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Introduction to Antennas

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					                            IEEE EMC Symposium, 2010




Introduction to Antennas

        Presented by
   Vince Rodriguez, Ph.D.


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                                  IEEE EMC Symposium, 2010




                Outline I
• What is an antenna
• How do they radiate
• Radiation pattern
  – What is it
  – E and H plane
  – Far and near field
  – Omnidirectional/Directional

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                                   IEEE EMC Symposium, 2010




               Outline II
• Radiation pattern (cont’)
  – Isotropic
  – Main, side and back lobes.
  – Half power and 3dB beamwidth
  – Side lobe level
  – Directivity and gain



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                            IEEE EMC Symposium, 2010




              Outline III
•   Antenna Factor
•   Input impedance
•   S parameters and VSWR
•   Polarization




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                                IEEE EMC Symposium, 2010




What is an antenna?


                  “An antenna is a
                  device that
                  provides a means
                  for radiating and
                  receiving radio
                  waves”
Book definition




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                          IEEE EMC Symposium, 2010




    What is an antenna?



                              Radiated
                              energy
Guided
energy




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                                                    IEEE EMC Symposium, 2010




How do antennas radiate?




Electromagnetic energy does not like to go from the guided
medium to the free space medium



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                                          IEEE EMC Symposium, 2010




        How do antennas radiate?




Guided waves in the cable
                            Free Space waves


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                                                                IEEE EMC Symposium, 2010




       How do antennas radiate?
                     A good antenna will ease all the
                    guided energy that it receives in to
                            a free space wave




                                  ANTENNA


Guided waves in the cable                                  Free Space waves




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                     IEEE EMC Symposium, 2010




How do antennas radiate?




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How do antennas radiate?

              So, one way of accomplishing
              radiation is to smoothly
              change from the transmission
              line (cable) environment to an
                            environment.
              open space environment

              The antennas will receive in
              the same way that they
              radiate.




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                                                      IEEE EMC Symposium, 2010




How do antennas radiate?
As the currents in the antenna change direction the wave
propagates outward as is the case when we shake a rope




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                                IEEE EMC Symposium, 2010




   Radiation Pattern


                  “a 3D plot that
                  displays the
                  strength of the
                  radiated fields or
                  power density as a
                  function of
Book definition
                  direction”


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                                             IEEE EMC Symposium, 2010




   Radiation Pattern




The radiation is then a representation of how
much Electromagnetic energy is concentrated in
each direction around the antenna

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                                        IEEE EMC Symposium, 2010




Radiation Pattern
          Because of the difficulty of plotting
          a 3D plot usually the patterns are
          shown as E and H planes
                                  90
           0
                            120                60

          -10

                      150                              30
          -20


          -30
           30


          -40   180                                          0


          -30


          -20
                      210                              330

          -10

                            240                300
           0
                                         Eplane18GHz
                                  270    Hplane18GHz




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                                   IEEE EMC Symposium, 2010



Radiation Pattern: E and H
          Plane

              The E plane is the plane that
              is parallel to the Electric field
              The      l    is the l
              Th H plane i th plane th t  that
              is parallel to the Magnetic field

              The Electric and Magnetic
              fields are perpendicular to
              each other




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                                                                   IEEE EMC Symposium, 2010



Radiation Pattern: Near and
         Far Field




  To put it on simple terms, the near field has
  spherical waves and standing wave behaviors


                                                  The far field the sphere is large that
                                                   it resembles a plane and the wave
                                                                            is traveling


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                                            IEEE EMC Symposium, 2010



     Radiation Pattern:
Omnidirectional and Directional
            OMNI = Latin for Every or All




          So, Omnidirectional
          radiates in “every”
          direction?




