Interface and cabling characterization for SKA

Paul van der Merwe

Prof. HC Reader

Stellenbosch University

Introductory background

2007• Masters program: -Fundamental EMC principles.

-RFI mitigation. -Accurate measurements.

• Cable trays Enclosures • Verify results screened room measurements

using computational analysis.2008

• PhD continuation cable tray measurements: -End-terminations and

mid-span connections.-Plane wave radiation.

• Simulation of pedestal base.

Cable tray connections: Visualizing Physics

• No clear definition end-terminations and mid-span connections.

• Two models, two methods inducing CM currents.

• Investigated connections specified standards and EMC literature.

• Screened room measurement.

• Explained results current and field arguments.

Model 1 Model 2

Cable tray connections (cont): Current analysis

Model 1 • Indirect current injection.

• Concentrated induced CM current.

• Separate excitation and victim loops.

• Mid- and end-connections major current diversion increased path impedance high coupling.

Model 2• Direct current injection.

• Uniform CM current distribution.

• Cable tray common conductor.

• Small current diversion for connections path impedance constant invariant coupling.

Model 1 Model 2

Cable tray connections (cont 2): Field analysis

Model 1 • Resultant magnetic field combination primary field and secondary field.• Current flow influence strength of secondary field. • Secondary subtract primary field.• Side straps and L-brackets – high coupling. Wide bottom connection or U-bracket –

low coupling.

Model 2

• Excitation and victim loop connected via cable tray common conductor.• Interference coupling to victim loop constant regardless of connection type.• Same field intersects victim loop regardless type of end- or mid-connection.

Model 1 Model 2

Cable tray connections (cont 3): Typical Results

Model 1

Model 2

0 10 20 30 40 50 60 70 80 90 100-140

-120

-100

-80

-60

-40

|S21

| in

dB

Frequency(MHz)

Magnitude of S21 coupling in dB

Bottom connectionL-bracketFull U-bracketSide straps

0 10 20 30 40 50 60 70 80 90 100-55

-50

-45

-40

-35

-30

-25

|S21

| in

dB

Frequency(MHz)

Magnitude of S21 coupling in dB

Bottom connectionCentre L-bracketSide strapsFull U-bracket

Radiation of cable trays: Visualizing the problem

• Lower frequency analysis complete, next step HF radiation.

• One model used: -Anechoic chamber

-OATS

• Absorbing anechoic chamber, ground reflections OATS.

• LPDA and horn antennas used.

• Purpose: Comparison measured data with computed data using plane wave excitation.

Radiation of cable trays (cont): Adaptation

Bad Luck• Initial measurement procedure using VNA.• VNA calibration.• VNA not performing satisfactorily on day of measurement.• Signal generator and SA.• Uncalibrated system cable loss added in post-processing.

Good Luck• Voltage calculated in computation - voltage available from

measurements.• Signal generator as transmitter, SA as receiver: voltage into 50 Ω

load calculated.

• Antenna input power known, compensate S11 mismatch.

Radiation of cable trays (cont 2): Compensation

OATS influences

• OATS has reflective ground plane.

• Minimize ground reflection, placing receiver close to ground.

• Interference removed by calculating phase change along reflected path.

• Computation included ground plane therefore multi-path interference.

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

x 109

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

Por

t vo

ltage

Frequency(Hz)

Voltage induced on pickup wire inside cable tray using measured and computed data

Voltage using US horn 846

CST voltage using horn

Next step

Why all the previous work?

• Want to know: Level of inter-cable coupling in pedestal base.• What to do: Implement measures minimizing external energy entering

pedestal.• Constraints: Cost effective and practical.

Finally

• Best end and mid-span connections using current and field arguments.

• HF radiation measurements of cable tray structure. • Anechoic chamber and OATS measurements.• Pedestal and concrete base study. • Best interface.