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MeerKAT Lightning Protection,
Earthing and RFI Mitigation01 December 2010
Dr. Gideon Wiid
Post-Doctoral Fellow
Host:
Prof. Howard Reader
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Engineering Search: Engineering Search: Nearby Galaxies and Nearby Galaxies and
PulsarsPulsars
• Galaxy nearby:
Cape Town
• Pulsar serviced,
no more timing
problems
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AcknowledgementsAcknowledgements
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
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• RFI Background
• Reverberation Chamber and
cable transfer impedance
• Lightning Protection
• RFI Mitigation
• Earthing
MenuMenu
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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 to RFIBackground to RFI
≠
RFIRFI
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• 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 – Electrons Background – Electrons
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Background – Making Background – Making WavesWaves
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• 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
Reverberation ChamberReverberation Chamber
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• Cable transfer impedance vs shielding effectiveness
• 15 SKA cables and metal braided sheath• GPS cable compromises Shielding chamber• Influence on overall shielding effectiveness
Cable Transfer ImpedanceCable Transfer Impedance
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• Elevation Motor assembly• Lightning current levels• Bonding efforts• Path of least resistance• Dirt road vs highway• Traffic jam!
Lightning Protection 1Lightning Protection 1
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• 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 2Lightning Protection 2
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Impedance: ZC = 1/jωC ZL = jωL ; HF Currents more concentrated
Lightning Protection 3Lightning Protection 3
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• Azimuth bearings• Shoes and LDC
conductor• Misaligned 2 vs 4• FEKO during PhD• Measurements Nov.
2010
RFI Mitigation 1 - ShoesRFI Mitigation 1 - Shoes
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• 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 2 - RFI Mitigation 2 - MeasurementsMeasurements
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RFI Mitigation 3 - RFI Mitigation 3 - ShieldingShielding
0 10 20 30 40 50 60 700
10
20
30
40
50
60
70
Curre
nt (d
BuA)
Position (number of points)
Sampled Current on Conductor for Different Frequencies with Shield
45 MHz Shielded80 MHz Shielded140 MHz Shielded 200 MHz Shielded
0 2 4 6 8 10
-40
-20
0
20
40
60
Position [#]
Mag
nitu
de [d
B A
]
Measured Current on Conductor for Hardened Pedestal
f=45 MHzf=80 MHzf=140 MHzf=200 MHz
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• Chiller plant no earthing philosophy – floating
• Cables going through to pedestal interior
• Possible path for induced currents
• Consider optimal earthing options
Chiller Plant Earthing 1Chiller Plant Earthing 1
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No redirection of current
Shunted connection redirects current
Chiller Plant Earthing 2Chiller Plant Earthing 2
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ConclusionConclusion
• 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!
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T H E E N DT H E E N D
Thank you for your timeThank you for your time
Any Questions?Any Questions?
Veni, Vidi, VelcroVeni, Vidi, Velcro
( I came, I saw, I stuck around )( I came, I saw, I stuck around )