ratio of imum sigal out of the frot of the atea to the imum sigal comig out of the back of the atea, expressed i db. This atea has a frot-to-back ratio of about 17 db. 5

6 Reasos For Watig Directive Ateas Lower oise whe lookig oly at a small sectio of space Stroger sigal whe lookig i the directio of the source Remote sesig (radar)- whe iterested i properties of a small sectio of space Ca be used to spatially filter out sigals that are ot of iterest Ca provide coverage to oly desired regio Atea directioal characteristics are sometimes expressed as a sigle scalar variable: beam width, beam area, mai-lobe beam area, beam efficiecy, directivity, gai, effective aperture, scatterig aperture, aperture efficiecy, effective height. Calculatig Atea Patters The ormalized field patter (dimesioless) is give by:,,, Ad the ormalized power patter (dimesioless) is give by: P, where S S, S,,, W m, is the Poytig vector / Z 0, 10 log P, which ca be put i terms of decibels: P db 10 6

7 3 db, or Half-Power, Beamwidth (aalogous to the 3 db badwidth) The beamwidth is the rage of agles for which the radiatio patter is greater tha 3 db below its imum value xample: what is the beamwidth for the radiatio patter below? Beamwidth = 56º 3 db below imum z R si R Solid Agle () The actual area traced out by ad is R si x y The solid agle represeted by ad is si The solid agle is rage idepedet The actual area is equal to R The solid agle of a sphere is 4 steradias, or sr. Solid agle is sometimes expressed i degrees: radia radia 38.8 deg /radia Thus there are , 53 square degrees i a sphere 7

8 Beam Area ( A ), or Solid Beam Agle This parameter provides a meas for specifyig directivity whe the atea is directive i both ad. Provides a alterative to specifyig beamwidth i both ad separately. Beam Area is give by the itegral of the ormalized power patter: A P, d quivalet solid agle A Actual patter 0 0 P, si d d (sr) A ca ofte be approximated by the half-power beamwidths i ad A HPHP (sr) Polar Plot of P( ) Half-Power Beamwidth Radiatio Itesity (U(,))) ad Directivity (D) Radiatio Itesity is the power radiated per solid agle, ad ulike the Poytig vector, it will be idepedet of rage. Its uits are (Watts/steradia), ad it is related to the Poytig vector magitude ad ormalized power by: U, S, P, U, S, Directivity is the ratio of the imum radiatio itesity to the average radiatio itesity: U D U, S, Average S Average (dimesioless) The average value of the Poytig vector is give by: 1 1 SAverage S, d S, si d d (Watts/m )

9 Directivity (D) Substitutig our expressio for S average ito our equatio for D: S, S, 1 D SAverage 1 1 S, S, d d 4 4 S, 1 4 This gives us the expected result that 1 A as the beam area decreases, the P, d 4 atea becomes more directive. xample: what is the beam area ad directivity of a isotropic atea (assumig oe existed)? Is otropic P, 1 A P, si d d 4 (Sr) A beam area of 4 implies that the mai beam subteds the etire spherical surface, as would be expected 4 D 1 Which is the smallest directivity that a atea ca have A 0 0 Directivity (D) ad Gai (G) Recallig our approximatio 4 4 (Sr) ( deg ) D A HP HP HP HP A HP HP (sr), we ca write D as: Note that the umber of square degrees i a sphere is rouded off The Gai of a atea, G, depeds upo its directivity ad its efficiecy. That efficiecy has to do with ohmic losses (the heatig up of the atea). For high-frequecy, low-power applicatios we geerally assume efficiecy to be high. G is related to D by G = kd, where k is efficiecy (0 k 1) Gai is ofte expressed i decibels, refereced to a isotropic atea. G dbi 10log G 10log Gisotropic 10 log is used, rather tha 0 log, sice G is based o power G 9

10 xample: Apply Our quatios O Some Published Atea Specificatios Sice the gai is less tha the directivity, the atea is ot 100% efficiet. The oe db differece ca be put ito liear uits. G kd D D or the atea is 79% efficiet Let s see if directivity agrees with beamwidths log10 10 log10 1 k (deg ) D 11.4 (60)(60) D dbi HP HP D 10 log10 10 log D isotropic TYP NO FRQ. RANG MHz VSWR INPUT IMPDANC DIRCTIVITY GAIN BAMWIDTH H PLAN BAMWIDTH PLAN SID AND BACK LOB LVL. CROSS POLARIZATION.0:1 MAX. 50 OHMS 11 dbi 10 dbi NOM. 60 NOM. 60 NOM. -15 db MIN. 0 db NOM. POWR HANDLING 100 WATTS CW 10