Introduction to Measurement Techniques

Transcription

1 Introduction to Measurement Techniques Andrés García Aguilar Outline 1. Introduction, 2. Far-field ranges, 3. Anechoic chambers, 4. Near-field systems: Spherical, planar & cylindrical, 5. Compact ranges, 6. Polarization measurements, 7. Measurement instrumentation, 8. Power and dynamic range, 9. Gain standards and Gain measurements, 10. Other measurement systems. 2

2 It Introduction ti 3 Introduction Definition of an antenna by the Webster ss Dictionary: a usually metallic device for radiating or receiving radio waves. Definition of an antenna by IEEE Standard Definitions of Terms for Antenna: a mean for radiating or receiving radio waves i.e. antenna is the transitional structure between free-space and a guiding device. Aims of Antenna Measurements: Evaluation of designed antennas, Empirical validation for antenna analysis methods. Antenna Parameters to be measured: Radiation pattern parameters: directivity, cross-polar radiation... Gain and antenna efficiency, Impedance and port isolations. Antenna measurement systems according to field regions: Outdoor far field ranges, Anechoic chambers: planar, cylindrical, spherical near-field systems, compact ranges 4

3 Field Regions Space surrounding S di an antenna is i subdivided bdi id d in: i Reactive near field region: that portion of the near field region g immediately y surrounding g the antenna, where the reactive field predominates. Radiating near field region: (Includes Fresnel zone): ) intermediate region, region where here the radiation fields predominate, but the angular field distribution depends upon the distance from the antenna. t Far Field (Fraunhofer) region: the region of the field of an antenna where the angular g field distribution is independent of the distance from the antenna. Far field distance satisfies: 2 D2 y r D: Maximum dimension of the antenna r 5 Example: radiated field of an antenna Simulated Vertical radiation pattern for a 9 resonant dipoles (h=210 cm) at 900 MHz MHz. Far field distance = 26.7 m: distance d: a) d = 27 m b) d = 13.5 m c) d = 6.75 m d) d = 2.7 m a) db db d) db c) b) db

4 Antennas and far-field distances For monopoles and dipoles (length λ), R far field 10λ is used, because reactive field is negligible. For large reflectors in microwave band with circular aperture with D >> λ, R far field 2D 2 /λ is used, because of the phase errors at the aperture. For base station antennas (arrays with height h >> λ), R far field 2h 2 /λ is used, because of the phase errors at the aperture. 7 Antenna Measurement Systems Compact Systems Far-Field Systems: Elevated & Ground Reflection Ranges Near-Field Systems: Planar, Cylindrical i l & Spherical 8

5 Far-field ranges 9 Far field ranges Antenna under test (AUT), usually in reception, is illuminated by a source (probe antenna). This antenna must be in far field distance. In this case, the incident wave is a plane wave. Source antenna AUT TheAUTcanbemeasuredin transmission or in reception. Radiation patterns and parameters are the same, according to the reciprocity theorem. 10

6 Anechoic chambers 11 Anechoic chambers Close areas (normally shielded) covered by electromagnetic absorbing material, that simulate free space propagation conditions, due to the absorption of the radiation absorbing material (RAM). Advantages: All weather operation. Control of the environment (temperature, cleanness...) Security. Freedom from interference. 12

7 Anechoic chambers: Microwave Absorbing materials Pyramidal absorbers (H>5): < -50 db. Convoluted absorbers: < -50 db in mm-wavelengths. Flat laminate absorbers: < -25 db (towers...). Wlk Walkway absorbers: b (pyramidal + foam + polystyrene laminate). Wedge absorbers: for compact ranges. All weather or vent absorbers. 13 Anechoic chambers: Examples a) Rectangular Test chamber: max <70º W/R>1/ R Source antenna main lobe can t illuminate the side walls, floor or ceiling. b) Tapered Test Chamber: W Interference signals eliminate the ripple in quiet zone. Used in VHF and UHF. The source is adjusted at each frequency. c) Semi-open chamber: AUT is protected by a building (without one wall). The antenna source is far away. 14

