© emss antennas, tdp awg, lax, 6-7/5/2009 the kat-7 feed package 1

© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009

The KAT-7 feed package

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EMSS ?

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• A group of companies, operating from Stellenbosch, SA

• Employee-owned, 100 bodies, T/O ~ $M6• CEM code FEKO: www.feko.info• EMSS Antennas (8 bodies)

– Contracted into MeerKAT project, cost-plus-material basis, shared risk

– Self-representative on subsystem level– Develop and implement feed technology, act as

custodians of EM issues


The KAT-7 reflector: Revisited

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• Design freeze late 2007– D = 12m (project decision, based on

SKA trends)– F/D = 0.38 (Calculated to suit typical

WBFs, also allowing a classicalhorn feed as fall-back)

– Prime focus: Four struts, (+),~2/3 out, cross-section shaped tominimise spillover

– Surface to be flame-sprayedAluminium or mesh, 1.5mm RMS

– Drives: Elevation over azimuth


The KAT-7 SPF

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The KAT-7 horn: Design freeze early 2008

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• Design briefs– 1200 – 1950MHz (48%)– Orthogonal linear polarisations– Optimise Ae / (Tspillover + X)

• Results– Choked wide-angle horn, Ø = 1m (oversized)– Irises (for impedance matching af flower)

– Theoretical TE11 mismatch: ≤ –33dB

– Theoretical Tspillover: 5 2K

– Theoretical illumination efficiency: 70 65%


1200 1400 1600 1800 2000-40

-35

-30

-25

-20

-15

-10

-5

0

Fre que ncy [MHz]

|S1

1|

[dB

]

1200 1400 1600 1800 20000

1

2

3

4

5

6

Fre que ncy [MHz]

Ts

pil

l [K

]

1200 1400 1600 1800 200042

43

44

45

46

Fre que ncy [MHz]G

0 [

dB

]

1200 1400 1600 1800 200060

65

70

75

Eff

icie

ncy

[%

]

The KAT-7 horn: Theoretical results

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Theoretical Ae / Tsys (no blockage)

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1200 1300 1400 1500 1600 1700 1800 1900 20002

2.2

2.4

2.6

2.8

3

Fre que ncy [MHz]

Ae/T

sy

s [m

2/K

]


The KAT-7 prototype horn

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The KAT-7 OMT

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• Design briefs (early 2008)– Smaller than typical bladed OMTs (to facilitatte

small cryostat)– High-fidelity, no hybrid devices

• Chosen candidate– Two crossed dipoles in shorted waveguide– Impedance-transforming feeder lines– Two Marchand baluns– Design method: Aggressive space mapping


The KAT-7 OMT size

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• Size does matter…– Final device quite small– 0.2λ3

– Thus small cryostat areal possibility

– Theoretical performancevery promising

– (Typical quadridgestructures = 3 – 6 λ3)


The KAT-7 OMT

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The Kat-7 horn: Pattern measurements

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• Pattern verification– 500m Ground-reflection

far-field range– Principal planes and

diagonal cuts made– Fed by prototype single-

mode transducer– Found excllent

agreement with CEM predictions


The Kat-7 horn: Pattern measurements

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The KAT-7 horn + OMT: Port properties

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The KAT-7 horn + OMT: Port properties

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• Loss?– Crude method: Short waveguide, divide

insertion loss by two– Looks like 0.1+ dB– I.e., ~8K at ambient,

and ~2K at 77Kphysical

– Since verified by twoOMTs back to back,transmission –0.2dB,confirming 0.1dB loss


The KAT-7 cryostat

• Design briefs– Small vessel, robust– Stirling-cycle cryo-cooler

• Sunpower GT unit: 15W at 77K (neck at 30°C)• Cold fluid up from pedestal, for cooling the cooler

– No mechanical vacuum maintenance• Ion pump, carbon traps, limited cryo-pumping at 77K

– Host to LNAs and noise injection circuitry– Life support from pedestal below

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The KAT-7 cryostat

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Conclusions

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• Good horn performance from prime focus

• Very good performance from small OMT

• Getting there with a small robust cryostat

• Full development will be captured in a project memo, towards end of 2009

• Thank you.

© emss antennas, tdp awg, lax, 6-7/5/2009 the kat-7 feed package 1

Documents

emss antennas

tdp awg

azimuth slide

db loss slide

horn omt

prototype horn

aggressive space mapping

classical horn feed