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                                           IEEE EMC Symposium, 2010



     Radiation Pattern:
Omnidirectional and Directional
             No, Omnidirectional radiates in every
             direction on one of the principal planes




                                   A dipole antenna




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                                 IEEE EMC Symposium, 2010




     Radiation Pattern:
Omnidirectional and Directional
                        Omni
              directional on
              the H plane. It
            radiates equally
                        on all
               directions on
                   this plane


            But not on this
            plane




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                                       IEEE EMC Symposium, 2010



     Radiation Pattern:
Omnidirectional and Directional
           Can you radiate
           equally on all
           directions




           Isotropic, Greek, isos
           meaning same,
           tropos meaning
           direction,



                                    ΑΡΗΣΤΟΤΕΛΗΣ

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                                           IEEE EMC Symposium, 2010



     Radiation Pattern:
Omnidirectional and Directional
              There is no such thing in real life. An
                isotropic radiator is a mathematical
                concept, an idea that belongs in the
             world of ideas like my professor, Plato,
                                          would say.
                                          p
              It is however use as a comparison to
              determine the Directive Gain of a real
                                            antenna




                   ΑΡΗΣΤΟΤΕΛΗΣ

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                                                                          IEEE EMC Symposium, 2010



     Radiation Pattern:
Omnidirectional and Directional
Directional?, well that is plain English,
The antenna radiates mainly in one
direction. Lets look again at the
beautiful EMCO 3117                                                 90
                                             0
                                                              120                   60

                                            -10

                                                        150                                  30
                                            -20


                                            -30

                  Radiation is
                                            -40   180                                              0
                 mainly in this
                     direction              -30


                                            -20
                                                        210                                  330

                                            -10

                                                              240                   300
                                             0
                                                                              Eplane18GHz
                                                                    270       Hplane18GHz




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                                                 IEEE EMC Symposium, 2010



Radiation Pattern: Main, Side
          and Back                  90
              0
              Side            120         60
              Lobes:
            -10
              smaller
              than the
            -20
                    150                         30
              main lobe
            -30
                                                 Is a side
Main                                             lobe that
Lobe:       -40   180                            happens
                                                    0
Is the                                           to be on
strongest   -30                                  the
in level.                                        opposite
            -20                                  direction
                        210                     330
                                                 than the
            -10
                                                 main lobe
                              240         300
             0
                                    270


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                                                             IEEE EMC Symposium, 2010



     Radiation Pattern: Half Power
              Beamwidth
                                                       90
   Half power                   0
                                                 120            60
            1
   Half =     = 0.5            -10
            2
                                           150                               30
   In decibels                 -20


           ⎛1⎞
10 × log10 ⎜ ⎟ = −3.02 ≈ −3dB
                               30
                              -30

           ⎝2⎠                -40    180                                           0
   -3dB= half power
                               -30


                               -20
                                           210                               330

                               -10

                                                 240            300
                                0
                                                       270
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                                                                                                                            IEEE EMC Symposium, 2010



        Radiation Pattern: Half Power
                 Beamwidth
Let’s represent the pattern in Cartesian coordinates, for clarity



                                                                               C omput e d pat t er n 18 GH z 3 1 17



                                                 0
                         90
                                                -3
  0
                   120         60
                                                -6
 -10                                            -9

             150                     30        -12
 -20
                                               -15
 -30                                           -18

                                               -21
 -40   180                                 0
                                               -24
 -30                                           -27

                                               -30
 -20
             210                     330
                                               -33
 -10                                           -36
                   240         300             -39                                                     18G Hz 3117 H plane
  0
                         270                   -42
                                                  -180   -150   -120   -90   -60      -30          0          30       60     90   120   150    180
                                                                                        P h i (d e g r e e s)




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                                                                                                    IEEE EMC Symposium, 2010



    Radiation Pattern: Half Power
             Beamwidth                  C omput e d pa t t e r n 1 8 GH z 3 1 1 7




-3dB      0

Half     -3

         -6
power    -9
        -12
        -15
        -18

         21
        -21
        -24
        -27
        -30
        -33

        -36
        -39                                                       1        1
                                                                   8G Hz 31 7 H plane
        -42
           -180   -150   -120   -90   -60       -30           0           30        60   90   120      150     180
                                                  P h i (d e g r e e s)




                                                                    About 25 degrees
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                                                                                              IEEE EMC Symposium, 2010



Radiation Pattern: Sidelobe
           Level
                                                  Comput ed pa t t er n 1 8GHz 3 11 7