8 Near-field systems: Spherical, planar, cylindrical 15 Near field systems The radiated field is measured in a surface (plane, cylinder or sphere) near to the AUT, and the far field is obtained using a transformation algorithm. Advantages: Less use of space, Indoor systems advantages (independent of weather conditions ), The far field is obtained without the error due to the finite distance. Drawbacks: More complex and precise exploring systems are required, Transformation software based on modal analysis (with plane, cylindrical or spherical waves), A probe calibration is necessary. 16

9 Near field theory If we know the tangential component of the fields over a closed surface including all the sources, it is possible to know the fields wherever outside it. A.D. Yaghiang, An Overview of Near-Field Antenna Measurements, IEEE Transactions on Antennas And Propagation, Vol. AP-34. Nº 1, January Types of near-field scanning Planar System XY (Transformation: FFT) Cylindrical System Spherical lsystem 18

10 Planar near-field system: theoretical analysis In the planar near-field system, the coupling equation relates the measured values with the probe correction coefficients and the AUT transmission coefficients. Since they are related by a Fourier Transform,, it can be solved applying ppy an Inverse Fourier Transform. Ideal Probe situation (ideal dipole) A.D. Yaghiang, An Overview of Near-Field Antenna Measurements, IEEE Transactions on Antennas And Propagation, Vol. AP-34. Nº 1, January Planar near-field system: theoretical analysis Real Probe situation Far-Field A.D. Yaghiang, An Overview of Near-Field Antenna Measurements, IEEE Transactions on Antennas And Propagation, Vol. AP-34. Nº 1, January

11 Planar near-field system: practical aspects Angular validity of the measurement: Measurement tplane Sampling acquisitions has to be limited to a finite rectangle in the measurement plane. This truncation limits the validity of measurement result in far field to angles lower to θ v. v L x D v atan 2z0 ABP z 0 If E(L x /2) < -40 db, the truncation error is negligible. ibl Maximum sampling rate Sampling Theorem D Lx 2 x 2k 2 y 2 (In some conditions, it can be higher) 21 Planar near-field system 22

12 UPM antenna measurement ranges Planar System: Dimensions: 6 meters lower horizontal guide 1 meter supporting cart 5.5 meters tower upper horizontal guide at 2 meters - 3 high precision linear elements assure the scanner high precision. - The lower horizontal guide is a linear ball spline, that allows a free rotation of the vertical tower. - The tower leans on the upper horizontal guide. - Scan area: 4.75 x 4.75 meters - Frequency band: GHz - Horizontal axis velocity: 10 cm/sec - Vertical axis velocity: 33 cm/sec - z errors < 0,34mm peak to peak in the scan area 23 Cylindrical near-field system In the cylindrical near-field system, the coupling equation relates the measured values with the probe correction coefficients and the AUT transmission coefficients. With 2 set of measured values for each polarization & the probe correction coefficients, the AUT transmission coefficients could be derived. Then, with these AUT transmission coefficients, the θ-components & φ-components of the far-field field can be obtained. 24

13 Cylindrical near-field system: theoretical analysis Ideal Probe situation Real Probe situation Far-Field A.D. Yaghiang, An Overview of Near-Field Antenna Measurements, IEEE Tansactions on Antennas And Propagation, Vol. AP-34. Nº 1, January Cylindrical near-field system: practical aspects Angular validity of the measurement: Measurement tcylinder Sampling acquisitions has to be limited to a finite rectangle in the measurement plane. This truncation limits the validity of measurement result in far field to angles lower to θ v. v L x D v atan 2z0 ABP z 0 If E(L x /2) < -40 db, the truncation error is negligible. ibl Maximum sampling rate Sampling Theorem Vertical direction: the same than planar system, Horizontal direction: the same than the spherical system. D Lx 26

14 Cylindrical near-field system 27 UPM antenna measurement ranges Cylindrical and Spherical System: - Sharing elements with the planar system. - Cylindrical: AUT on Azimuth positioner and probe on scanner y-axis. - Spherical system: AUT on Roll over Azimuth - Frequency band: 1 40 GHz - Linear slide to adjust measurement distance. CYLINDRICAL SYSTEM 28

