Offset Quadruple-Ridge Orthomode Transducer, Mode Splitter/Combiner X-Band OMT Design Review October 1, 2009 Gordon Coutts
Introduction 2
Low-Band EVLA Circular Polarizers • Circular to Square Transition • Quadruple-Ridge OMT (separates orthogonal linearly polarized signals) • Quadrature Hybrid • Phase-Matched cables connecting the OMT to Quadrature Hybrid the hybrid 3
High-Band EVLA Circular Polarizers • Circular to Square Transition • Sri’s corrugated waveguide Phase Shifter • 45 Degree offset mode splitter • Bøifot OMT (separates orthogonal linearly polarized signals)
X-Band Design Challenges • Two options using conventional technology from existing EVLA receivers: – Cascaded Bøifot OMT/ mode splitter/ phase shifter • This would scale to an impractically large size at X-band – Direct scaling of the C-Band Polarizer to work at X-Band • This would result in very small dimensions (20 mil chamfer, 30mil ridge gap) • Manufacturing tolerances would be a significant percentage (of the order of 10%) of the scaled dimensions • Narrow ridge dimensions would not readily accommodate set screws/coaxial feeds • Phase matching to an external hybrid would be extremely difficult due to the required cable length adjustments (1.9mil/degree at 12GHz) 5
Proposed X-Band OMT Design 6
Novel X-Band OMT Design • The new X-Band OMT uses a 45 degree offset quadruple- ridge design • The novel polarizer design combines concepts from low- band and high band circular polarizer designs • The OMT combines the function of the ‘45 degree twist’ mode splitter and Bøifot OMT used in the high frequency designs 7
Novel X-Band OMT Design • Ridges are offset from the square waveguide input by 45 degrees • Square Waveguide Input: 0.947” x 0.947” • Detects circularly polarized signals when used in conjunction with Sri’s waveguide phase shifter • No external quadrature hybrid or phased matched cables in this design 8
High-Band EVLA Circular Polarizers • Circular to Square Transition • Sri’s corrugated waveguide Phase Shifter • 45 Degree offset mode splitter • Bøifot OMT (separates orthogonal linearly polarized signals)
Proposed EVLA X-Band Circular Polarizer • Circular to Square Transition • Sri’s corrugated waveguide Phase Shifter • 45 Degree offset quadruple-ridge OMT
Compact Design of X-Band OMT • Compact design: OMT Length is 6.12” 11
X-Band OMT Dimensions • Chamfer profile similar to C-band OMT for manufacturability • 125 mil Ridge Width • 62 mil Ridge Gap • 40 mil Chamfer flat section • Locator block sets ridge gap and maintains symmetry 12
X-Band OMT Dimensions • The quadruple-ridge waveguide dimensions: – optimum impedance at low- band edge – Eliminate higher order modes • 0.047” semi -rigid coaxial feeds • 62.5mil spaced shorting pins for impedance matching and TE11 trapped-mode resonance suppression • One 2-56 set screw for each sorting pin, with set screws for adjacent pins on opposing ridges 13
Theory of Operation 14
Circularly Polarized Electromagnetic Waves • LCP (Astronomy Definition) • RCP (Astronomy Definition) 15
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y x z y x LCP signal
Theory of Operation Apparent motion of electric field vector of circularly polarized electromagnetic waves as viewed from the receiver (astronomy definition). y y x x LCP signal RCP signal
Theory of Operation: Phase Shifter Direction of Direction of Propagation Propagation y y x x LCP signal RCP signal
Theory of Operation: OMT Port 2 Port 1 Port 2 Port 1 y' y x x' (mode 1) (mode 2)
Theory of Operation: OMT Port 2 Port 1 Port 2 Port 1 LCP RCP signal signal output output y' y x x' LCP signal RCP signal
HFSS Simulated OMT Performance 0 -0.5 -1 -1.5 Transmission Mag. (dB) -2 S{2,TE10'} -2.5 S{2,TE01'} -3 S{1,TE10'} S{1,TE01'} -3.5 -4 -4.5 -5 8 8.5 9 9.5 10 10.5 11 11.5 12 Freq. (GHz) • HFSS simulated modal transmission S-parameter magnitude from OMT input to the coaxial OMT output ports 39
HFSS Simulated OMT Performance 0 -5 -10 -15 Reflection Mag. (dB) -20 -25 S11 -30 S22 -35 -40 -45 -50 8 8.5 9 9.5 10 10.5 11 11.5 12 Freq. (GHz) • HFSS simulated reflection OMT S-parameters 40
Measured X-Band OMT Performance 41
