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Offset Quadruple-Ridge Orthomode Transducer, Mode Splitter/CombinerX-Band OMT Design Review October 1, 2009
Gordon Coutts
2
Introduction
Low-Band EVLA Circular Polarizers
3
• Circular to Square Transition
• Quadruple-Ridge OMT (separates orthogonal linearly polarized signals)
• Quadrature Hybrid
• Phase-Matched cables connecting the OMT to the hybridQuadrature
Hybrid
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
6
Proposed X-Band OMT Design
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
14
Theory of Operation
Circularly Polarized Electromagnetic Waves• LCP (Astronomy Definition)
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• RCP (Astronomy Definition)
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LCP signal
Theory of Operation
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Apparent motion of electric field vector of circularly polarized electromagnetic waves as viewed from the receiver (astronomy definition).
LCP signal RCP signal
Theory of Operation: Phase Shifter
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Direction ofPropagation
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Direction ofPropagation
LCP signal RCP signal
Theory of Operation: OMT
Port 1Port 2 Port 1Port 2
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(mode 1) (mode 2)
Theory of Operation: OMT
Port 2 Port 1
LCP signal output
Port 2 Port 1
RCP signal output
LCP signal RCP signal
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x'
HFSS Simulated OMT Performance
• HFSS simulated modal transmission S-parameter magnitude from OMT input to the coaxial OMT output ports
39
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
8 8.5 9 9.5 10 10.5 11 11.5 12
Tran
smis
sion
Mag
. (d
B)
Freq. (GHz)
S{2,TE10'}
S{2,TE01'}
S{1,TE10'}
S{1,TE01'}
HFSS Simulated OMT Performance
• HFSS simulated reflection OMT S-parameters
40
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
8 8.5 9 9.5 10 10.5 11 11.5 12
Refle
ction
Mag
. (dB
)
Freq. (GHz)
S11
S22
41
Measured X-Band OMT Performance
Measured OMT Performance
42
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
8 9 10 11 12
Tran
smis
sion
Mag
. (dB
)
Freq. (GHz)
X-Band OMT Transmission - First Prototype
Mode2 - FPMode2 - BPMode1 - FPMode1 - BP
Measured OMT Performance
43
-35
-30
-25
-20
-15
-10
-5
0
8 9 10 11 12
Refle
ction
Mag
. (dB
)
Freq. (GHz)
X-Band OMT Reflection - First Prototype
Mode2
Mode1
Specification
Measured OMT Performance
44
-70
-60
-50
-40
-30
-20
-10
0
8 9 10 11 12
Isol
ation
Mag
. (dB
)
Freq. (GHz)
X-Band OMT Isolation - First Prototype
Measured OMT Performance
45
-5
-4.5
-4
-3.5
-3
-2.5
-2
-1.5
-1
-0.5
0
8 9 10 11 12
Tran
smis
sion
Mag
. (dB
)
Freq. (GHz)
X-Band OMT Transmission - Second Prototype
Mode2 - FPMode2 - BPMode1 - FPMode1 - BP
Measured OMT Performance
46
-25
-20
-15
-10
-5
0
8 9 10 11 12
Refle
ction
Mag
. (dB
)
Freq. (GHz)
X-Band OMT Reflection - Second Prototype
Mode2
Mode1
Specification
Measured OMT Performance
47
-60
-50
-40
-30
-20
-10
0
8 9 10 11 12
Isol
ation
Mag
. (dB
)
Freq. (GHz)
X-Band OMT Isolation - Second Prototype
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
50
50
60
70
80
90
100
110
8 9 10 11 12
Phas
e (D
egre
es)
Freq. (GHz)
Machined Phase Shifter #1: Measured Relative Phase Shift
Relative Phase Shift
Ideal Phase Difference
Measured Axial Ratio Performance
51
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
8 9 10 11 12
Axi
al R
atio
(dB)
Freq. (GHz)
Axial Ratio - OMT Proto #2 - Machined Phase Shifter #1
FP Out (Meas.)
BP Out (Meas.)
