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					Antennas

          Dr. John S. Seybold

                 November 9, 2004
    IEEE Melbourne COM/SP AP/MTT Chapters




Introduction
 The antenna is the “air interface” of a communication
 system
 An antenna is an electrical conductor or system of
 conductors that performs;
    Transmission - radiates electromagnetic energy into space
    Reception - collects electromagnetic energy from space
 In two-way communication, the same antenna can
 be used for transmission and reception with the use
 of suitable receiver protection
 The principle of reciprocity states that the transmit
 and receive characteristics of an antenna are
 identical
                              RF Propagation
         Copyright © August 2002, John S. Seybold, All Rights Reserved   2
Types of Antennas
 Isotropic antenna (idealized)
    Radiates power equally in all directions
 Dipole antennas
    Half-wave dipole antenna (or Hertz antenna)
    Quarter-wave vertical antenna (or monopole antenna)
 Aperture antennas
    Parabolic reflective antenna
    Horn antenna
    Cassegrain antenna
    Lens antenna
 Directive beam antenna
 Many variations, folded dipoles, end-fed dipoles, loop, inverted
 Vee, phased array, ….
                               RF Propagation
          Copyright © August 2002, John S. Seybold, All Rights Reserved       3




Dipole Antennas
                                                               λ          λ
                                                               4          4




 The half-wave dipole consists of two radiators, each a quarter
 wavelength long
 Other lengths may be used, with different radiation patterns,
 gains and radiation resistances
 Has a nominal broadside gain of 2.14 dB
 Reduced gain off the ends of the dipole
 Forms the basis for the standard quarter wave antenna
 Antenna gains may be expressed in dB (dBi) or dBd
                               RF Propagation
          Copyright © August 2002, John S. Seybold, All Rights Reserved       4
            Quarter-Wave Antennas
             Essentially one-half of a dipole antenna
             Ideally relies on a reflective ground plane to provide an image
             of the antenna to complete the dipole (monopole antenna)
             Real-world antennas tend to use counterpoises instead
             Some antennas may be shorter than a quarter wavelength but
             are electrically equivalent to a quarter wave (rubber duck)
             The gain of a quarter-wave antenna varies considerably
             depending upon its deployment
                A “rubber duck” is typically – 3dBi
                When handheld near the head, a quarter wave antenna will have a
                nominal gain of –10 dBi


                                                    RF Propagation
                            Copyright © August 2002, John S. Seybold, All Rights Reserved   5




            Monopoles


                                    Radiators

Traps
                Radiators                                   Insulating
                                                            Supports




                                   Electrical
                                   Counterpoise
                                   s
                                                          Conductive
                                                          Support
Insulator
                                                          Matching Network


                  Shielded                                  Shielded
                  Coaxial                                   Coaxial
                  Feedline              Insulator           Feedline

                                                    RF Propagation
                            Copyright © August 2002, John S. Seybold, All Rights Reserved   6
Aperture Antennas
  Asymmetrical gain patterns can be achieved
  by using asymmetrical apertures or
  illumination functions
  Tapering the aperture illumination reduces
  side lobe levels at the expense of main lobe
  broadening and gain (efficiency) reduction
  There is a Fourier transform relationship
  between the aperture illumination taper
  function and the antenna radiation pattern
                                    RF Propagation
               Copyright © August 2002, John S. Seybold, All Rights Reserved   7




Yagi Antenna
                        Driven Element
                                                               Reflector




Direction of
maximum gain



                    Directors




                                    RF Propagation
               Copyright © August 2002, John S. Seybold, All Rights Reserved   8
        Reflector Antennas
                                                            Main Reflector
Reflector
                         LNB Lens


                      LNB                          Feed Horn               Sub Reflector



                                                               Sub Reflector Supports
            Feed Support

    Offset Feed
    Reflector Antenna                                Cassegrain Antenna

                                        RF Propagation
                   Copyright © August 2002, John S. Seybold, All Rights Reserved            9




        Radiation Patterns
            Radiation pattern
               Graphical representation of radiation
               properties of an antenna
               Depicted as two-dimensional cross section
               May be plotted in rectangular or polar
               coordinates
            Reception pattern
               Receiving antenna’s equivalent to radiation
               pattern
                                        RF Propagation
                   Copyright © August 2002, John S. Seybold, All Rights Reserved           10
                      Antenna Pattern( )

