Millimetre Spherical Wave Antenna Pattern Measurements at NPL. Philip Miller May 2009

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1 Millimetre Spherical Wave Antenna Pattern Measurements at NPL Philip Miller May 2009

2 The NPL Spherical Range The NPL Spherical Range is a conventional spherical range housed within a 15 m by 7.5 m by 7.5 m temperature controlled anechoic chamber It is equipped with a HP 8510C RF system operating up to 110 GHz. This is being upgraded to an Agilent PNA-X System

3 The NPL Spherical Range - Layout Transmit Tower Ram Screens Receive Positioner System Range Pit Transmit Tower Rails

4 The NPL Spherical Range - Positioner 1m maximum with 0.6m Travel 3.7m Possesses Alignment Aids Counterbalance Adjustable Slide

5 The NPL Spherical Range - Photograph

6 Millimetre Wave Measurements Major Error Sources RF Stability Errors Equal Signal and Reference Path Lengths Temperature Controlled Chamber Stability better than 1 deg at 72 GHz Can be tracked using a Tie Scan Antenna Positioner Errors High Accuracy Positioner System Antenna Stability Errors Possibly More of a Problem in Polar Mode

7 Millimetre Wave Measurements Positioner Errors Three Sources of Error Misalignment Errors Manufacturing Errors Positioning Errors Examine the effects of the errors on a 42 dbi directivity antenna

8 Millimetre Wave Measurements Misalignment Errors Bottom Turntable can be aligned to degree Large effect due to Tower Axes can be aligned to 0.1 mm Phase error ε p = 25 sin(θ) deg at 94 GHz Reduces to 0.5 deg over main beam area (Range Length = 6 m)

9 Millimetre Wave Measurements Manufacturing Errors Axis Positioning Errors and Axis Tilt Errors Tilt Error (Degs) Azimuth Axis Tilt Error Fully Loaded Horizontal Plane Vertical Plane Tilt Error (degs) Elevation Axis Tilt Error Across Slide Along Slide Position (Degs) Position (degs)

10 Millimetre Wave Measurements Manufacturing Errors Uncertainty Budget Error Quantity Uncertainty in Horizontal Plane Positional Uncertainty in Horizontal Plane, mm Azimuth Position Wobble deg Azimuth Positioner Radial Runout mm Azimuth Positioner Axial Runout mm Elevation Position Wobble deg Elevation Positioner Radial Runout 0.05 mm Elevation Positioner Axial Runout 0.02 mm % Probability 0.32

11 Millimetre Wave Measurements Manufacturing Errors Error Model Largest Major Frequency Component = 3 * θ Error Bounded by 1 mm in Azimuth Error Bounded by 2 mm in Elevation ( 3φ + φ ) sin( θ ) + 22*sin( 3* ( θ ) + ) Pherror = 11*sin 0 90 θ0 degrees

12 Millimetre Wave Measurements Positioning Errors Positioning Errors are due to two effects: - Stopping Window of Uncertainty when a Positioner is Sent to a Point Trigger Timing Error when Acquiring with a Moving Positioner For the Scan Axis Bus Delay = 15 ms corresponding to an deg uncertainty For Step Axis Positioning Uncertainty = deg

13 Millimetre Wave Measurements Antenna Near-Field Amplitude Data R = 5 m Near Field Theta Cut - Amplitude Level db Angle Degrees

14 Millimetre Wave Measurements Antenna Near-Field Phase Data R = 5 m Near-Field Theta Cut - Phase Phase Degrees Angle Degrees Rate of Change of Phase = 70 deg/deg at 1 deg Shows a 36 mm offset at 94 GHz

15 Millimetre Wave Measurements Error Analysis Techniques Transform Data with and without Errors Calculate the effective error signal from ε ( ) ( θ,φ) = 20.*log 10^ ( P ( θ,φ) /20) 10^ ( P ( θ,φ) /A /20) Re-pointing carried out using Lagrange Interpolation N PN(θ,φ) is the magnitude of the normalised error free pattern Pe(θ,φ) is the magnitude of the non-normalised pattern with errors Ae is the normalising amplitude constant chosen to minimise the peak error signal e e

16 Millimetre Wave Measurements Manufacturing Errors Phi Plane Manufacturing Errors Phi - Null on Boresight Level db No Error Case Error Signal Error Signal Repointed Angle Degrees

17 Millimetre Wave Measurements Manufacturing Errors Theta Plane Manufacturing Errors Theta - Null on Boresight Level db No Error Case Error Signal Error Signal Repointed Angle Degrees

18 Millimetre Wave Measurements Alignment Errors Phi Plane Alignment Error Phi Level db No Errors Error Signal Angle Degrees

19 Millimetre Wave Measurements Alignment Errors Theta Plane Alignment Error Theta Level db No Errors Error Signal Angle Degrees

20 Millimetre Wave Measurements Positioning Errors Large Error Case Modelled as a Timing Error between 0 to 15 ms at an axis speed of 5 deg/s Errors assumed to a have a Rectangular Distribution Both Amplitude and Phase Errors Modelled Small Error Case Limited to degree equivalent to Step Acquisition Case

21 Millimetre Wave Measurements Positioning Large Errors Phi Plane Timing Errors Phi - Large Errors Level db No Errors With Errors Error Signal Angle Degress

22 Millimetre Wave Measurements Positioning Large Errors Theta Plane Timing Errors Theta - Large Errors Level db No Errors With Errors Error Signal Angle Degrees

23 Millimetre Wave Measurements Positioning Small Errors Phi Plane Timing Errors Phi - Small Errors Level db No Errors Error Signal Angle Degrees

24 Millimetre Wave Measurements Positioning Small Errors Theta Plane Timing Errors Theta - Small Errors Level db ` No Errors Error Signal Angle Degrees

25 Millimetre Wave Measurements Positioning Errors Observations The Error Signal should reduce by ε red = 20.*log(.075/.005) = 24dB Actual Reduction Phi Cut reduction = 24 db to 55 db Theta Cut reduction = 11 db to 50 db (Possible Correlation between Successive Cuts)

26 Millimetre Wave Measurements Positioning Errors Observations Actual Phi Positioning Errors Step Case Phi Angle Error Distribution Probability Error Degrees

27 Millimetre Wave Measurements Positioning Errors Observations Actual Theta Positioning Errors Step Case Elevation Theta Angle Setting Accuracy Case Error Degrees Theta Angle Degrees

28 Millimetre Wave Measurements Positioning Errors Conclusions Receiver Timing Error Largest Source of Uncertainty In Step Scan Mode errors at 100 GHz Gain Uncertainty better than 0.05 db Sidelobe Uncertainty better than 0.4 db at 40 db NPL is Upgrading its Receiver to an Agilent PNX reducing Timing Error from 15 ms to 60 μs. Range could give Excellent Result up to 300 GHz