Presentation on theme: "An Introduction to RF Anechoic Chamber Technology"— Presentation transcript:
1 An Introduction to RF Anechoic Chamber Technology Vicente Rodriguez, Ph.D.ETS-Lindgren1301 Arrow Point Dr.Cedar Park, TX, 78613
2 SUMMARY The Chamber Family Absorber Materials The EMC Chamber The antenna ChamberTapered and Rectangular.RCS chambersReferences
3 Chamber Types: Two Main Families EMC testingProperties: Semi-free space or half free spaceAbsorber: FS-1500,1000,600,400 (polyurethane) and FS-1250,600,400 PS and EMC-24, (for High frequency applications other absorber may be used).Design tools: In-House SoftwareStd. Doc: ANSI C /1998; EN (semi-anechoic),-3(fully anechoic),-1(shielding test); VCCI V98.3 (Japanese);RCS and Antenna.Properties: Fully anechoic.Absorber: EHP series absorbersRCS: mostly military applications, is a chamber to measure radar cross section of a targetAntenna: military and commercial, these are chambers to measure the radiation pattern of a radiator, could be an antenna or an antenna plus other system.
4 The Absorber Family 1 Microwave Pyramidal absorber. EMC and EHP series Electric LossesPreferred technology for High frequenciesIt can be used for low frequencies if size (length) is increased
5 The Absorber Family 2 Ferrite Tile . Magnetic Losses Preferred technology for Low frequencies (up to 1GHz), it has low profile.It cannot be used for high frequencies
6 The Absorber Family 3 Hybrid Absorber . Electric and Magnetic Losses Preferred technology for EMCApplications. foam has to have special formula for good matching with ferrite tile at the bottom. At High frequencies its performance is not as good as MW pyramid of equal size. Flat top causes undesired reflections at MW range.
7 The Absorber Family 4 Flat laminate . Electric Losses Preferred technology for laboratory set ups. It is a sandwich of different foams.About 20dB absorption as frequency increases.
8 The Absorber Family 5 Wedge and pyramid Electric Losses A variant of pyramidal absorber wedge does not show backscattering. Preferred technology for QZ treatment and for RCS chambers.
9 Pyramidal Absorber Theory (Example) This material is volumetrically loaded having the same constitutive parameters through the volume of the pyramidPopular types of absorber have constitutive parameters of:Non magnetic materialLow permittivity with lossesWe will study how the electromagnetic wave behaves as is incident on to a wall of this type of absorber.
10 Pyramidal Absorber Theory (Example) At the tip of the absorberThe wave impedance is that of airNO SUDDEN CHANGE IN WAVEIMPEDANCE = LOWREFLECTIVITYAlong the length of the pyramid the wave impedance falls between those two values.At the base of the pyramidThe wave impedance becomes
12 Pyramidal Absorber Theory (Example) Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the heightLet’s assume a length of 30cmThe wavelength at 3GHz is10cmAnd at 10GHz is30mmFor 3GHzWavelength at 3GHzApproximate thickness of equivalent solid material
13 Pyramidal Absorber Theory (Example) Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the heightFor 10GHzWavelength at 3GHzApproximate thickness of equivalent solid materialIn practice the reflection coefficient may not be as small as this but it will be significantlySmaller than at 3GHz
14 EMC ChamberDesign is guided by the standards and the test that the customer is going to perform. Frequency range is from 30 to 1000MHzAt what distance is the measurement (3m, 5m,10m)? Are we testing immunity or emissions?The chamber must perform as if it was an infinite ground plane in an infinite open spaceNSA is a measurement of how close we are to this goal.Std.Docs call for NSA being within +- 4dB of the theoretical.4m1m1,3,5,10m
15 EMC ChamberThere are some rules that can be applied when sizing an EMC Chamber once the test distance and the quiet zone size are knownABDTx1Tx2QzdQzsabsRx2Rx1Dsin()aRx1 can be a minimum of 1.5mIt is desirable that > 0 so that reflections from the side wallsDo not arrive in phase to the test area.