Measured OMT Performance X-Band OMT Transmission - First Prototype 0 -0.5 -1 Transmission Mag. (dB) Mode2 - FP -1.5 Mode2 - BP Mode1 - FP -2 Mode1 - BP -2.5 -3 -3.5 -4 -4.5 -5 8 9 10 11 12 Freq. (GHz) 42
Measured OMT Performance X-Band OMT Reflection - First Prototype 0 -5 Mode2 Mode1 -10 Specification Reflection Mag. (dB) -15 -20 -25 -30 -35 8 9 10 11 12 Freq. (GHz) 43
Measured OMT Performance X-Band OMT Isolation - First Prototype 0 -10 -20 Isolation Mag. (dB) -30 -40 -50 -60 -70 8 9 10 11 12 Freq. (GHz) 44
Measured OMT Performance X-Band OMT Transmission - Second Prototype 0 -0.5 -1 Transmission Mag. (dB) Mode2 - FP -1.5 Mode2 - BP Mode1 - FP -2 Mode1 - BP -2.5 -3 -3.5 -4 -4.5 -5 8 9 10 11 12 Freq. (GHz) 45
Measured OMT Performance X-Band OMT Reflection - Second Prototype 0 -5 Reflection Mag. (dB) Mode2 Mode1 -10 Specification -15 -20 -25 8 9 10 11 12 Freq. (GHz) 46
Measured OMT Performance X-Band OMT Isolation - Second Prototype 0 -10 Isolation Mag. (dB) -20 -30 -40 -50 -60 8 9 10 11 12 Freq. (GHz) 47
Measured Circular Polarization Performance using Machined Prototype Phase Shifters 48
Machined Phase Shifters • Prototype X-Band phase shifters were fabricated in-house • Used to evaluate circular polarization performance of the new X-Band OMT • The X-Band OMT was connected to the phase shifter and measured using the PNA 49
Machined Phase Shifter Measured Performance Machined Phase Shifter #1: Measured Relative Phase Shift 110 Relative Phase Shift 100 Ideal Phase Difference Phase (Degrees) 90 80 70 60 50 8 9 10 11 12 Freq. (GHz) 50
Measured Axial Ratio Performance Axial Ratio - OMT Proto #2 - Machined Phase Shifter #1 5 4.5 4 FP Out (Meas.) 3.5 BP Out (Meas.) Axial Ratio (dB) 3 FP Out (Calc. From Meas. Data) BP Out (Calc. From Meas. Data) 2.5 2 1.5 1 0.5 0 8 9 10 11 12 Freq. (GHz) 51
Circular Polarization Performance Circular Co-Polarization Response: Septum Polarizer Input, Quad-Ridge OMT Output (P.S.#1, OMT#2) 0 -1 Transmission Mag. (dB) -2 -3 -4 Meas Co-Pol (NB OMT L - WB Proto FP Out) -5 Meas Co-Pol (NB OMT R - WB Proto BP Out) Co-Pol (CP IN - WB Proto BP Out) - Calc from Meas. Data -6 Co-Pol (CP IN - WB Proto FP Out) - Calc from Meas. Data -7 8 8.2 8.4 8.6 8.8 9 Freq. (GHz) 52
Circular Polarization Performance Circular Co-Polarization Response: Septum Polarizer Input, Quad-Ridge OMT Output (P.S.#1, OMT#2) 0 -10 -20 Transmission Mag. (dB) -30 -40 -50 Meas X-Pol (NB OMT R - WB Proto FP Out) -60 Meas X-Pol (NB OMT L - WB Proto BP Out) Calc X-Pol (CP IN - WB Proto BP Out) -70 Calc X-Pol (CP IN - WB Proto FP Out) -80 -90 8 8.2 8.4 8.6 8.8 9 Freq. (GHz) 53
Measured Circular Polarization Performance using Scaled Ku-Band Phase Shifter Experimental Data 54
Scaled Phase Shifter Performance Measured Ku-Band Phase Shifter Response Scaled to X-Band 110 Relative Phase Shift 100 Ideal Phase Difference Phase (Degrees) 90 80 70 60 50 8 9 10 11 12 Freq. (GHz) 55
Measured Axial Ratio Performance using Scaled Ku-Band Phase Shifter Data Axial Ratio - OMT Proto #2 - Scaled Ku Band Phase Shifter 5 4.5 4 3.5 Axial Ratio (dB) FP Out (Calc. From Meas. Data) 3 BP Out (Calc. From Meas. Data) 2.5 2 1.5 1 0.5 0 8 9 10 11 12 Freq. (GHz) 56
Circular Polarization Performance Circular Co-Polarization Response: Omt Proto. 2 with Scaled Ku-Band Phase Shifter 0 -0.2 -0.4 Transmisssion Mag. (dB) -0.6 -0.8 -1 -1.2 -1.4 Co-Pol Transmission Calc. from Measured Data - FP Out -1.6 Co-Pol Transmission Calc. from Measured Data - BP Out -1.8 -2 8 9 10 11 12 Freq. (GHz) 57
Circular Polarization Performance Circular Cross-Polarization Response: Omt Proto. 2 with Scaled Ku-Band Phase Shifter 0 X-Pol Transmission Calc. from Measured Data - FP Out -10 X-Pol Transmission Calc. from Measured Data - BP Out Transmisssion Mag. (dB) -20 -30 -40 -50 -60 -70 8 9 10 11 12 Freq. (GHz) 58
Circular Polarization Performance using Measured OMT Data and Ideal Phase Shifter 59
OMT Contribution to Axial Ratio Axial Ratio - OMT Proto #2 - Ideal Phase Shifter 1 0.9 0.8 0.7 Axial Ratio (dB) FP Out (Calc. From Meas. Data) 0.6 BP Out (Calc. From Meas. Data) 0.5 0.4 0.3 0.2 0.1 0 8 9 10 11 12 Freq. (GHz) 60
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