Waveguide Antennas

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Waveguide Antennas
EE609 Radiating Systems Term Paper Presentation

• Ravindra.S.Kashyap (06307923)
• Sarvagya.P.Dwivedi (06307909)

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Outline

• Basics of Slot Radiators
• Waveguide-Fed Slots
• Open Waveguide Antennas
• Introduction to Waveguide Slot Arrays
• Design of Waveguide Slot Arrays
• Circularly Polarized Waveguide Slot Arrays
• Applications of Waveguide Antennas
• Summary

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Basics of Slot Radiators

• Slots are cuts in a metallic sheet
• They are dual structures of half-wave dipoles
• Analysis of slot makes use of a fictious
  magnetic sheet
• The form of currents for a dipole and a slot
  are same

Boundary Conditions to be Satisfied
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Basics of Slot Radiators

• Due to this duality, slots are referred to as
  magnetic dipoles
• Far field components of the slots are thus found
  out by the duality principle
• Radiation pattern is exactly same as that of a
  dipole except for the reversed polarization
• Horizontal Slot gives Vertical Polarization
• Vertical Slot gives Horizontal Polarization

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Slot Radiators
Far Field Components of Slot Radiation
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Slot Radiators

• Bookers Relation
• Our speculation is that the design equation for
  dipole can be used for single slots also
• The terminal impedance at the slot center would
  be however high even though purely real

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Waveguide-Fed Slots

• Any slot will radiate only if it cuts the surface
  currents
• A longitudinal slot placed on the waveguide axis
  would not radiate
• A slot in the transverse wall perpendicular to
  axis would also not radiate

Surface currents for dominant mode excitation
Radiating Non-radiating waveguide slots
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Waveguide-Fed Slots

• Slot excitation and hence radiation increases
  when
• Displacement of longitudinal slot is higher
• Angular displacement of the broad wall slot is
  higher
• Inclination of transverse-wall slot is higher

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Waveguide-Fed Slots

• A longitudinal-shunt slot would not radiate if a
  short circuit is at from the slot centre
• A longitudinal-series slot would not radiate if a
  short circuit is at from the slot center
• Long slots should be fed at the center
  to prevent excitation of higher order modes

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Waveguide-Fed SlotsPolarization Mechanism

• Slots placed across the waveguide axis will have
  opposite polarization
• A pair of slots in space quadrature displaced
  from the axis gives circular polarization
• Sense of polarization can be reversed by placing
  the pair in its image position

Slot for linear polarization
Source www.antenna.ee.titech.ac.jp/research/anima
tion-e.html
Crossed slot for circular polarization
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Open Waveguide Antennas

• An open end of rectangular or circular waveguide
  can be used as a radiator
• Waveguides are usually excited with dominant
  modes due to its favorable radiation pattern
• It is a rudimentary form of horn antennas without
  any flaring
• They do not give high directivity and are usually
  used to feed reflector antennas
• Owing to the missing flare, impedance matching
  with the free space is not good and hence causes
  reflections

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Open Waveguide Antennas
Radiation pattern of three rectangular waveguides
Radiation pattern of three circular waveguides
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Open Waveguide Antennas

• H-plane pattern is smoother than E-plane pattern
  owing to the larger aperture in that plane
• Larger waveguides are needed to get high
  directivities and gain
• Gain is less due to considerable back radiation
• Precautions has to be taken to kill higher order
  modes

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Open Waveguide Antennas...Gain Enhancements
Waveguide antenna with HIGP
Radiation pattern
Schematic representation
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Open Waveguide Antennas...Gain Enhancements
Antenna geometry
Radiation patterns for various spacing and
dielectric permittivity
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Open Waveguide Antennas...Array Configurations
Two circular waveguide antenna array
Four circular waveguide antenna array
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Waveguide Slot Arrays

• Resonant arrays consists of slots spaced
  apart and alternate slots placed on the opposite
  sides of the axis
• Waveguide is terminated with a short circuit at
  from the last slot
• They are broadside arrays
• These arrays are inherently narrow band
• Arrays should be designed for lower side lobe
  level (SLL) than required to cater for
  manufacturing tolerances

