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?