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Antennas

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... to convert electrical energy traveling along a transmission line into ... A receiving antenna will have a current induced in its elements(conductors) from ... – PowerPoint PPT presentation

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Title: Antennas


1
Antennas
  • A transmitting antenna is used as a mechanism to
    convert electrical energy traveling along a
    transmission line into electromagnetic waves and
    launch them into space.
  • A receiving antenna will have a current induced
    in its elements(conductors) from the electric and
    magnetic fields in space.

2
Half Wave Dipole
  • The ideal isotropioc radiator will radiate
    equally well in all directions, however is
    theoretical in nature and could not be built.
  • The fundamental practical antenna is the half
    wave dipole.
  • Dipole implies two parts or elements however does
    not have to be ½ wavelength long however it is
    more suitable for impedance matching.

0.95 accounts for end capacitance
3
Visualizing the action of the Half Wave Dipole
  • Consider a parallel wire transmision line
    terminated with an open circuit.
  • There will be a voltage maximum at the open end
    and ¼ wavelength from the end a current maximum
    exists with a high SWR.
  • Now visualize the two conductors separating as
    they pivot out from a point ¼ wavelength from the
    end.
  • The result is the stretching away of the electric
    field from the wires until some of the field
    detaches and develops electromagnetic waves.

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Radiation Resistance
  • Recall that an open circuited lossless line
    presents itself as a short circuit ¼ wavelength
    from the open end.
  • Points on either side of this location will
    appear reactive.
  • In terms of FEEDPOINT impedance there will never
    be a non-zero resistive component because an open
    circuited line can not DISSISPATE power.
  • However power for the most part is RADIATED from
    the antenna and causes the input impedance to
    have a resistive component. (1/2 wave dipole is
    about 70 ohms)

7
Efficiency
  • The portion of the antennas power delivered into
    space is called Radiation Resistance.
  • There is a portion consumed by Ohmic losses in
    the conductors (elements) and therefore cause the
    antenna to have an EFFICIENCY of less than one.

8
Radiation Patterns
  • Antenna geometry and resulting field pattern will
    be referenced to the 3 dimensional coordinate
    system.
  • The x-y axis is horizontal and angles referenced
    by phi while the z axis is vertical and
    referenced to the horizontal in terms of angle by
    theta toward the zenith.
  • Zenith represents the angle of elevation.
  • Azimuth would be the angular displacement about
    the horizontal plane.

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13
Polar Plots
  • Antenna radiation patterns can be presented as
    x-y presentations or in polar format.
  • Polar plots are scaled either as linear or
    logarithmic.
  • Because it is two dimensional two plots are
    required.
  • One for the vertical plane (elevation to zenith)
    and the other in the horizontal plane (azimuth)
    to illustrate the antenna E and H field patterns.

14
Dipole Polarization
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16
dBi/dBd
  • The gain of a lossless dipole in the direction of
    maximum radiation is 2.14 dBi with respect to an
    isotropic radiator.
  • The gain of any antenna in dBd is 2.14 dB less
    than when it is expressed in dBi.

17
EIRP and ERP
  • EIRP is the product of the actual power going
    into the antenna and its gain with respect to an
    isotropic radiator.
  • ERP(Effective Radiated Power) is the product of
    the applied power to the antenna and its gain
    with respect to a dipole.

18
Field Regions
  • Reactive Near Field
  • Near Field or Fresnel Field
  • Far Field or Fraunhofer Field
  • Rayleigh Distance

19
Reactive Near Field
  • Region close to the antenna where mutual coupling
    exists with a reactive field dominating over a
    radiating field.
  • In this region mutual impedances will be observed
    with test antennas and field strength meters.
  • Due to reactive nature it will tend to store
    energy rather than propagate it.
  • Field strength decreases at a complicated rate.
  • Distance from source defined by

20
Reactive and Radiating Field Regions
21
Near Field Region
  • The RADIATING NEAR FIELD the radiation fields
    dominate.
  • Falls between a boundary that defines the
    Reactive near field and the Radiating Far Field.
  • The boundary between the Near Field and the Far
    Field is defined by the Rayleigh distance as

Where L is longest dimension of physical antenna
22
Radiating Far Field
  • Region beyond the Near field occupying the space
    from the boundary to infinity.
    .
  • Far field is dominated by the radiating field and
    the radiation pattern is independent of distance.
  • Power Density varies as the inverse square of the
    radial distance.

23
Gain and Directivity
  • Directivity is the calculated Gain of a lossless
    antenna.
  • Real antennas have losses. Gain is the product of
    directivity and antenna efficiency.

24
Impedance
  • Radiation resistance of a ½ wave dipole in free
    space is approximately 70 ohms.
  • Impedance is completely resistive at resonance,
    that is at the frequency that the physical
    elements are cut to.
  • If frequency is above resonance the feedpoint
    impedance will have an INDUCTIVE component.
  • If the frequency is below resonance then the
    feedpoint impedance will have a capacitive
    component.

25
Effects of Ground
  • Interactions between Antenna and GroundWaves
    radiate from the antenna directly downward and
    reflect vertically from the ground and induce a
    voltage in passing the antenna elements.
  • Magnitude and Phase of the resulting current
    depends on the height of the antenna above
    ground.
  • There are two current components the power of
    the transmitter with the free space impedance and
    the induced current from reflections of the
    ground surface.
  • Therefore feedpoint resistance is affected by
    antenna height above ground due to this MUTUAL
    COUPLING.

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Effects of Ground
  • Reactive Near Field antenna acts as
    lumped-constant component inductor or capacitor
    where energy is stored and a minimal amount
    radiated. Interaction with ground reflections
    develop mutual impedance altering feedpoint Z and
    therefore could increase losses.
  • Radiating Far Field In this region ground
    reflections influence the radiation pattern.
    HORIZONTALLY polarized antennas have their
    pattern SHAPE affected in the elevation plane
    depending on height above ground. VERTICALLY
    polarized antennas have SHAPE and STRENGTH
    affected.
  • Other than height above ground the dielectric
    constant and the conductivity of the ground
    composition can have a significant effect on the
    behavior of the antenna.
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