Investigating EMC policies for the SKA

Paul van der Merwe

Prof. HC Reader

Stellenbosch University

Introductory background

What was done during 2009?

• Cable tray measurements• Pedestal interfaces• Karoo site visit• European visit investigating 2-PAD and EMBRACE

aperture arrays

Cable tray radiation measurements

• Determine the shielding ability of cable trays in high

frequency RFI environments.

• Make use of known gain antennas and anechoic

chamber.

• S21 measured – induced voltage and gain calculated.

• Define two methods using computational analysis.

Shielding analysis of cable trays: Method 1

• The cable tray structure is radiated using a plane wave definition with known E-field level.

• Voltage induced on victim cable in cable tray measured at port.

Shielding analysis of cable trays: Method 2

• The cable tray structure is excited and far field radiation pattern is measured

• Gain computed and compared to calculated gain using measured S21

Comparison: Measurement & Computation

• Comparing measured and computed data validates the accuracy of the computational model.

• Sequence have to be repeated for various positions of receiving antenna

0 1 2 3 4 5 6

x 109

-80

-75

-70

-65

-60

-55

-50

-45

-40

-35

-30

Vol

tage

(V

)

Frequency (Hz)

Plotting the measured and computed values for the cable tray being radiated on a linear scale

Measured voltage radiated at 90 deg

CST voltage radiated at 90 deg

0 1 2 3 4 5 6

x 109

-25

-20

-15

-10

-5

0

5

10

15

Gai

n of

cab

le t

ray

(dB

)

Frequency (Hz)

Calculating the gain of the cable tray from measurements using K1 over 6 GHz bandwidth vertically radiated

Cable tray gain

CST computed gain

Method 1 Method 2

Pedestal interfaces: what can be done?

• Power provision for dish needs to enter pedestal via some cable interface.

• CM current on power cable able to enter pedestal.

• Bonding of meshed floor to pedestal at point of entry reduces interference.

On-site measurements

• Evaluate site RFI and interference generated by infrastructure.• Radiated interference measured – E-field antenna.• Conducted interference measured – CM current probe.• Proposed measurements on dish cables.

Investigating 2-PAD and EMBRACE: Purpose of visit

• Exchange interaction between South African SKA effort and European aperture array development.

• Become familiar with European SKADS, AAVP setup.

• EMI investigations of both 2-PAD and EMBRACE systems.

• Improve RFI mitigation for both systems.

• Gain knowledge on aperture arrays – measurements experience

2-PAD EMBRACE

Grounding schematic for array

• Evaluation of induced CM current on system by radiated RFI.• Processing bunker shielding measurements• Transfer impedance measurements of different sections of system• Balun investigation

Interface for cables

with 360° connectors

Ground plane

Power filter connected to transformer.

Transformer supplies ground reference

Antenna elements

Non-metallic support frame Suggested

grounding points for chamber

Non-metallic sub-frame

Capacitive connection of

ground plane to ground

GroundEquipotential bonding conductor

A

B C

I

D

A/D converter

Test Points

F

EG

PC

EM Injection Clamp

Bunker shielding: How much does it protect?

• Effective shielding of processing bunker important.• Bunker forms major part of RFI mitigation.• Effective diversion of CM current – Measurements conducted.• Validation of effective shielding at interfaces.

LPDASignal

generator

EMC chamber

Current probe

Transfer impedance measurement

• Transfer impedance:

• Identify transfer impedances in EMBRACE grounding system.

• Influence of transfer impedance can be approximated:

Antenna ground plane

Signal source

Antenna cable

Capacitive coupling to ground

IDM75 Ω SA

SA

ICM

EM injection clamp

Signal gen

ICM

Connector

CM

DMtI

VZ

)( CMDMtDMLx IIZIZV

ICM

Vx VZt

ZL

Circuitry inside bunker

Ztransfer

ICM

Bunker interface

75 ΩSA

EM injection clamp

Signal gen

SA

ICM

IDM

Ground

Vx VZt

ZL

Circuitry inside bunker

Ztransfer

ICM

IDM

ICM

Bunker interface

Antenna ground plane

Signal source

Antenna cable

Capacitive coupling to ground

Connector

Finally

• Shielding ability of cable trays at higher frequencies: can they be used?

• Two methods defined two analyze them. • Improving the pedestal interfaces for cable entry.• Initial site evaluation of RFI. • Investigated 2-PAD and EMBRACE aperture array systems• Measured the induced CM currents, bunker shielding and transfer

impedance.