MeerKAT Lightning Protection,

Earthing and RFI Mitigation01 December 2010

Dr. Gideon Wiid

Post-Doctoral Fellow

Host:

Prof. Howard Reader

Engineering Search: Engineering Search: Nearby Galaxies and Nearby Galaxies and

PulsarsPulsars

• Galaxy nearby:

Cape Town

• Pulsar serviced,

no more timing

problems

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

• RFI Background

• Reverberation Chamber and

cable transfer impedance

• Lightning Protection

• RFI Mitigation

• Earthing

MenuMenu

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

• 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

Background – Making Background – Making WavesWaves

• 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

• 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

• Elevation Motor assembly• Lightning current levels• Bonding efforts• Path of least resistance• Dirt road vs highway• Traffic jam!

Lightning Protection 1Lightning Protection 1

• 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

Impedance: ZC = 1/jωC ZL = jωL ; HF Currents more concentrated

Lightning Protection 3Lightning Protection 3

• Azimuth bearings• Shoes and LDC

conductor• Misaligned 2 vs 4• FEKO during PhD• Measurements Nov.

2010

RFI Mitigation 1 - ShoesRFI Mitigation 1 - Shoes

• 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

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

• 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

No redirection of current

Shunted connection redirects current

Chiller Plant Earthing 2Chiller Plant Earthing 2

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!

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 )