                                           Antenna Radiation Pattern
             0

            10

            20

            30
Gain (dB)




            40

            50                                                                                  Beamwidth
            60
                                                                                                Front-to-back ratio
            70
                                                                                                Side lobe level
            80
                 30   25   20   15         10    5       0       5    10   15   20    25   30
                                                     Angle (deg)

                                                                     RF Propagation
                                       Copyright © August 2002, John S. Seybold, All Rights Reserved           11




                      Antenna Pattern Parameters
                       Beam width (or half-power, 3 dB, beam width)
                            Measure of directivity of antenna
                            Usually assumed to be the half-power or 3 dB width of the
                            pattern measured in degrees or milliradians
                       Front-to-back ratio
                            The ratio of the gain at 0 degrees to the gain at 180 degrees
                            Provides a measure of how well unwanted signals from the
                            rear can be rejected
                       Side lobe level
                            Usually taken to be the peak gain of the first side lobe
                            relative to the main lobe in dB
                            Has a significant impact on a systems ability to eliminate
                            spatially diverse sources of interference
                                                                     RF Propagation
                                       Copyright © August 2002, John S. Seybold, All Rights Reserved           12
Circular Aperture Antenna
Pattern




                                     RF Propagation
                Copyright © August 2002, John S. Seybold, All Rights Reserved   13




Antenna Directivity and Gain
 The directivity of an antenna is a metric of its radiation coverage
                Power density at d in max direction
           D=
                    mean power density at d
 When the antenna losses are included, this becomes the antenna gain
                      Power Density at d in max direction
            G =η ⋅
                                  PT
                                     4π d 2
 PT        is the power applied to the antenna terminals
 4πd   2    is the area of a sphere with radius d
 η          is the total antenna efficiency, which includes resistive and
             taper losses of the antenna (η = ηTηR)

                                     RF Propagation
                Copyright © August 2002, John S. Seybold, All Rights Reserved   14
Antenna Gain
  Antenna gain
      Power output, in a particular direction, compared to that
      produced in any direction by a perfect omnidirectional
      antenna (isotropic antenna)
  Can be expressed in
      dBi, decibels relative to an ideal isotropic radiator
      or dBd, decibels relative to an ideal dipole
  Effective area
      Related to physical size and shape of antenna
                                          Ae = η Ap
      Where Ap is the physical area of the antenna

                                    RF Propagation
             Copyright © August 2002, John S. Seybold, All Rights Reserved   15




Calculating Antenna Gain
If specific information on antenna gain is not available, the gain of an
aperture antenna can be estimated using
                          4π APηTη R               4π Ae
                   G=                        =
                                λ   2
                                                    λ2
    Where η is often assumed to be approximately 0.6
    Works best for aperture antennas, difficult to apply to wire antennas or beams
    (effective height)
 If the physical area is not known, but the azimuth and elevation beam
 widths are known, we can use a rule of thumb

                                        20,000
                                G≅
                                        θ AZθ EL
     Where the 3 dB beam widths are expressed in degrees (Skolnik,
     Introduction to Radar Systems)
                                    RF Propagation
             Copyright © August 2002, John S. Seybold, All Rights Reserved   16
Aperture Antenna Gain Example
   Given a circular aperture of 30 cm diameter, what is the gain at 39 GHz?

   A 30 cm diameter circular aperture has a physical area of 0.0707 m2. Assuming
   an aperture efficiency of 60% yields

                                       Ae = 0.0424 m2

   Using the expression for antenna gain and the wavelength of 7.69 mm yields

                                           G = 9005

   Or expressed as decibels relative to an isotropic radiator as 10log(9005), or 39.5
   dBi of gain.



                                    RF Propagation
               Copyright © August 2002, John S. Seybold, All Rights Reserved                  17




Antenna Regions
Far-Field (Fraunhoffer) Region                                                           2D 2
    Where D is the largest linear dimension of the antenna                          r>
    This is the region where the wavefront becomes approximately planer                   λ
    The apparent gain of the antenna is a function only of the angle
    (i.e. the antenna pattern is completely formed)


Radiating Near-Field (Transition region)                                   λ     2D 2
    The region between near and far field                                    <r<
    The antenna pattern is taking shape but is not fully formed           2π      λ
    Gain measurements will vary with distance


Reactive Near-Field
                                                                                     λ
    Region where E and H are not orthogonal,                                   r<
    Anything within this region will couple with the antenna                        2π
    and distort the pattern
    Gain is not a meaningful parameter here
                                    RF Propagation
               Copyright © August 2002, John S. Seybold, All Rights Reserved                  18
Antenna Radiation Regions

        Reactive Near-Field
                       λ
                d≤
                      2π




           Radiating Near-                     Far-Field Region
           Field



                     d=2D2/λ


                                     RF Propagation
                Copyright © August 2002, John S. Seybold, All Rights Reserved              19




Radiation Region Example
   How much separation is required between a 140 MHz quarter-wave monopole
   antenna and a 450 MHz quarter-wave monopole antenna if both are mounted
   on the roof of an automobile and near-field coupling is to be avoided?

