Departement Elektriese en Elektroniese Ingenieurswese

Department of Electrical and Electronic Engineering

Development of a Wideband Ortho-Mode

Transducer

Dirk de Villiers

University of Stellenbosch

03 December 2008

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Agenda

• Introduction: OMT overview

• Trapped modes in QRWG OMT’s

• Offset ridges to increase modal separation

• Design method

• Simulated results

• Construction

• Measured results

• Conclusions

Introduction: OMT overview

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Introduction: OMT overview

• An ortho-mode transducer (OMT) is a polarisation filter which separates orthogonal polarisations within the same frequency band

• Typically a three port device as shown:

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OMT1

2

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Introduction: OMT overview

• Classic Bøifot junction type

• 1990 – Bøifot, Lier and Schaug-Pettersen

• 1.5:1 Bandwidth

• Well suited to X-band and higher due to bulky size

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Wollack 1996: K-band design

Introduction: OMT overview

• Balanced probe fed types• Often more compact than waveguide types• Bandwidth 1.5:1• Must be very carefully constructed to ensure a balanced feed

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Engargiola and Navarrini 2005: K-band design with scaling possible to high mm-wave band

Grimes et al. 2007: Planar C-band design

Introduction: OMT overview

• Turnstile junction waveguide type• Popular at higher frequencies at or above the K-band• Commercially available up to 94 GHz• Again, about 1.5:1 bandwidth due to the singe mode waveguide

limitation

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Navarrini and Plambeck 2006: K-band design with scaling possible to high mm-wave band

Introduction: OMT overview

• Finlines and ridged waveguides have wider single mode operation

• Several finline OMT’s have been reported showing bandwidths of up to 2.4:1

• Cross coupling levels are high due to the asymmetry in the structure

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Skinner and James 1991: L-band design

Introduction: OMT overview

• Quad Ridged Waveguide (QRWG) types are the most popular for lower frequency (L-band to C-band) applications

• 2.4:1 bandwidth possible

• Good isolation between ports

• They have a problem with trapped modes though…

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Skinner and James 1991: L-band design

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Trapped modes in QRWG OMT’s

fc = 0.509 GHz

fc = 0.516 GHz

fc = 0.912 GHz

fc = 0.958 GHz

fc = 1.949 GHz

fc = 3.233 GHz

Offset ridges to increase modal separation

• This problem does not occur in DRWG due to loss of the axial symmetry• Use rectangular waveguide (Similar properties to circular but easier to

construct)

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fc = 0.999 GHz

fc = 1.412 GHz

fc = 0.600 GHz

fc = 1.035 GHz

fc = 0.300 GHz

fc = 0.999 GHz

Offset ridges to increase modal separation

• Still need both polarisations• Don’t use QRWG with close modal separation in the feed area –

asymmetry in feed excites the higher order mode• Move one of the ridges in the axial direction to achieve an

approximate DRWG structure in the feed area…

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Offset ridges to increase modal separation

• Still need both polarisations• Don’t use QRWG with close modal separation in the feed area –

asymmetry in feed excites the higher order mode• Move one of the ridges in the axial direction to achieve an

approximate DRWG structure in the feed area…

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Offset ridges to increase modal separation

• Still need both polarisations• Don’t use QRWG with close modal separation in the feed area –

asymmetry in feed excites the higher order mode• Move one of the ridges in the axial direction to achieve an

approximate DRWG structure in the feed area…

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Offset ridges to increase modal separation

• Still need both polarisations• Don’t use QRWG with close modal separation in the feed area –

asymmetry in feed excites the higher order mode• Move one of the ridges in the axial direction to achieve an

approximate DRWG structure in the feed area…

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Offset ridges to increase modal separation

• Check effect of the orthogonal ridges on the DRWG modal cutoffs• How far down the taper can the ridge be moved to still ensure single mode

operation?