16 EMC ChamberThe FCC asks that the performance of that anechoic chamber matches that of an out door range.The Normalize Site Attenuation (NSA) is measured.Another common measurement for qualification is the field uniformity measurementAHDTx11mabs1 – 4 meter scan1.5m2m
17 Chamber Validation Requirements EMC ChamberChamber Validation RequirementsA typical standard will have wording similar to this:ANSI C § , Alternate test Sites: Measurements can be made at a location other than an OATS, ... Provided that the alternate site meets the site attenuation requirements of over the volume occupied by the EUT, … and the ground plane requirements of 5.4.3What this means is that it must be shown that the chamber performs like an infinite ground plane with no obstructions anywhere.
20 EMC ChamberTesting the Uniform Area according EN For chambers where immunity measurements will be performed it may be required to test the FU according to a given standard. this will show that the reflections from the wall do not affect the field.This plane includes the uniform area, 12 from 16 points of E-Field are within +6/-0 dB.
21 EMC Chamber: Partially lined chambers Mil Std chambers and some other aircraft and SAE EMC standard documents call for partial absorber treatment chambers.Frequencies for use start in the 100 of MHz.Not looking for a half free space.Absorber is loading the cavity (chamber) to reduce any resonant behavior.EMC24 absorber is enough for these applications.(A pyramidal specially loaded absorber)EUT Bench1m
22 EMC Chamber: Mil Std Chamber Mil Std 461 Defines the size of the chamber in terms if the EUT being measured, EUT size determines the size of the chamber. The sketches bellow show the standard MIL-STD chamber offered by ETS-Lindgren.
23 Mil Std 461EWhat is it?“Department of Defense: Requirements for the control of electromagnetic interference characteristics of subsystems and equipment”So it is a complete Standard for all different EMC measurements
24 Mil Std 461E Mil Std 461 E came to be in It superseding both AugustIt superseding bothMil Std 461D and Mil Std 462DWhich passed away(rest in peace)
25 Mil Std 461E Emissions Conducted Susceptibility Mil Std 461E Is made of many parts(which one are YOU interested in)EmissionsRadiatedSusceptibility
26 Mil Std 461E Conducted CE 101 Emissions CE 102 CE 106 Conducted CS 101 SusceptibilityCS 105CS 116CS 109
27 Mil Std 461E Conducted Emissions CE 101Conducted Emissions, power leads 30HZ-10KHzNo chamber requiredShielded room recommended.LISN, receivers, O’cope, data recorders, sig gens,Current probe, etc are part of the required EquipmentCECE 102Conducted Emissions, power leads 10KHz-10MHzCE 106Conducted Emissions, Antenna terminal, 10KHz to 40GHz
28 Mil Std 461E Conducted Susceptibility (Immunity) CS 101Conducted Susceptibility, Power leads, 30Hz, to 150KHzCS 103Conducted Susceptibility, Antenna Port, Intermodulation, 15KHz to 10GHzCS 104Conducted Susceptibility, Antenna Port, rejection of Undesired signals, 30Hz to 20GHzCS 105Conducted Susceptibility, Antenna Port, Cross-modulation, 30Hz to 20GHzCSCS 109Conducted Susceptibility, Structure Current, 60Hz to 100KHzCS 114Conducted Susceptibility, Bulk Cable Injection, 10kHz to 200MhzCS 115Conducted Susceptibility, Bulk Cable Injection, Impulse excitationCS 116Conducted Susceptibility,Damped Sinusoidal transients, cables and power leads, 10KHz to 100MHz
29 Mil Std 461E Radiated RE 101 Emissions RE 103 RE 105 More details about equipment and facilities will be givenRadiatedRS 101RS 103SusceptibilityRS 105
30 Mil Std 461E Radiated Emissions RE 101Radiated Emissions, Magnetic Field, 30Hz to 100KHzRERE 102Radiated Emissions, Electric Field, 10KHz to 18GHzRE 105Radiated Emissions, Antenna Spurious and harmonic Outputs, 10KHz to 40GHz
31 Mil Std 461E Radiated Susceptibility (Immunity) RS 101Radiated Susceptibility, Magnetic Field, 30Hz to 100KHzRERS 103Radiated Susceptibility, Electric Field, 2MHz to 18GHzRS 105Radiated Susceptibility, Transient Electromagnetic field
32 Mil Std 461E Where do we test? Paragraph 4.3.2“To prevent interaction between the EUT and the outside environment, SHIELDED ENCLOSURES will be usually required for testing”Paragraph“RF absorber material… shall be used when performing RE and RS testing inside a shielded enclosure…The RF absorber shall be placed above, behind and on both sides of the EUT, and behind the radiating or receiving antenna”
33 The Absorber Family EMC-24 Mil Std 461E requirements TABLE I page 10 6dB absorption 80Mhz to 250Mhz10dB absorption 250Mhz and above
34 Mil Std Chamber side view Mil Std 461 Defines the size of the chamber in terms if the EUT being measured, EUT size determines the size of the chamber. The sketches bellow show the standard MIL-STD chamber offered by ETS-Lindgren.
