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Development of a Wideband Ortho-Mode Transducer
Dirk de Villiers
University of Stellenbosch
03 December 2008
Departement Elektriese en Elektroniese Ingenieurswese

Department of Electrical and Electronic Engineering
Agenda
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Introduction: OMT overview
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Trapped modes in QRWG OMT’s
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Offset ridges to increase modal separation
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Design method
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Simulated results
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Construction
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Measured results
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Conclusions
Introduction: OMT overview
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Introduction: OMT overview
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An ortho-mode transducer (OMT) is a polarisation filter which separates orthogonal
polarisations within the same frequency band
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Typically a three port device as shown:
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1
OMT
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Introduction: OMT overview
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Classic Bøifot junction type
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1990 – Bøifot, Lier and Schaug-Pettersen
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1.5:1 Bandwidth
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Well suited to X-band and higher due to
bulky size
Wollack 1996: K-band design
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Introduction: OMT overview
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Balanced probe fed types
Often more compact than waveguide types
Bandwidth 1.5:1
Must be very carefully constructed to ensure a balanced feed
Engargiola and Navarrini 2005: K-band design
with scaling possible to high mm-wave band
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Grimes et al. 2007: Planar C-band design
Introduction: OMT overview
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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
Navarrini and Plambeck 2006: K-band design with scaling possible to high mm-wave band
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Introduction: OMT overview
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Finlines and ridged waveguides have wider single mode operation
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Several finline OMT’s have been reported showing bandwidths of up to 2.4:1
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Cross coupling levels are high due to the asymmetry in the structure
Skinner and James 1991: L-band design
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Introduction: OMT overview
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Quad Ridged Waveguide (QRWG) types are the most popular for
lower frequency (L-band to C-band) applications
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2.4:1 bandwidth possible
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Good isolation between ports
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They have a problem with trapped modes though…
Skinner and James 1991: L-band design
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Trapped modes in QRWG OMT’s
fc = 1.949 GHz
fc = 0.912 GHz
fc = 0.509 GHz
fc = 3.233 GHz
fc = 0.958 GHz
fc = 0.516 GHz
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Offset ridges to increase modal separation
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•
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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)
fc = 0.999 GHz
fc = 0.600 GHz
fc = 0.300 GHz
fc = 1.412 GHz
fc = 1.035 GHz
fc = 0.999 GHz
Offset ridges to increase modal separation
•
•
•
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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…
Offset ridges to increase modal separation
•
•
•
13
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…
Offset ridges to increase modal separation
•
•
•
14
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…
Offset ridges to increase modal separation
•
•
•
15
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…
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?
First higher order mode
g
a
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Design method
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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.
2.
3.
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Feed
Impedance tapers
Full structure integration
Design method: Feed
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•
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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
Design method: Tapers
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•
•
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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
Design method: Full structure integration
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•
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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
Width = 140 mm
Space = 106 mm
fc = 1.002 GHz
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Width = 150 mm
fc = 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
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•
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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
Construction
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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
Construction
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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
Construction
•
•
•
•
•
•
•
27
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
Measured results
•
•
•
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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:
Measured results
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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:
Conclusions
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A QRWG OMT with offset orthogonal ridge was designed, manufactured and
measured
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Performance is similar to traditional QRWG OMT’s
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No trapped modes
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No need for optimisation in the design
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Construction and assembly is straight forward
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Only problem with construction is the LASER/waterjet cutting of the ridges – machine
them in future
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Article accepted in the Electronics Letters: “Broadband offset quad-ridged waveguide
orthomode transducer”
Acknowledgements
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CST in Darmstadt, Germany for the use of software licenses
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Wessel Crouwkamp and Lincoln Saunders at the University of Stellenbosch for help
with the construction of the prototype