RFI Characterisation of Karoo Array Telescope Structure
P. Gideon Wiid (PhD)
Howard C. Reader
Riana H. Geschke
Stellenbosch University
December 2008
Where do I fit in?
• RFI - Radio Frequency
Interference
• Extremely low RFI needed
• Earth-bound RFI, e.g. direct
and indirect lightning strikes
• Scale model measurements
• Computational modelling Fig 1: Lightning strike
What am I doing and how?
Correctly define and minimise RFI on
KAT structure:
• Lightning policies – RAO sites
• RFI measurements – XDM @
HartRAO
• CEM Modelling and Measurement –
KAT scale models
• MoM w.r.t. lightning, shielding
• Result analysis for RFI minimisation Fig 2: ADM design (June 2008)
What have I done? 1
• Site Visits to:
– Astron, LOFAR &
Westerbork in Netherlands
– Jodrell Bank & Manchester
University in UK
– Shielding of enclosures
• Measurements at
HartRAO on XDM
– Importance of loops Fig 3: Westerbork and HartRAO Observatories
What have I done? 2
• 2nd Model - 1/40th scale as per XDM
• Focus: structure, conduit and soil
• CADFEKO and CST verified by
measurement
• Memory and license limitations
• Eskom load shedding
• CHPC runtimes 4 days, 1 day
• Towards ADM for KAT-7
• Specific concerns re RFI on ADM Fig 4: XDM model with saltwater
What have I done? 3
• 3rd 1/20th Scale model – Willem
Esterhuyse
• June 2008 ADM KAT-7, simplified
• Measurement – anechoic chamber
• Geometry replicated in CADFEKO
• CHPC runtimes / unknowns
• CADFEKO verified by measurement
• MoM investigations: lightning,
shielding Fig 5: ADM Scale Model
What have I done? 4
Fig 6: Excitations on ADM model with LDC
• Indirect lightning strike –
EM plane wave
• Direct lightning strike –
current injection
• Consider LDC, shielding
• Electric field strengths
• Current densities
• Runtimes / file-sizes
• CHPC and SU HPC
What have I done? 5
• EM Plane Wave
• Current density
• Magnetic flux cutting
closed conducting loop
• Top & bottom struts “hot”
• Other positions
• Edges “hot”
• Shadowing
• Bearings Fig 7: Current densities on dish struts
What have I done? 6
• Current injection
• Current distribution struts
• Injection source loop
influences levels
• Shielding – top closed
• Raised structure: “hot”
• Cables entering: “hot”
• Partial shields for cost
• Feedback to SKA Fig 8: Electric field around and inside structure
Where am I going?
• Project plan: 2009
– CADFEKO Investigations:
• Lightning protection over
bearings
• Pedestal shielding,
earthing interfaces
– Result analysis: cost-effective RFI solutions
– Feedback to SKA offices, influencing design
Confucius says
• Confidence built for CADFEKO usage
• E-fields, Current density
• Shielding, earthing interfaces
• LDC layout and design
• LDC over bearings
• Cost-effective solutions
Conclusions
• Confidence built for CADFEKO usage
• E-fields, Current density
• Shielding, earthing interfaces
• LDC layout and design
• LDC over bearings
• Cost-effective solutions
Whether you believe you can do a thing or not, you are right. - Henry Ford
Acknowledgements
- SA SKA for bursary and travel grants
- Eskom TESP for bursary grants
- CST for use of Microwave Studio
- EMSS for CADFEKO license and usage
- Mel van Rooyen at EMSS for CADFEKO support
- Willem Esterhuyse for enormous mechanical insights in scale model
- Prof. DB Davidson, Danie Ludick and Evan Lezar for help in setting up simulations and CADFEKO on CHPC and SU HPC
- CHPC and SU HPC for usage of facility
Are we done yet?