Transcript Feed Systems for the KAT Radio Astronomy Telescopes

The KAT-7 feed package
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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EMSS ?
• 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-plusmaterial basis, shared risk
– Self-representative on subsystem level
– Develop and implement feed technology, act
as custodians of EM issues
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 reflector: Revisited
• Design freeze late 2007
– D = 12m (project decision, based on
SKA trends)
– F/D = 0.38 (Calculated to suit typical
WBFs, also allowing a classical
horn feed as fall-back)
– Prime focus: Four struts, (+),
~2/3 out, cross-section shaped to
minimise spillover
– Surface to be flame-sprayed
Aluminium or mesh, 1.5mm RMS
– Drives: Elevation over azimuth
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 SPF
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 horn: Design freeze early 2008
• 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%
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 horn: Theoretical results
0
6
46
75
-5
5
-10
45
4
70
Efficiency [%]
44
0
3
G [dB]
[K]
spill
-20
T
11
|S | [dB]
-15
-25
2
-30
65
43
1
-35
-40
1200 1400 1600 1800 2000
Frequency [MHz]
0
1200 1400 1600 1800 2000
Frequency [MHz]
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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60
1200 1400 1600 1800 2000
Frequency [MHz]
6
Theoretical Ae / Tsys (no blockage)
3
2.6
2.4
e
A /T
sys
[m 2 /K]
2.8
2.2
2
1200
1300
1400
1500
1600
1700
Frequency [MHz]
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
1800
1900
2000
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The KAT-7 prototype horn
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 OMT
• 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
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 OMT size
• Size does matter…
– Final device quite small
– 0.2λ3
– Thus small cryostat a
real possibility
– Theoretical performance
very promising
– (Typical quadridge
structures = 3 – 6 λ3)
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 OMT
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The Kat-7 horn: Pattern measurements
• Pattern verification
– 500m Ground-reflection
far-field range
– Principal planes and
diagonal cuts made
– Fed by prototype singlemode transducer
– Found excllent
agreement with CEM
predictions
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The Kat-7 horn: Pattern measurements
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 horn + OMT: Port properties
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 horn + OMT: Port properties
• 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 two
OMTs back to back,
transmission –0.2dB,
confirming 0.1dB loss
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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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
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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The KAT-7 cryostat
© EMSS Antennas, TDP AWG, LAX, 6-7/5/2009
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Conclusions
• 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
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