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- 1. Basic Antenna Concepts An antenna is a transducer of electrical and electromagnetic energy electromagnetic electrical electrical When We Design An Antenna, We Care About• Operating frequency and bandwidth – Sometimes frequencies and bandwidths• Input impedance (varies with frequency)• Radiation pattern (Gain)• Polarization• Efficiency• Power handling capacity• Size and weight – Fits into component packaging (aesthetics)• Vulnerability to weather and physical abuse• Cost 1
- 2. Consider One Way To Launch A WaveStart with a twin-lead or parallel-plate transmission line (theyboth look the same from the side) E HSource TEM wave in the guide By bending and terminating the guide the fields are forced to “leap” into free space One Way To Launch A Wave (2)By bending the transmission line so that it forms a 90º angle,you get the classic dipole antenna E HSource If the total length of the dipole is /2, the input of the dipole(the gap between the two “legs”) will be 73+j42 2
- 3. Dipole Impedance versus Length Half-wave dipoleNetwork Analyzer Smith Chart Display 3
- 4. Antenna PatternsAntenna patterns indicate how the radiation intensity (E-field, H-field, or power) from an antenna varies in space (usually specifiedin spherical coordinates. The pattern is usually plotted againstone spherical angle ( or ) at a time. Example: The radiation pattern for a half-wave dipole Physical antenna The angle(s) at which maximum radiation occurs is called “boresight” Normalized radiation pattern Antenna Patterns (2) • Patterns usually represent far-field radiation – Far enough away from the antenna so that the 1/R2 & 1/R3 field terms have dropped out – Rule of thumb: the far field begins at 2d2/, where d is the maximum dimension of the antenna (or antenna array) • Antennas are generally categorized as isotropic (equal radiation in all directions) or directional – There is no such thing as an isotropic antenna (i.e., one that is isotropic in and • In some cases we are only interested in particular components (e.g., E(,), H(,), etc.) 4
- 5. Antenna Pattern Example (horn antenna) Side lobes Main lobe Antenna Pattern Example (4-element Yagi) (4- Azimuth PatternElevation Pattern Definition: Front-to-back ratio is the ratio of maximum signal out of the front of the antenna to the maximum signal coming out of the back of the antenna, expressed in dB. This antenna has a front-to-back ratio of about 17 dB. 5
- 6. Reasons For Wanting Directive Antennas• Lower noise when “looking” only at a small section of space• Stronger signal when “looking” in the direction of the source• Remote sensing (radar)- when interested in properties of a small section of space• Can be used to spatially filter out signals that are not of interest• Can provide coverage to only desired regionAntenna directional characteristics are sometimesexpressed as a single scalar variable: beam width, beamarea, main-lobe beam area, beam efficiency, directivity,gain, effective aperture, scattering aperture, apertureefficiency, effective height. Calculating Antenna PatternsThe normalized field pattern (dimensionless) is given by: E , E , n E , maxAnd the normalized power pattern (dimensionless) is given by: S , P , n S , max E2 , E2 , where S , is the Poynting vector Z0 W / m 2which can be put in terms of decibels: P , dB 10 log10 P , n 6
- 7. 3 dB, or Half-Power, Beamwidth Half- (analogous to the 3 dB bandwidth) The beamwidth is the range of angles for which the radiation pattern is greater than 3 dB below its maximum value Example: what is the beamwidth for the radiation pattern below? Beamwidth = 56º 3 dB below maximum z Solid Angle () ( R sin The actual area traced out R by and is R 2 sin The solid angle represented y by and is sin x The solid angle is range independent The actual area is equal to R 2 The solid angle of a sphere is 4 steradians, or sr. Solid angle issometimes expressed in degrees: 2 180 180 1 radian 57.3 1 radian 3282.8 deg /radian 2 2 2 Thus there are 3282.8 4 41, 253 square degrees in a sphere 7
- 8. Beam Area (A), or Solid Beam AngleThis parameter provides a means forspecifying directivity when the antenna isdirective in both and . Provides analternative to specifying beamwidth in both and separately.Beam Area is given by the integral of the Equivalentnormalized power pattern: solid angle AA Pn , d Actual pattern 2 Half-Power 0 0 Pn , sin d d (sr) BeamwidthA can often be approximated by thehalf-power beamwidths in and A HPHP (sr) Polar Plot of P( ) Radiation Intensity ( ,)) and Directivity ( ) (U( ) (D)Radiation Intensity is the power radiated per solid angle, andunlike the Poynting vector, it will be independent of range. Itsunits are (Watts/steradian), and it is related to the Poynting vectormagnitude and normalized power by: U , S , P , n U , max S , maxDirectivity is the ratio of the maximum radiation intensity to theaverage radiation intensity: U , max S , max D (dimensionless) U Average S AverageThe average value of the Poynting vector is given by: 2 1 1 S , d 4 S , sin d d (Watts/m 2 S Average ) 4 0 0 8
- 9. Directivity ( ) (D) Substituting our expression for Saverage into our equation for D: S , max S , max 1 D S Average 1 1 S , 4 S , d 4 S , max d 1 4 This gives us the expected result that 1 A as the beam area decreases, the 4 Pn , d antenna becomes more directive.Example: what is the beam area and directivity of an isotropicantenna (assuming one existed)? 2 Isotropic Pn , 1 A P , sin d d 4 n (Sr) 0 0A beam area of 4 implies that the main beam subtends theentire spherical surface, as would be expected 4D 1 Which is the smallest directivity that an antenna can have A Directivity ( ) and Gain ( ) (D) (G)Recalling our approximation A HPHP (sr), we can write D as: 4 4 (Sr) 41000 (deg2 ) Note that the number of squareD A HP HP HPHP degrees in a sphere is rounded offThe Gain of an antenna, G, depends upon its directivity and itsefficiency. That efficiency has to do with ohmic losses (theheating up of the antenna). For high-frequency, low-powerapplications we generally assume efficiency to be high. G isrelated to D by G = kD, where k is efficiency (0 k 1)Gain is often expressed in decibels, referenced to an isotropicantenna. G GdBi 10 log10 10 log10 G G isotropic 10 log is used, rather than 20 log, since G is based on power 9
- 10. Example: Apply Our Equations On Some Published Antenna SpecificationsSince the gain is less TYPE NO. 201164than the directivity, the FREQ. RANGE 225-400-antenna is not 100% MHz VSWR 2.0:1 MAX.efficient. The one dB INPUT IMPEDANCE 50 OHMSdifference can be put DIRECTIVITY 11 dBiinto linear units. GAIN 10 dBi G kD 0.1 NOM.10 log10 10 log10 1 k 10 0.79 BEAMWIDTH H 60° NOM. D D PLANE or the antenna is 79% efficient BEAMWIDTH E 60° NOM. PLANELet’s see if directivity agrees with beamwidths SIDE AND BACK -15 dB 2 LOBE LEVEL. MIN. 41000 (deg ) 41000 D 11.4 CROSS 20 dB HPHP (60)(60) POLARIZATION NOM. POWER HANDLING 100 D WATTSDdBi 10 log10 10 log10 11.4 10.6 CW D isotropic 10

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