Offset Quadruple-Ridge Orthomode Transducer, Mode Splitter/CombinerX-Band OMT Design Review October 1, 2009

Gordon Coutts

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Introduction

Low-Band EVLA Circular Polarizers

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• 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)

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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

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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

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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”

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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

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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

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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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HFSS Simulated OMT Performance

• HFSS simulated modal transmission S-parameter magnitude from OMT input to the coaxial OMT output ports

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-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

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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

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-50

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-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

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Measured X-Band OMT Performance

Measured OMT Performance

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-5

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

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Tran

smis

sion

Mag

. (dB

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Freq. (GHz)

X-Band OMT Transmission - First Prototype

Mode2 - FPMode2 - BPMode1 - FPMode1 - BP

Measured OMT Performance

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-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

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-70

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8 9 10 11 12

Isol

ation

Mag

. (dB

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Freq. (GHz)

X-Band OMT Isolation - First Prototype

Measured OMT Performance

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-5

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-2.5

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-1.5

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-0.5

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Tran

smis

sion

Mag

. (dB

)

Freq. (GHz)

X-Band OMT Transmission - Second Prototype

Mode2 - FPMode2 - BPMode1 - FPMode1 - BP

Measured OMT Performance

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-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

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-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

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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

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Machined Phase Shifter Measured Performance

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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

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2.5

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3.5

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4.5

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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

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-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

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-90

-80

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-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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100

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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

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0

0.5

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2.5

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3.5

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4.5

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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

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-2

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

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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

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-70

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-40

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-20

-10

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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

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0

0.1

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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

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-1

-0.9

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-0.7

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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

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-70

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8 9 10 11 12

Tran

smis

ssio

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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

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