Planar 4 x 9 array synthesis example excluding the reflective ground plane.

We are pleased to announce the new release of Antenna Magus Version 4.4. This release sees the addition of 5 new antennas:

• Horn-fedtruncatedreflectorantenna• Shunt-fed slanted V-dipole pair• OffsetPattern-fedCassegrainreflectorantenna• Monopole dielectric resonator antenna• Bifilarhelixantenna

Thearraysynthesistoolhasalsobeenextendedtoincludetheeffectsofareflectivegroundplanewhencalculatingtheradiationpatternofa synthesized array.

July 2013Newsletter 4.4

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Antenna Magus version 4.4 released!

Array synthesis reflective ground plane addition

Anewadditiontothearraysynthesistoolistheoptionofincludingtheeffectofareflectivegroundintheradiationpatterncalculation.Thisfeatureisespeciallyusefulwheremeasuredorsimulatedsingle-elementpatterndataisavailable,whiletheelementswillbeoperatingoveralargegroundplaneorclosetoalargeconductingstructureinthearrayenvironment.Thepatterndatacanbeimportedandpositionedacertaindistanceawayfromareflectivegroundplaneandtheresultantradiationpatternsynthesisedwithinafewseconds.Belowisanexampleofa4x9planardipolearrayshowingthelayoutandsynthesisedradiationpatternsincludingandexcludingareflectivegroundplane.

Planar 4 x 9 array synthesis example including a reflective ground plane at z = - 0.25 λ.

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Truncated reflector

Truncatedversionsoftheparabolicreflectormaybetermed‘shaped-beam parabolic antennas’ and have the advantage that they maybedesignedforspecific(anddifferent)EandHbeamwidths.Thisisusefulinsystemssuchasmechanicallyscannedsearchradars,airportsurveillanceradars,airtrafficcontrolradarsandmilitaryheightfinderradars - where fan beam radiation patterns are required.

Inordertorealiseafanbeam,thereflectoristruncated(eithercut-offhorizontally,orusinganellipticalintersection)andfedbyanon-axissectoralhornantenna.Theantennaisusuallyfocusedexperimentallybyfindingtheoptimalaxialposition(andtransversepositioniftherearealignmenterrors)whichminimizesthenullbetweenthemainlobeand first side lobe. This is required, as the reflector’s focus is notlocatedatasingulargeometricalpoint,butshiftswithfrequency.

Antenna Magus offers various design options for this antenna:beamwidth (both E and H plane), peak gain, or gain with heightrestriction may be chosen. The following two images compare three different H-plane 3dB beamwidth designs (2°, 3.5° and 5° respectively) with a constant E-plane beamwidth of 6°, while thethird image shows the typical 3D radiation pattern of the antenna designedforapeakgainof30dBi.

Zoomed pattern cuts for three 3 dB H-plane beamwidth designs: (a) 2°, (b) 3.5° and (c) 5°.

Typical 3D radiation pattern at the centre frequency.

Comparing three 3 dB H-plane beamwidth designs: (a) 2°, (b) 3.5° and (c) 5°.

Shunt-fed Slanted V-Dipole pair

The Shunt-fed Slanted V-dipole pair implemented in Antenna Magus consistsoftwoV-dipolesseparated0.25λ,supportedbyahorizontalmast.Thisconfigurationprovidesaverymechanicallyrobust,simple,low cost circularly polarised antenna that can be used in high-power transmitapplicationsandharshenvironmentalconditions.Although

each dipole is linearly polarised, an omnidirectional circularlypolarised pattern is obtained by adjusting the slant angles and diameter of the dipoles. These antennas are typically used for FM and TVbroadcastingwherecircularpolarisation is required,andcanbeusedinlineararraystoachieveanarrow-beamdoughnutradiationpattern.

Typical LHC and RHC gain patterns at the centre frequency.

New antennas in Version 4.4

LHC RHC

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Feed antenna pattern properties accounted for by Antenna Magus when designing the Pattern-fed Offset Cassegrain re-flector.

Normalised radiation pattern of the Pattern-fed Offset Cassegrain dual reflector.

3D radiation pattern of the Pattern-fed Offset Cassegrain.

Offset Pattern-fed Cassegrain reflector antenna

Summary of the dual reflector antennas in Antenna Magus.