15 Spherical near-field system: theoretical analysis Ideal Probe situation Real Probe situation Far-Field A.D. Yaghiang, An Overview of Near-Field Antenna Measurements, IEEE Tansactions on Antennas And Propagation, Vol. AP-34. Nº 1, January Example of an spherical near-field system 30

16 UPM antenna measurement ranges Spherical System: Dimensions: 7.3 x 4.3 x 4.3 m Frequency band: GHz ORBIT Controller and positioners Agilent HP8530A VNA Approved for Space Measurements (ESA) at 53GH 5.3 GHz using ERS panel AUT Positioner. Roll over Azimuth on longitudinal table Polarization Positioner 31 Compact ranges 32

17 Compact ranges The idea is to form a planar wave around the AUT using reflector systems. They are used for measuring antennas in far field and for measuring object RCS. Don t need field transformation, the measurements are obtained in far-field. field LIMITATIONS: Complex & big structures needed, so the chamber dimensions must be higher. Their precisions are, in general, lower than in near field systems. Mainly related with the flatness of the field in the quiet zone: Desired amplitude constant to a fraction of a db, Desired phase flat to few degrees. At higher frequencies, limited by the tolerances of the reflectors surfaces. At lower frequencies, limited by the electrical size of the absorber pyramids. 33 UPM antenna measurement ranges Compact Range System: Gregorian System Measurements of Antennas and RCS Dimensions: Main chamber: 15.2 x 7.9 x 7.3 m Subreflector chamber: 6 x 3 x 2.4 m Frequency band: 6 60 GHz Rotary joints at 40 GHz Quiet zone at X band: 2.5 m.diameter (0.25 db, 3º) AUT Plane phase front Elliptic Sub-reflector Double chamber Gregorian System Parabólic Reflector D=4.5 m feeder Main Reflector Feeder Subreflector 34

18 Polarization measurements, Measurement instrumentation, Power and dynamic range. 35 Polarization measurements AUT Roll -axis Source antenna ˆ ˆ Azimuth -axis With a double polarization probe, it is possible to obtain E y E simultaneously, but an accurate calibration of both channels is required. With a single polarization probe, each component is acquired in one scan with a 90º rotation of source antenna. Components CP-XP, CPC-XPC are obtained with field expressions. 36

19 Measurement instrumentation 37 Power and dynamic range P P Rmax Rmin DR dbmp dbm 20log GT dbi G R dbi PSat T λ 4π R dbm S dbm S NdB Rx db P dbm P dbm Rmax Rmin P T R P R P sat = Saturation of the transmitter S Rx = Sensitivity of the receiver S/N = Required signal to noise (measurement errors) DR = dynamic range of the measurement (SLL or XP requirements) S/N Amp. error Phase error 20 db ±0.9 db ±5.7 º 30 db ±0.28 db ±1.8º 40 db ± dB ±0.57º 38

20 Gi Gain measurements 39 Gain standards In microwave bands, rectangular horns are used as gain standards. The gain is almost equal to the directivity given by the manufacturer. The error of this value uses to be in 0.3 db If a better precision is required, it is necessary to calibrate the gain standard, using: Absolute gain measurement. Integrating the radiation pattern to obtain the directivity Calibrated gain of a X band horn. Calibration done in a Spherical range 40

21 Gain measurements IEEE Standard Definitions of Terms for Antennas: GAIN in a given direction: The ratio of the radiation intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically. U, 2 * r 1 2E H G Paccepted 4π U isotropic P 4 radiated REALIZED GAIN: The gain of an antenna reduced by the losses due to the mismatch of the antenna input impedance to a specified impedance. G R = G (1 Γ in 2 ) 41 Gain measurements The most classical method is gain substitution technique,, illuminating the AUT and a gain standard (usually, a Standard Gain Horn, SGH) with the same source antenna, for example a probe. AUT vs Probe SGH vs Probe Far-field substitution technique: ( Gaut ) db d aut prb 20log ( G sgh ) db dsghprb P 1 Γsgh Raut prb 10log 10log P Rsghprb 1 Γaut