35 Mil Std Chamber Side View Mil Std 461 Defines the size of the chamber in terms if the EUT being measured, EUT size determines the size of the chamber. The sketches bellow show the standard MIL-STD chamber offered by ETS-Lindgren.
36 Mil Std Chamber RE and RS Equipment RECOMMENDEDREQUIREDCurrent ProbeEMCO MCHAMBEREMCO 7604EMCO 4-TR
37 Mil Std Chamber RE and RS Equipment REQUIREDEMCO MCHAMBEREMCO 4-TREMCO 7-TREMCO 3115EMCO 3110BEMCO 3301BEMCO 3106MIL STD 461E does not longer accept Log Periodic and spiral Logs only double ridge horns above 200MHz
38 Mil Std Chamber RE and RS Equipment RECOMMENDEDREQUIREDCurrent ProbeEMCO MCHAMBEREMCO 7605/7606EMCO 4-TR
39 Mil Std Chamber RE and RS Equipment REQUIREDEMCO MCHAMBEREMCO 4-TREMCO 7-TREMCO 3115EMCO 3109EMCO 3301BEMCO 3106MIL STD 461E does not longer accept Log Periodic and spiral Logs only double ridge horns above 200MHz
40 Mil Std Chamber RE and RS Equipment paragraph , states that an accepted method is the mode tuned reberveration chamber, the range is 200MHz to 40GHz, for the reverb log periodics can be used since pattern is no longer an issue
41 CISPR 25“Limits and methods of measurement of radio disturbance characteristics for the protection of receivers used on board vehicles”This means that we measure the emissions that would affect any receiver in the vehicle. Is another self immunity standard, of how vehicle receivers are immune to radiated emissions from its own systemsSAE J and SAE J are equivalent standards
42 CISPR 25 “Covers the frequency range from 150KHz- 1000MHz When an absorber lined chamber is used the absorption of the material has to be better than 6dB for the range 70MHz and up.For the chamber testing of subsystems a monopole is used for the range 150KHz to 30MHz, for 30MHz to 200MHz a biconical antenna is used, the log periodic is used for the range 200MHz-1000MHz. For equipment testing a TEM cell can be used.
43 EMC Chamber: CISPR 25 Chamber The CISPR-25 calls for reflectivity in the EUT area to be better than 6dB.There is no method for testing this.Normal incidence performance of absorber is the best way to determine the reflectivity in the area given the test geometry
44 EMC Chamber: CISPR 25 Chamber A recommended practice is to map the field along the cable harness although the standard does not mentions anything about any chamber validation method. This method can help compare the results in two different chambers.
48 EMC Chamber: CISPR 25 Chamber Apart from component testing the CISPR 25 rooms could accommodate some vehicle testing if the floor is reinforced.
49 CISPR 25 TEM CellsAdditionally CISPR 25 allows for testing of equipment in TEM cells and other TEM like devices
50 Automotive Testing: A Short Introduction 50Every manufacturer has its own requirements (usually very difficult to meet).Automotive standards are actually rather simple.The most common are SAE, ISO and 95/54 EC. These usually are copies of each other with small difference.The previous slides look at the FACT 25 chamber which can be used for automotive component testing for all these standards.A short introduction to emission testing of whole vehicles is presented now
51 Automotive Testing: A Short Introduction 5110 metersLPDABICONICALHORNThe 10m emission testing locates the antenna 10m from the outer shell of the vehicleThe antenna is not scan but located at 3m height. (For 3m testing the antenna is located at 1.8meters.Both sides of the vehicle and both polarizations are tested10 meters
52 Automotive Testing: A Short Introduction 52Plane of longitudinal symmetry10 metersmid point of enginecompartmentAntenna in line withThe antenna is to be in line with the middle point of the engine compartment.A two antenna position chamber makes the test much easier10 metersThe antenna not in use is setAt a different polarization toreduce coupling between antennas10 meters
53 EMC ChamberThere are a lot of different Standards in EMC. When a different standard request appears the RF engineer must analyze the requirements of the standard and recommend a proper solution.Also customers may have their own special requirements. Their company may have internal requirements for testing. RF engineering analysis must be conducted to see how to meet these requirements (or if is even possible to meet them.Most chambers are required to meet several standards.