   To avoid mutual, near-field coupling, each antenna must be outside of the
   reactive near-field of the other. The reactive near-field of the 140 MHz antenna
   is larger than that of the 440 MHz antenna, so it determines the minimum
   required separation.
                                                 λ
                                      d min ≤
                                                2π
   where                           λ = 2.14 m

The result is that the minimum required separation is                     dmin = 0.341 m


                                     RF Propagation
                Copyright © August 2002, John S. Seybold, All Rights Reserved              20
Antenna Impedance
 Every antenna will present a certain amount of
 radiation resistance (impedance) to its source
 This impedance may be a function of frequency
 For best operation, the transmitter and receiver
 should be matched to the antenna impedance
 Mismatched components result in signal reflection,
 SWR and reduced power transfer (loss)
 Many antennas are 50 ohms as that is relatively
 standard in RF and microwave work
 Sometimes to achieve a match, loading coils or
 matching networks are used
                                                RF Propagation
                    Copyright © August 2002, John S. Seybold, All Rights Reserved                              21




Typical Characteristics for
Various Aperture Illuminations
  Type of distribution, |z| < 1         Relative gain     Half-power beamwidth   Intensity of first sidelobe
                                                          In degrees             dB below maximum intensity
  Uniform; A(z) = 1                            1                     51λ/d                     13.2

  Cosine; A(z) = cosn (πz/2)
         n=0                                   1                     51λ/d                   13.2
         n=1                                 0.810                   69λ/d                    23
         n=2                                 0.667                   83λ/d                    32
         n=3                                 0.575                   95λ/d                    40
         n=4                                 0.515                   111λ/d                   48

  Parabolic; A(z) = 1 -(1 - ∆)z2
         ∆ = 1.0                               1                      51λ/d                  13.2
         ∆ = 0.8                             0.994                    53λ/d                  15.8
         ∆ = 0.5                             0.970                    56λ/d                  17.1
         ∆=0                                 0.833                    66λ/d                  20.6

  Triangular; A(z) = 1 - |z|                  0.75                    73λ/d                  26.4

  Circular; A(z) = (1-z2)0.5                 0.865                   58.5λ/d                 17.6

  Cosine-squared plus pedestal;
  0.33 + 0.66 cos2(πz/2)                       0.88                    63λ/d                 25.7
  0.08 + 0.92 cos2(πz/2), Hamming              0.74                   76.5λ/d                42.8
  d = aperture width (diameter)
  λ = wavelength
  Source: Introduction to Radar Systems, Merrill I. Skolnik, McGraw-Hill 1980



                                                RF Propagation
                    Copyright © August 2002, John S. Seybold, All Rights Reserved                              22
Antenna Polarization
    Electromagnetic waves can be characterized by their
    polarization
    Elliptical polarization is a generalized polarization that
    encompasses linear and circular polarization
    Linear polarization means the the electric field
    portion of the electromagnetic wave exists in a plane
    (normal to the direction of propagation)- usually
    vertical or horizontal
    Circular polarization can be mathematically
    represented as the sum of a vertical and a
    horizontally polarized wave, 90 degrees out of phase

                                                 RF Propagation
                     Copyright © August 2002, John S. Seybold, All Rights Reserved           23




Circular Polarization
      y Ey
                                                           The axial ratio is an important
                                                           antenna parameter that describes
                                                           the shape of the polarization ellipse.
                                                            The axial ratio is defined as the
                            x Ex
                                                           ratio of the major axis to minor axis
                                                           of the polarization ellipse when the
                                                           phase angle between the linear
z                y                                         polarization components, δ, is +90°.
            E2                                             By Definition, the axial ratio is
       Ey
                                                           always > 1 (0 dB)
                           Tilt
                                                           Since it is a ratio of amplitudes (not
                           Angle   τ
                                                    x
                                                           powers), it can be expressed in dB
                                            E1             using 20log(AR).
                                       Ex



