Wave Ant

Waveguide AntennasRavindra.S.Kashyap (06307923)Sarvagya.P.Dwivedi (06307909)EE609: Radiating SystemsTerm Paper Presentation

EE609: Radiating Systems

OutlineBasics of Slot RadiatorsWaveguide-Fed SlotsOpen Waveguide AntennasIntroduction to Waveguide Slot ArraysDesign of Waveguide Slot ArraysCircularly Polarized Waveguide Slot ArraysApplications of Waveguide AntennasSummary


Basics of Slot RadiatorsSlots are cuts in a metallic sheetThey are dual structures of half-wave dipolesAnalysis 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

DipoleSlot


Basics of Slot RadiatorsDue to this duality, slots are referred to as magnetic dipolesFar 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 polarizationHorizontal Slot gives Vertical Polarization Vertical Slot gives Horizontal Polarization


Slot RadiatorsFar Field Components of Slot Radiation


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

Slot Radiators


Waveguide-Fed SlotsAny slot will radiate only if it cuts the surface currentsA longitudinal slot placed on the waveguide axis would not radiateA slot in the transverse wall perpendicular to axis would also not radiate

Surface currents for dominant mode excitationRadiating & Non-radiating waveguide slots


Waveguide-Fed SlotsSlot excitation and hence radiation increases whenDisplacement of longitudinal slot is higherAngular displacement of the broad wall slot is higherInclination of transverse-wall slot is higher


Waveguide-Fed SlotsA longitudinal-shunt slot would not radiate if a short circuit is at from the slot centreA longitudinal-series slot would not radiate if a short circuit is at from the slot centerLong slots should be fed at the center to prevent excitation of higher order modes


Waveguide-Fed SlotsPolarization MechanismSlots placed across the waveguide axis will have opposite polarizationA pair of slots in space quadrature & displaced from the axis gives circular polarizationSense of polarization can be reversed by placing the pair in its image positionSource :www.antenna.ee.titech.ac.jp/research/animation-e.htmlSlot for linear polarizationCrossed slot for circular polarization


Open Waveguide AntennasAn open end of rectangular or circular waveguide can be used as a radiatorWaveguides are usually excited with dominant modes due to its favorable radiation patternIt is a rudimentary form of horn antennas without any flaringThey do not give high directivity and are usually used to feed reflector antennasOwing to the missing flare, impedance matching with the free space is not good and hence causes reflections


Open Waveguide AntennasRadiation pattern of three rectangular waveguidesRadiation pattern of three circular waveguides


Open Waveguide AntennasH-plane pattern is smoother than E-plane pattern owing to the larger aperture in that planeLarger waveguides are needed to get high directivities and gainGain is less due to considerable back radiationPrecautions has to be taken to kill higher order modes


Open Waveguide Antennas...Gain EnhancementsSchematic representationWaveguide antenna with HIGPRadiation pattern


Open Waveguide Antennas...Gain EnhancementsRadiation patterns for various spacing and dielectric permittivityAntenna geometry


Open Waveguide Antennas...Array ConfigurationsTwo circular waveguide antenna arrayFour circular waveguide antenna array


Waveguide Slot ArraysResonant arrays consists of slots spaced apart and alternate slots placed on the opposite sides of the axisWaveguide is terminated with a short circuit at from the last slotThey are broadside arraysThese arrays are inherently narrow bandArrays should be designed for lower side lobe level (SLL) than required to cater for manufacturing tolerancesResonant broadside array


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

Waveguide Slot Arrays


Design of Waveguide Slot ArraysArrays comprises different types of slots, already mentioned, arranged in a patternThe lengths & positions of slots are adjusted to get the required amplitude and phase distributionThis specific distribution will yield the required radiation patternThe combined admittance of all the slots should also result in perfect impedance match at the waveguide input


Design of Waveguide Slot ArraysDesign ObjectivesCenter frequency of operationAngle of maximum radiationDirectivity and SLLOutline 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


Design of Waveguide Slot ArraysOutline of design procedureAn initial value of slot displacement and lengths are assumedCurve-fit polynomials are generated for experimentally generated graphsBased 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


Waveguide Slot ArraysObservationsChoice of slot width is sufficient for bandwidth controlProviding the exact displacements are not necessary, the values can be rounded offMutual coupling is more in case of planar slot array than linear arrayLinear arrayPlanar array


Waveguide Slot ArraysIntuitive AnimationsSource :www.antenna.ee.titech.ac.jp/research/animation-e.htmlMagnetic field strength inside the waveguideFarfield Radiation : E-fieldFarfield Radiation : H-field


Circularly Polarized Waveguide Slot ArraysThe array makes use of crossed slots as individual elementsCrossed slots displaced from each other is more popular due to ease in numerical computation

Displaced crossed slot for CPIterative array design procedure


CP Waveguide Arrays

ParameterValueFrequency7.5GHzDielectric fillingTeflonWaveguide WR90AR

Wave Ant

Documents

Transcript of Wave Ant