54 Antenna Chamber: Requirements The purpose of these chambers is to measure the radiation pattern and characteristics of a radiatorFrequency range:Far field Requirement:Quiet Zone Size:This may determine if a tapered or rectangular chamber should be usedDirectly related to the previous requirement since is related to the wavelength at the lowest frequency:The far field distance will determine the path length and hence the chamber length.Requirementsd
55 Antenna Chamber: Requirements 3. Quiet Zone Size:4. Source Antenna Selection:Test region where the level of reflected energy is equal or smaller than certain specified valueMust be large enough to encompass the largest antenna being measured:It also determines the size of the chamber, the rule of thumb is that width and height of chamber be at least 3 times the Qz diameter or side.Can be:SphericalCubical volumeCylindricalRequirements
56 Antenna Chamber: Requirements 4. Source Antenna Selection:5. Back wallConsiderations:Rectangular chamber: the source antenna must have a pattern that illuminates the whole Qz while avoiding the side walls.Tapered chambers: Small antennas better. 15dB gain at least. No LPDA as phase center moves.RequirementsAbsorber normal incidence must be at least equal to Qz level, avoid lights, cameras and doors, ( for both rectangular and tapered.)
57 Antenna Chambers: Rectangular and Tapered Free Space conditionWhat Antennas can be measured? Omni-directional and directional.TaperedQuasi-free Space.Absorber treatment is used to create a far field free space behavior of the waves at the location of the antenna under test.Lower frequency antenna patterns can be measuredIt can be used for High frequency testing but positioning of the source antenna is critical
58 Antenna Chambers: Rectangular and Tapered CRITERIARECTANGULARTAPEREDAntenna PatternsPoor at LF, good at mid and HFGood at Low Mid and High FSource placementNot criticalCritical at HFSource antennaLimited by Far field15dB directivityAxial ratio<0.1dB<0.5dBCross polarization> 35dB>25dBSwept frequency measurementsIdeal configurationNot recommendedAmplitude taper (Qz)Freq. independentFreq. DependentPhase deviation (Qz)Boresite errorlowPotentially high
59 Antenna Chamber Rectangular I Top (or side view)PyramidABQz2Path lengthPyramid
60 Antenna Chamber Rectangular II Design of Rectangular chambers:The application of the chamber will determine the Qz size and the Path length and with it the size of the chamberDetermining the specular performance:Based on the thickness of absorber thebehavior at different incident angles can becomputed.QzAssume a chamber with: width “B”; path length “L”; Qz radius “r”, thenPath lengthrBdIt is desirable to have <45º
61 Antenna Chamber Rectangular III With the value of it is possible (based on the thickness of the absorber in terms of wavelengths) to determine the expected reflectivity.With the known directivity of the antenna andThe knowledge of it is possible to computeThe gain of the antenna in that directionThe reflection at the edge of the quiet zoneIs given by:Where R is the absorber reflectivity and G is the gain of the source antennaQzPath length
62 Antenna Chamber: The Absorber Treatment Back wall(receive end wall)Normal Reflectivity better than QZ levelSide wallOblique incidence Reflectivity with off main beam gain better than QZ level
63 Antenna Chamber: The Absorber Treatment Side wall absorber is only needed on those areas where a specular reflection exists between the source and the QZEverywhere else shorter absorber can be used
64 Antenna Chamber: The Absorber Treatment Transmit end wall absorber can have a reflectivity that when added to the front to back ratio of the source antenna it meets the required QZ level
65 Antenna Chamber: The Absorber Treatment For a given pyramid element size chosen there is no expected backscattering component. The scattered field is a sum of all the possible grating lobe waves which propagate in different directions, Only those where the following condition is satisfied contribute to the scattering at a distance [*]For m=0 and n=0 we have specular reflection only. For higher order modes to propagate we see that the period of the structure has to be larger than the wavelength[*] W. Sun, C. Balanis “Analysis and Design of Periodic Absorbers by Finite-Difference Frequency-Domain Method” report No. TRC-EM-WS-9301 Telecommunications Research Center, Arizona State University, Tempe, AZ 1993.