The Pattern-fed Offset Cassegrain reflectoristhe8thdual-reflectorincluded inAntenna Magus and the 4th Cassegrain-type reflectorantenna template.

Dual-reflectors are compact and offer a lot of design flexibilitythoughtheyaremorecomplextodesignandmanufacturethansinglereflectorantennas. Sub reflector shapingcanbeused to increasethe focus-depth or to optimise illumination for an existing feedantennaandmainreflector.Byusingadualreflectorwithanoffsetfeed,apertureblockagecanbedecreasedandmountingonaflatorrotatingplatformissimplified.Therearehoweversomefactorslikespillover,radiationpatternasymmetry,feed/subandmain-reflectoralignment and othermanufacturing complexities that have to beconsideredwhenchoosinganoffset-feddualreflectortopology.

When compared with the Horn-fed Offset Cassegrain,thepattern-fedoptionreducessimulationtimeandcomplexityandmakesprovisionfordesignsbasedonfeedpropertiessuchasfeedbeamwidth,edgetaper and feed distribution efficiency as illustrated on the right.Properties of any existing feed antenna can be approximatedusingthepattern-feedapproach,orthedesiredradiationpatternproperties of an ideal feed antenna can be determined based on the reflectordesign.Thoughaphysicalfeedantennaisnotincludedinthepattern-feddesign,approximateantennadimensionsareusedtoensurethatminimalblockageoccurs.

Side lobe level

First Null beamwidth/2

0° 90°ϴ

Edge taper

1 2 Number of sidelobes

Aperture DistributionFeed Distribution

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Monopole dielectric resonator antenna

The Hybrid monopole dielectric resonator antenna is an attractiveoption for designers who want a simple, compact antenna thatcan achieve wider bandwidths. This antenna uses an annular ringdielectricresonator(DRA)withareportedbandwidthofupto3:1.Thewidebandwidthperformance isactuallyachievedby3distinctresonances(f1,f2andf3)withassociatedomnidirectionalpatterns.These resonances can be associated with the physical dimensions of theantenna:theheightofthemonopole(f1),combinedeffectofthemonopoleandDRA(f2)andtheDRAbyitself(f3).Themiddleresonance(f2)isachievedbytheDRAeffectivelyloadingthemonopolesothatitachievesthecurrentdistributionofaslightlyshortermonopole. Thefollowingfiguresshowtheradiationperformanceandreflectioncoefficientina50Ωsystemofadesignfor2:1bandwidthusingaDRAwith εr=20.The lastfigurecomparesnearfieldcutsatdifferentfrequencies.Notehowthepositionsofthepeakradiationchangewithfrequency.

Total gain pattern at fmin,1.5 fmin and 2 fmin on a ground plane with a diameter of 1.3λ fmin.

Bifilar helix

The Bifilar helix is constructed using two volutes with an equalnumberofturns,andtheirstartingpointspositioned180°apart.Theendsofthevolutesareconnectedwithashortingwirewhichaddstothe structural integrity of the antenna.

Bifilar helix antennas are often constructed using thickmetal rodsor pipes, making them mechanically robust and able to withstandstrongwindsandharshenvironmentalconditions.Theseconstructionsmake them ideal for shoreline installations.Theseantennascanbepurchased off-the-shelf for many marine communication frequency bands and are well suited for rapid installation.

The 180° phase shift between the two volutes allows for circularpolarizationwithanend-firebeaminthedirectionofthehelixaxis.Theradiationpatternisstablewithawell-definedend-firelobeandlowsideandbacklobesacrosstheoperationband.Atypicalradiationpattern is shown in the following image.

Typical reflection coefficient versus frequency for a 2:1 design.

Near field cuts around the antenna at low (f1), center (1.5x f1) and up-per (2x f1) frequencies.

Image of the Bifilar helix antenna.

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Theinputimpedanceexhibitsoscillatorybehaviouratthehigh-frequencyendofthebandastheinputresistanceincreasesfrom200Ωto300Ωasillustratedinthefigureontheright.The-10dBS11bandwidthina200Ωsystemisroughly1.85:1.

Typical circularly polarised 3D gain pattern at the centre frequency.

(Bifilar helix continued...)

Typical circularly polarised radiation pattern cut.

Typical impedance vs. normalised frequency.