                                                 RF Propagation
                     Copyright © August 2002, John S. Seybold, All Rights Reserved           24
Polarization Loss
                   Linear antennas that are misaligned in orientation
                   Circular antennas that are not truly circular
                   Loss actually occurs between the wave and the
                   receiving antenna, when circular polarization is non-
                   ideal
                   For linear polarization, the polarization loss factor
                   (power) is F = cos2(τ)
                   For nearly cirular polarization, the polarization loss
                   factor is
                                          F=
                                                (1 + AR )(1 + AR ) + 4 AR AR + (1 − AR )(1 − AR )cos(2[τ
                                                             2            2                                2          2
                                                                                                                                − τ r ])
                                                                       2(1 + AR )(1 + AR )
                                                         w            r           w   r                w          r         w
                                                                                              2            2
                                                                                          w            r




                                                                    RF Propagation
                                  Copyright © August 2002, John S. Seybold, All Rights Reserved                                             25




Polarization Loss
                                                                                                  Incident Wave
                                                                                                  Axial Ratio
                   1.3
                                 Polarization Loss Factor vs. Axial Ratio
                                                                                                                          Polarization loss
                                                                                                   5 dB
                   1.2                                                                                                    factor versus axial
                   1.1                                                                                                    ratio for several
                    1
                                                                                                    4 dB
                                                                                                                          different axial ratios
                   0.9                                                                                                    (one for wave, the
                   0.8
                                                                                                   3 dB                   other for Rx
Loss Factor (dB)




                   0.7                                                                                                    antenna)
                                                                                                    2 dB
                   0.6

                   0.5
                                                                                                   1 dB
                   0.4
                                                                                                   0 dB
                   0.3

                   0.2

                   0.1

                     0
                         0   1               2              3                 4               5
                                         Antenna Axial Ratio (dB)


                                                                    RF Propagation
                                  Copyright © August 2002, John S. Seybold, All Rights Reserved                                             26
Pointing Loss
     When directional antennas are used,
     the link analysis should allow for some
     pointing loss (or tracking loss for
     dynamic systems)

                Antennas Aligned




               Antennas Misaligned

                                     RF Propagation
              Copyright © August 2002, John S. Seybold, All Rights Reserved   27




Some Antenna References
1.    M. I. Skolnik, Introduction to Radar Systems, 3rd Ed., McGraw-Hill, New York,
      2001
2.    C. A. Balanis, Antenna Theory, Analysis and Design, 2nd Ed., Wiley, New
      York, 1997
3.    K. Siwiak, Radiowave Propagation and Antennas for Personal
      Communications, 2nd ed., Artech House, Norwood, 1998
4.    W. L. Stutzman, G. A. Thiele, Antenna Theory and Design, 2nd Ed., Wiley,
      Hoboken, 1998
5.    J. D. Kraus, R. J. Marhefka, Antennas for All Applications, 3rd Ed., McGraw-
      Hill, 2002
6.    J.Liberti, Jr., T. S. Rappaport, Smart Antennas for Wireless Communications:
      IS-95 and Third Generation CDMA Applications, Prentice-Hall, Upper Saddle
      River, 1999
7.    J. S. Hollis, T. J. Lyon, L. Clayton, Microwave Antenna Measurements, 2nd
      Ed., Scientific Atlanta, Atlanta, 1970, Chapter 3
8.    ARRL Antenna Handbook
9.    R. C. Johnson, Antenna Engineering Handbook,McGraw-Hill, 1992
                                     RF Propagation
              Copyright © August 2002, John S. Seybold, All Rights Reserved   28
Summary
 Antenna gain is the gain over an isotropic radiator in the direction of maximum
 intensity – usually expressed in dBi
 Antenna gain for an aperture antenna can be estimated from
                                       4π Ae
                                 G=
                                        λ2

 The antenna pattern is not well formed until the far-field region is reached
                                       2D 2
                                  r>
                                        λ
 Beam width is generally the angular distance between the –3 dB points on the
 antenna pattern main lobe

 Antenna reception and radiation patterns are identical
 Polarization mismatch between Tx and Rx antennas reduces the received signal
 level


                                 RF Propagation
            Copyright © August 2002, John S. Seybold, All Rights Reserved       29

				
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Description: The antenna is the "air interface" of communication system