66 Antenna Chamber: The Absorber Treatment At high frequencies the antenna under test may re-scattered the backscattered energy from the pyramidal absorber surrounding it
67 Antenna Chamber: The Absorber Treatment Traditionally in RCS chambers the backscatter of the side walls (and ceiling/floor pair) is to be reduced using Wedge. By using wedge around the QZ section of the chamber we can improve the quality of the measurements at high frequencies
68 Antenna Chamber: The Absorber Treatment Top (or side view)PyramidWedge2QzPyramidBPyramidWedgePyramidA
69 QZ FF Far Field and QZ Absorber reflectivity, chamber size, Gain of source antennaQZFFAntenna size and frequency of operation
70 QZ reflectivity can be found. For any frequency. Far Field and QZKnowing the chamber size, absorber layout,Gain of source antenna, and QZ diameterQZ reflectivity can be found. For any frequency.But that does not imply that the whole QZ is in the FF
71 Far Field and QZ If the path length is fixed That it is possible to determine what is the QZ diameter that will be in the far field assuming illumination by spherical waves from a point source at the location of the source antenna
72 Far Field and QZ So for a given size So for a given sizeChamber we can provide the QZ reflectivity for a set of frequencies and different source antenna gains.The example shows a 20ft by 10ft by 10 ft chamber with 18” and 24” absorber and a 2ft diameter QZ.
73 Far Field and QZBut of the given 2ft of QZ and for a fixed 13ft path length only smaller spheres are in the FF of the source antenna
74 Antenna Chamber Tapered I Tapered chamber concept was develop to avoid the deficiencies of the Rectangular chambers at low frequenciesAt frequencies below 500MHz: Horns are no longer an option (very large).Less efficient antennas must be used.The thickness of the side wall absorber has to be increased to allow for good performance (and the chamber size increases to accommodate absorber).Tapered chambers do not eliminate the specular reflection. The specular region is located close to the aperture of the source antenna.The resulting Quiet Zone amplitudeand phase tapers approach thoseExpected in free-space hence the termQUASI-FREE-SPACETop (or side view)PyramidQz2Wedge
75 Antenna Chamber Tapered II This area absorber is criticalPyramidTop (or side view)This area absorber is less than ¾ thickness2WedgeQzWedgePyramidPyramidWedgePyramidTesting antennaStd Horn or ridge horn dipole minimum, No Log Periodic (phase center moves away from the side walls)
76 Antenna Chamber Tapered III Qz needs to be 1/3w clear from the sideshalf wavelength clear from absorber tipsApex angle less than 28 deg. Hence lengthTop (or side view)PyramidQz2Wedge
77 RCS Chambers I To measure Radar cross section We only want reflection from the targetWedge is used on part of the walls, ceiling and floor to reduce reflections from incidence on the flat part of the pyramids.The target illumination mechanism changes depending on frequency and type of radar.Top (or side view)WedgePyramidA/3ABPyramidPyramid
78 The RCS ChamberTo measure Radar cross section (That is, the energy that bounces back from a target to the source of the original incident signal MONOSTATIC, or to a receiver located at a different point BISTATIC)We only want reflection from the target coming backWedge is used on part of the walls, ceiling and floor to reduce reflections from incidence on the flat part of the pyramids.
79 RCS Chambers IIReflection of the back wall will limit how small of a RCS can be measured in the chamber.Assuming a good target illumination system is used the RCS of the back wall can be given by the RCS of an infinite reflective wallMinus the normal incidence reflectivity of the absorber placed on the back wall.D
80 RCS Chamber: Target Illumination Top (or side view)PyramidWedgeA/3ABTop (or side view)PyramidWedgeA/3ABAlways try to reduce the any energy going to the side walls.Two antennas (one receive one transmit)One antenna (receive and transmit)Compact range reflectorTop (or side view)PyramidWedgeA/3AB
81 ReferencesBrownell F. P. “Radio Frequency Anechoic Chambers” lecture materials, Microwave Antenna Measurement short Course, fb Consultants Camarillo,California.Kraus J. D. Antennas, 2d Ed. McGraw Hill: Boston, Ma, 1988.Balanis C. A. Antenna Theory: Analysis and design, 2d Ed, Wiley: New York, NY, 1997.Liu K. Private Communication