Resonant broadside array
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Waveguide Slot Arrays

• Non resonant arrays consists of slots that are
  not placed apart and the waveguide is
  terminated in a matched load
• Maximum radiation can not be in broadside but
  limited to a small angle from the axis

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Design of Waveguide Slot Arrays

• Arrays comprises different types of slots,
  already mentioned, arranged in a pattern
• The lengths positions of slots are adjusted to
  get the required amplitude and phase distribution
• This specific distribution will yield the
  required radiation pattern
• The combined admittance of all the slots should
  also result in perfect impedance match at the
  waveguide input

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Design of Waveguide Slot Arrays

• Design Objectives
• Center frequency of operation
• Angle of maximum radiation
• Directivity and SLL
• Outline of design procedure 1
• Choose an approximating polynomial based on the
  required directivity and SLL(Dolph-Chebyshev,
  Binomial, Taylor, Modified Taylor, Bayliss)
• Get the normalized array coefficients for slot
  excitations

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Design of Waveguide Slot Arrays

• Outline of design procedure
• An initial value of slot displacement and lengths
  are assumed
• Curve-fit polynomials are generated for
  experimentally generated graphs
• Based on these polynomials, initial values of
  slot displacement and lengths are checked for
  conformity with the required array excitation
  (taking mutual impedances into account)
• The sum of all normalized admittances is checked
  for sum being unity
• Design proceeds iteratively by altering the slot
  parameters simultaneously satisfying the array
  requirements and sum being unity

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Waveguide Slot ArraysObservations

• Choice of slot width is sufficient for bandwidth
  control
• Providing the exact displacements are not
  necessary, the values can be rounded off
• Mutual coupling is more in case of planar slot
  array than linear array

Linear array
Planar array
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Waveguide Slot ArraysIntuitive Animations
Magnetic field strength inside the waveguide
Source www.antenna.ee.titech.ac.jp/research/anima
tion-e.html
Farfield Radiation E-field
Farfield Radiation H-field
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Circularly Polarized Waveguide Slot Arrays

• The array makes use of crossed slots as
  individual elements
• Crossed slots displaced from each other is more
  popular due to ease in numerical computation

Displaced crossed slot for CP
Iterative array design procedure
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CP Waveguide Arrays
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Applications of Waveguide Antennas
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WLAN (802.11b)

• Frequency of operation (USA Canada) 2.41
  2.46 GHz

8 elements
16 elements
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Slot Applicators for Microwave Heating

• Waveguide slot antennas are used as applicators
  for heating lossy dielectric
• Proposed structure is a resonant array with
  broadside radiation
• Empirical design method has been compared against
  analytical methods

Empirical Method
Analytical Method
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Substrate Integrated Waveguide Antennas

• A rectangular waveguide is created by packing a
  substrate in between two metal ground planes and
  fencing it with closely spaced vias
• A slot cut in such a waveguide can be used as a
  radiator called as cavity backed slot (CBS)

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Substrate Integrated Waveguide Antennas
Geometry of SIW Antenna
Reflection Coefficient
Radiation Pattern
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Radial Waveguide Slot Array

• Linearly polarized RWSA has been designed for
  Bluetooth
• RSSI performance is better than monopole and
  hence is a good alternative

Geometry of RWSA
Radiation pattern of RWSA
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Summary

• Waveguide antennas have good potential for its
  use in varied areas of application
• Waveguide antennas suffers from the same
  disadvantages faced by waveguides including their
  usage at low frequencies
• These antennas are favorable for high power
  applications
• They are mechanically robust
• They can be used as low-profile antennas for
  millimeter wave applications
• As far as the personal communication is
  concerned, waveguide antennas are a good
  alternative for wire antennas

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Selected References

• 1 Robert.S.Elliot, Antenna Theory Design,
  IEEE Press and Wiley Interscience, 2003

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Thank You