                    0

The power          -3

level at the       -6

                   -9
strongest         -12
side-lobe in      -15

relation to the   -18

                  -21
level of the      -24
main lobe         -27

                  -30
                  -33
In this case      -36
15dB              -39                                                    18GHz 3117 H plane
                  -42
                     -180   -150   -120   -90   -60      -30         0         30       60    90   120   150   180
                                                           P h i (d e g re e s)




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                                                                                              IEEE EMC Symposium, 2010



 Radiation Pattern: Front to
         Back Ratio
Similar to the                                    Comput ed pa t t er n 1 8GHz 3 11 7

side lobe
level, but it       0
                   -3
applies to         -6

those especial     -9

side lobes, the   -12
                  -15
back lobes
b k l b or        -18
to the            -21

radiation level   -24
                  -27
opposite the      -30
main lobe         -33

                  -36

                  -39                                                    18GHz 3117 H plane
                  -42
                     -180   -150   -120   -90   -60      -30         0         30       60     90    120   150        180
                                                           P h i (d e g re e s)




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                                               IEEE EMC Symposium, 2010



       Radiation Pattern: Front to
               Back Ratio                90
It may be         0
                                   120         60
easier to see
                 -10
in polar
coordinate                   150                           30
                 -20
representation
                 -30


                 -40   180                                       0


                 -30


                 -20
                             210                           330

                 -10

                                   240         300
                  0
                                         270


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                                         IEEE EMC Symposium, 2010



   Radiation Pattern: Directivity
            and Gain
General Gain definition




Power in                                Power out


                             Powerout
                      Gain =
                             Powerin
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                          IEEE EMC Symposium, 2010




           Gain(IEEE)


                        Power out=
                        Total
                        radiated
Power in
                        power

                  Pr
           Gain =
                  Pin

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                                        IEEE EMC Symposium, 2010



   Traditional Gain(or directive
        gain or directivity)
                                       WHAT??
                 Fmax
D=
   average over sphere of F (θ , ϕ )
   maximum radiated power
D=
      total radiated power
       surface of a sphere



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                                                                    IEEE EMC Symposium, 2010



         Traditional Gain(or directive
              gain or directivity)      90
            0
                            120                   60

          -10
                                                                   Oh! OK.
                      150                                30
          -20


          -30


          -40   180                                            0


          -30
Maximum
radiation -20         210                                330
density                           Total radiated power add the
          -10
                                  radiation on every direction
                            240
            0                     and divided it by300
                                                    the
                                         270
                                                       π
                                  spherical surface 4π radians
                                                                                   34
                                            IEEE EMC Symposium, 2010




Traditional Gain vs. IEEE Gain

   IEEE gain takes in to account the losses in the
   antenna since it looks at how much power is
   radiated versus how much power goes in.

   Traditional gain looks at how Directive the
   antenna is how much power we receive (or
   transmit) in one direction versus the total
   radiated power. It is a measure of how the
   antenna concentrates the radiation.



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                                        IEEE EMC Symposium, 2010




      Antenna Factor

                  Antenna factor – an
                  antenna sensor
                     lib ti      hi h
                  calibration which permitsit
                  measuring an unknown
                  electric field strength.
                  Antenna factors are
                  commonly expressed in
                  terms of dB (1/m).
Book definition




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                                            IEEE EMC Symposium, 2010




                       Antenna Factor




The receiver,
(spectrum analyzer,                     The antenna is
RF voltmeter, vector                    receiving an Electric
voltmeter, etc)                         field unbounded
measures signal                         wave that is in V/m,
level in volts, it                      (Volts per meter)
measures potential

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                                              IEEE EMC Symposium, 2010




                    Antenna Factor
•   The AF is the electric field at a given
    distance divided by the voltage
    measured at the antenna input. It is a
    value that relates the voltage at the
    antenna input to the field seen by the
    antenna



          r
          E


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                         IEEE EMC Symposium, 2010




Antenna Factor
           Recall that the Electric and
           magnetic field can be
           related to each other by
           the impedance of free
           space

           ηo = 120π Ω ≈ 377 Ω




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                                           IEEE EMC Symposium, 2010




