Transcript MeerKAT Lightning Protection, Earthing and RFI Mitigation

MeerKAT
Lightning Protection,
Earthing and RFI Mitigation
01 December 2010
Dr. Gideon Wiid
Post-Doctoral Fellow
Host:
Prof. Howard Reader
Engineering Search:
Nearby Galaxies and Pulsars
• Galaxy nearby:
Cape Town
• Pulsar serviced,
no more timing
problems
Acknowledgements
EM research group at SU:
Prof. Howard Reader, Joely Andriambeloson,
Dr. Braam Otto, Dr. Paul van der Merwe, Philip Kibet
SKA SA Teams for support in measurements:
Japie Ludick, Richard Lord, Carel van der Merwe,
Luyanda Boyana, Siya Tshongweni
NRF and SKA SA Sponsorship
TESP funding for equipment
Menu
• RFI Background
• Reverberation Chamber and
cable transfer impedance
• Lightning Protection
• RFI Mitigation
• Earthing
Background to RFI
≠
RFI
Contrary to popular belief:
• RFI ≠ Black magic
NB: Electrons and Energy
• Opposite charges attract
• Electric field in between two polarities
• Electron experiences force in E-field
Background – Electrons
• Free electrons – sneaky little rascals
• Always takes the path of least resistance!
• Very Excitable – especially with
time-varying E-field applied
• Electron E-field lines: stationary vs. moving
Background – Making Waves
Reverberation Chamber
• Joely Andriambeloson – Transfer impedance of cables
• Reverberation chamber – Stirring or mixing waves
• Characterise avg or max
• Confirm field uniformity
• Standard radiator
• Compare to calibrated data
• Compare to GTEM
• Future: OATS
Cable Transfer Impedance
• Cable transfer impedance vs shielding
effectiveness
• 15 SKA cables and metal braided sheath
• GPS cable compromises Shielding chamber
• Influence on overall shielding effectiveness
Lightning Protection 1
•
•
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•
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Elevation Motor assembly
Lightning current levels
Bonding efforts
Path of least resistance
Dirt road vs highway
Traffic jam!
Lightning Protection 2
• Rogowski coil used for current
measurement (pool noodle)
• Structure currents spread out
• LDC and Elevation currents
concentrated
• Bearings grease changes
impedance
• FEKO simulations currently busy
Lightning Protection 3
Impedance: ZC = 1/jωC ZL = jωL ; HF Currents more concentrated
RFI Mitigation 1 - Shoes
• Azimuth bearings
• Shoes and LDC
conductor
• Misaligned 2 vs 4
• FEKO during PhD
• Measurements Nov.
2010
RFI Mitigation 2 - Measurements
•
LDC two extra connections
•
Clamped on other LDC and plate
•
Measurements inside and outside
•
Current levels outside higher on avg
for 4 vs 2 connections
•
Less current toward inside of pedestal
•
Path of least resistance!
RFI Mitigation 3 - Shielding
Sampled Current on Conductor for Different Frequencies with Shield
Measured Current on Conductor for Hardened Pedestal
70
60
45 MHz Shielded
80 MHz Shielded
140 MHz Shielded
200 MHz Shielded
60
f=45 MHz
f=80 MHz
f=140 MHz
f=200 MHz
40
Magnitude [dBA]
Current (dBuA)
50
40
30
20
20
0
-20
10
-40
0
0
10
20
30
40
50
Position (number of points)
60
70
0
2
4
6
Position [#]
8
10
Chiller Plant Earthing 1
• Chiller plant no earthing philosophy –
floating
• Cables going through to pedestal
interior
• Possible path for induced currents
• Consider optimal earthing options
Chiller Plant Earthing 2
 No redirection of
current
 Shunted connection
redirects current
Conclusion
• Reverberation chamber method – relatively
quick for testing cable shielding effectiveness
or transfer impedance
• Lightning protection measures can be
incorporated at MeerKAT design stage
• Elevation assembly might be reconsidered
• Pedestal RFI can be mitigated with correct
shielding and cable terminations
• Correct earthing and terminations will aid in
redirection of induced currents
• Electrons are sneaky!
THE END
Thank you for your time
Any Questions?
Veni, Vidi, Velcro
( I came, I saw, I stuck around )