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

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14

Theory of Operation

Circularly Polarized Electromagnetic Waves

• LCP (Astronomy Definition)

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• RCP (Astronomy Definition)

y

x

z

y

x

z

y

x

z

y

x

z

y

x

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y

x

z

y

x

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y

x

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y

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y

x

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y

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y

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y

x

z

y

x

z

y

x

z

y

x

z

y

x

z

y

x

z

y

x

LCP signal

Theory of Operation

y

x

y

x

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

y

x

Direction of

Propagation

y

x

Direction of

Propagation

LCP signal RCP signal

Theory of Operation: OMT

Port 1Port 2 Port 1Port 2

y y' x

x'

(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

y y' x

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

sio

n M

ag.

(dB

)

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

Ref

lect

ion

Mag

. (d

B)

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

sio

n M

ag. (

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

Re

fle

ctio

n M

ag. (

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

Iso

lati

on

Mag

. (d

B)

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

sio

n M

ag. (

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

Re

fle

ctio

n M

ag. (

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

Iso

lati

on

Mag

. (d

B)

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

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Machined Phase Shifter Measured Performance

50

50

60

70

80

90

100

110

8 9 10 11 12

Ph

ase

(De

gre

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

sio

n M

ag. (

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

-70

-60

-50

-40

-30

-20

-10

0

8 8.2 8.4 8.6 8.8 9

Tran

smis

sio

n M

ag. (

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

55

50

60

70

80

90

100

110

8 9 10 11 12

Ph

ase

(De

gre

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

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

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