22 Gain measurements Correction for impedance mismatch: Γ 1 Γ 1 Γsgh 1 Γ aut rx rx ΔZ 10log 10log ΓautΓrx 1 ΓsghΓrx To improve the impedance matching, an attenuator after the AUT is used. In this case: rx 0. ΔZ 2 1 Γsgh 10log 2 1 Γ aut 43 Absolute gain measurements 3 main types Two antenna method, Three antenna method, Radio source technique A) Two antennas method: based on using identical antennas and the Friis transmission i formula. AUT 1 AUT 2 B) Three antennas method eliminates the need for identical antennas by making three measurements and solving the three equations. AUT AUT SGH SGH PROBE PROBE 44

23 Absolute gain measurements C) Radio source is suited to very large, high gain antennas that cannot be measured any other way. The gain can be calculated either by comparing the level to a known noise source or by computation from the known noise figure and bandwidth of the receiver. 45 Others measurement systems 46

24 UPM antenna measurement ranges Arc System: Semi-Anechoic chamber Measurements of Antennas on scaled ships (1:50 and 1:100 models) Dimensions: 6.5 x 5.5 x 2.7 m Frequency band: 200 MHz 3 GHz Positioning system: azimuth for ship and elevation for probe. 47 Systems based on thermographical techniques Thermal intensity measurement in the planar system, Phase reconstruction, Radiation pattern extraction. 48

25 Linear Array Antennas Measurement System An ad-hoc measurement system designed and built for linear antennas by GR UPM for INDRA. The AUT is fixed on the structure. The probe moves along the linear slide, stopping in front of each array element, acquiring the near field radiated by the AUT (in amplitude and phase). This acquired field is processed to obtain the radiation pattern in the far field for the plane of the array. A post-processing is performed to obtain the radiation pattern main parameters. The measurement range is formed by the following main subsystems: Linear slide and antenna array supporting structure. Anechoic and reflector systems, including an elevator for antenna array installation. Radio frequency subsystem. The system also includes an ad-hoc acquisition, processing and representation software. 49 Linear Array Antennas Measurement System 50

26 CEAR Cylindrical Near-Field Range 51 CEAR Cylindrical Near-Field Range System designed and built by GR UPM Geometric features: Tower length: meters Tower usable length: 15.7 meters Distance between AUT axe and Probe: 5.5 or 7.5 meters 15.7 m Operation requirements: Frequency Range: L band ( MHz) 69.1º m System S t can workk in i windy i d situations it ti (< 30 kkm/h) /h) 5.35 m Error tolerances: 3.71 m 33.2º Directivity maximum error: 0.5 db SLL Error at 30 db: 2 db SLL Error at 40 db: 3 db Azimuth Beam Pointing Error: 0.05º ~1 m 3.42 m Elevation Beam Pointing Error: 0.1º 52

27 CEAR KEY POINTS Optimized i acquisition iti process 2 Polarizations 2 Radiation Patterns (monopulse curves) Z axis linearity ensured by an automatic laser based system Residual error <±1mm Temperature drifts corrected by a hw-sw system CEAR XY POSITIONING ERROR CORRECTION 2000 ERRORES DE POSICIONAMIENTO EN EL EJE X 1500 MORORIZED xy TABLE 1000 QUADRANT DETECTOR 500 TOWE R Err ror en micras 0 LASER -500 AND OPTICAL PLUMMBET Altura en cm 54

28 Systems for small antennas (mobile) EPFL-LEMA system (Lausanne - Switzerland) Chalmers-Bluetest system (Göteborg Sweeden) 55 SATIMO Stargate System Acquisition & Processing Stargate system made of 64 probes Probe Calibration 56

29 MVG Large Measurement System 57 Millilab Hologram based CATR for 650 GHz 58

30 Reverberation chamber For radiation efficient measurements (among other applications): Bluetest Chamber Magdeburg large reverberation chamber 59 RCS measurements 60

31 RCS measurements 61 RCS bistatic measurements 62

32 Acquisition, Processing and Representation Software 63 Software PROCENCA (GR-UPM) Measurement definition 64

33 Software PROCENCA (GR-UPM) Acquisition 65 Software PROCENCA (GR-UPM) Results definition 66

34 Software PROCENCA (GR-UPM) Results 67