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g

a

First higher order mode

Design method

• Use offset ridges so that the structure can be approximated as a DRWG structure

• Approximate formulas are available for the characteristic impedance of DRWG – Hoefer and Burton 1982 using a VI relationship

• This greatly simplifies the design of the feed and the impedance taper

• Design can be split into three parts:1. Feed2. Impedance tapers3. Full structure integration

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Design method: Feed

• Operating band: 1.2 – 2 GHz (1.66:1 Bandwidth)• Design ridges to give 50 Ω impedance to be matched to the coax feed• Place feed as close as possible to the open circuit at the back for maximum

bandwidth• Not very sensitive to shape and size of cavity – play around a bit to find a

suitable form• CST simulation model and results are shown below

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Design method: Tapers

• Use a Klopfenstein taper• Length of taper most important factor in obtainable bandwidth• Results shown for a 400 mm taper in a 150 mm DRWG

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Design method: Full structure integration

• Simulate the full structure with orthogonal ridge inserted• Some higher order resonances are excited within the operating band• These can be suppressed by

1. reducing the waveguide width in the unsymmetrical part of the structure (modes with E-field zeros at the waveguide walls)

a. When waveguide width is reduced the fundamental cutoff must be kept the same as that of the full size waveguide – use ridged waveguide

b. Tapered QRWG is needed to keep the fundamental cutoff constant as the width is increased

2. inserting a mode suppression plate behind the second ridge (modes with E-field maxima at the waveguide walls)

a. Shorten the cavity to increase resonant frequency to outside the band of interest

b. Thin plate in the centre of the guide does not interfere with the orthogonal mode

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Design method: Full structure integration

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Width = 140 mmSpace = 106 mmfc = 1.002 GHz

Width = 150 mmfc = 0.999 GHz

Design method: Full structure integration

• Simulate the full structure with orthogonal ridge inserted• Some higher order resonances are excited within the operating band• These can be suppressed by

1. reducing the waveguide width in the unsymmetrical part of the structure (modes with E-field zeros at the waveguide walls)

a. When waveguide width is reduced the fundamental cutoff must be kept the same as that of the full size waveguide – use ridged waveguide

b. Tapered QRWG is needed to keep the fundamental cutoff constant as the width is increased

2. inserting a mode suppression plate behind the second ridge (modes with E-field maxima at the waveguide walls)

a. Shorten the cavity to increase resonant frequency to outside the band of interest

b. Thin plate in the centre of the guide does not interfere with the orthogonal mode

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Design method: Full structure integration

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Simulated results

• To keep the prototype costs down a S-band scale model was designed to operate from 2.4 to 4 GHz

• Reflection is below -20 dB across the entire operating band for both ports

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Construction

• The structure was constructed with aluminium• Outer waveguide was CNC milled out of 20 mm slabs• 6 mm plate was used for the ridges• Construct the ridges as single pieces • LASER and waterjet cutting was tried – both were somewhat

inaccurate• Rather mill ridges as separate pieces in the future• Press fit centre conductor of feedline into ridges – worked well

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Construction

• The structure was constructed with aluminium• Outer waveguide was CNC milled out of 20 mm slabs• 6 mm plate was used for the ridges• Construct the ridges as single pieces • LASER and waterjet cutting was tried – both were somewhat

inaccurate• Rather mill ridges as separate pieces in the future• Press fit centre conductor of feedline into ridges – worked well

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Construction

• The structure was constructed with aluminium• Outer waveguide was CNC milled out of 20 mm slabs• 6 mm plate was used for the ridges• Construct the ridges as single pieces • LASER and waterjet cutting was tried – both were somewhat

inaccurate• Rather mill ridges as separate pieces in the future• Press fit centre conductor of feedline into ridges – worked well

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Measured results

• Prototype was measured in an anechoic chamber with a square pyramidal horn attached

• Horn is 5 wavelengths long at the bottom of the operating band and has a 3 wavelength aperture – this gives reflection below -30 dB from the horn

• Full OMT with horn:

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Measured results

• Prototype was measured in an anechoic chamber with a square pyramidal horn attached

• Horn is 5 wavelengths long at the bottom of the operating band and has a 3 wavelength aperture – this gives reflection below -30 dB from the horn

• Measured results:

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Conclusions

• A QRWG OMT with offset orthogonal ridge was designed, manufactured and measured

• Performance is similar to traditional QRWG OMT’s

• No trapped modes

• No need for optimisation in the design

• Construction and assembly is straight forward

• Only problem with construction is the LASER/waterjet cutting of the ridges – machine them in future

• Article accepted in the Electronics Letters: “Broadband offset quad-ridged waveguide orthomode transducer”

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Acknowledgements

• CST in Darmstadt, Germany for the use of software licenses

• Wessel Crouwkamp and Lincoln Saunders at the University of Stellenbosch for help with the construction of the prototype

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