FP Out (Calc. From Meas. Data)
BP Out (Calc. From Meas. Data)
Circular Polarization Performance
52
-7
-6
-5
-4
-3
-2
-1
0
8 8.2 8.4 8.6 8.8 9
Tran
smis
sion
Mag
. (dB
)
Freq. (GHz)
Circular Co-Polarization Response: Septum Polarizer Input, Quad-Ridge OMT Output (P.S.#1, OMT#2)
Meas Co-Pol (NB OMT L - WB Proto FP Out)
Meas Co-Pol (NB OMT R - WB Proto BP Out)
Co-Pol (CP IN - WB Proto BP Out) - Calc from Meas. Data
Co-Pol (CP IN - WB Proto FP Out) - Calc from Meas. Data
Circular Polarization Performance
53
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
8 8.2 8.4 8.6 8.8 9
Tran
smis
sion
Mag
. (dB
)
Freq. (GHz)
Circular Co-Polarization Response: Septum Polarizer Input, Quad-Ridge OMT Output (P.S.#1, OMT#2)
Meas X-Pol (NB OMT R - WB Proto FP Out)
Meas X-Pol (NB OMT L - WB Proto BP Out)
Calc X-Pol (CP IN - WB Proto BP Out)
Calc X-Pol (CP IN - WB Proto FP Out)
Measured Circular Polarization Performance using Scaled Ku-
Band Phase Shifter Experimental Data
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Scaled Phase Shifter Performance
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50
60
70
80
90
100
110
8 9 10 11 12
Phas
e (D
egre
es)
Freq. (GHz)
Measured Ku-Band Phase Shifter Response Scaled to X-Band
Relative Phase Shift
Ideal Phase Difference
Measured Axial Ratio Performance using Scaled Ku-Band Phase Shifter Data
56
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
8 9 10 11 12
Axi
al R
atio
(dB)
Freq. (GHz)
Axial Ratio - OMT Proto #2 - Scaled Ku Band Phase Shifter
FP Out (Calc. From Meas. Data)
BP Out (Calc. From Meas. Data)
Circular Polarization Performance
57
-2
-1.8
-1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
8 9 10 11 12
Tran
smis
ssio
n M
ag. (
dB)
Freq. (GHz)
Circular Co-Polarization Response: Omt Proto. 2 with Scaled Ku-Band Phase Shifter
Co-Pol Transmission Calc. from Measured Data - FP Out
Co-Pol Transmission Calc. from Measured Data - BP Out
Circular Polarization Performance
58
-70
-60
-50
-40
-30
-20
-10
0
8 9 10 11 12
Tran
smis
ssio
n M
ag. (
dB)
Freq. (GHz)
Circular Cross-Polarization Response: Omt Proto. 2 with Scaled Ku-Band Phase Shifter
X-Pol Transmission Calc. from Measured Data - FP Out
X-Pol Transmission Calc. from Measured Data - BP Out
Circular Polarization Performance using Measured OMT Data and
Ideal Phase Shifter
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OMT Contribution to Axial Ratio
60
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
8 9 10 11 12
Axi
al R
atio
(dB)
Freq. (GHz)
Axial Ratio - OMT Proto #2 - Ideal Phase Shifter
FP Out (Calc. From Meas. Data)
BP Out (Calc. From Meas. Data)
OMT CP Insertion Loss
61
-1
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
8 9 10 11 12
Tran
smis
ssio
n M
ag. (
dB)
Freq. (GHz)
Circular Co-Polarization Response: OMT Proto. 2 with Ideal Phase Shifter (OMT Insertion Loss)
Co-Pol Transmission Calc. from Measured Data - FP Out
Co-Pol Transmission Calc. from Measured Data - BP Out
OMT CP Isolation
62
-70
-60
-50
-40
-30
-20
-10
0
8 9 10 11 12
Tran
smis
ssio
n M
ag. (
dB)
Freq. (GHz)
Circular Cross-Polarization Response: Omt Proto. 2 with Ideal Phase Shifter (OMT CP Isolation)
X-Pol Transmission Calc. from Measured Data - FP Out
X-Pol Transmission Calc. from Measured Data - BP Out
Conclusions• A novel 45 degree offset quadruple-
ridge OMT design is proposed for the new EVLA wideband X-Band receivers
• Two prototypes have been fabricated and tested, and exceed specifications by a wide margin
• The compact design is amenable to cooling with a Model 22 refrigerator
• Measured results show that the novel design exhibits good axial ratio and circular polarization performance
• As with the other EVLA quadruple-ridge OMT designs, the new X-Band design is focused on excellent performance, ease of tuning and manufacturability
• The OMT electromagnetic design is ready for production
63