                   Antenna Factor
•   Antenna factor – for magnetic
    antennas the magnetic antenna
    factor (MAF) is measured. It relates
    the magnetic field to the voltage at
    the antenna input.
•   Because we can relate the E and
    the H, we can relate the AF to the
    MAF
          r
          H


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                                   IEEE EMC Symposium, 2010




                Input Parameters



At the input port of the antenna
we can
Measured a set of parameters
that are related to
How good the antenna is.
These are measured without
looking at the radiated power

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                         IEEE EMC Symposium, 2010




      Input Parameters
1. S11 parameter

2. The VSWR

3. The input
   impedance




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                                                             IEEE EMC Symposium, 2010




                    VSWR and S11
         When entering a different medium the waves bounces back




        Pref
S11 =
        Piin



                                                            V peak
                                             VSWR =
                                                            Vvalley
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                                                                  IEEE EMC Symposium, 2010




                      VSWR and S11
                                VSWR and S11
       These two are related to the amount of energy that does not go in
              to the antenna but it is reflected back to the generator




100% of power in to antenna 0% of power in to antenna 64% of power in to antenna



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                                                           IEEE EMC Symposium, 2010




               VSWR and S11
                         VSWR and S11
These two are related to the amount of energy that does not go in
       to the antenna but it is reflected back to the generator




S11=<-50                    S11=0dB                       S11=-4.6
VSWR=1:1                    VSWR=>30                      VSWR=4:1


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                                                             IEEE EMC Symposium, 2010




               Input Impedance
The reason that the energy bounces back at the input to the
    antenna is that the impedance of the antenna is not the same
    as the one of the cable the slight mismatch causes part of the
    wave to bounce and part to travel




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                                                          IEEE EMC Symposium, 2010




                     Input Impedance
                                            Connector 50 ohm impedance
Input impedance is the
impedance that can be
measured at the input of the
antenna.

Since most equipment uses 50
ohm cables and 50 ohm
receivers we want our antenna
input impedance to be as close
as possible to 50 ohms

NOTE: the antenna connector
may be 50 ohms, but what is
behind the connector is what
matters when talking about the
input impedance

                                 ridges ??ohm impedance
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                            IEEE EMC Symposium, 2010




Polarization


        When a propagating wave
           ill            i l l
        oscillates on a single plane
        then it is called a linear
        polarized wave




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                            IEEE EMC Symposium, 2010




Polarization


        A linearly polarized wave will
        radiate linearly polarized
        waves




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                            IEEE EMC Symposium, 2010




Polarization
        An antenna that can generate
        two linearly polarized waves
        simultaneously at the same
        time is called a dual polarized
        antenna




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                            IEEE EMC Symposium, 2010




Polarization
        When a linearly polarized antenna
        is set in a horizontal position it
        produces a horizontal polarized
        wave




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                                    IEEE EMC Symposium, 2010



                 Polarization
When a linearly polarized antenna
is set in a vertical position it
produces a vertical polarized
wave




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                                     IEEE EMC Symposium, 2010



                   Polarization
A circular or elliptical polarized
wave the plane of oscillation
rotates as the wave propagates




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                                                   IEEE EMC Symposium, 2010




          Polarization

                                   An antenna that radiates a
                                   circular polarized wave is
                                   called a circularly polarized
                                   antenna



This is one of our conical log spiral
antennas. They can be
manufactured to be left or right
hand circularly polarized.



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                  IEEE EMC Symposium, 2010




Polarization


Left Hand Circular Polarization




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               IEEE EMC Symposium, 2010




Polarization

 Right hand circular
 polarization




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                                      IEEE EMC Symposium, 2010




Polarization
Transmits and receives 100% of linear
polarization that is co-polarized
Receives ½ of circular polarized waves


Transmits and receives 100% of linear
polarization that is co-polarized
Receives ½ of circular polarized waves


Transmits and receives 100% of linear
polarization and 100% of co-polarized
circular polarized wave
Receives 0% of cross polarized circular
polarized waves

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             IEEE EMC Symposium, 2010




Questions?




                            58

				
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