An Introduction to RF Anechoic Chamber Technology

Name: An Introduction to RF Anechoic Chamber Technology
Uploaded: 2017-06-27T19:20:13+00:00
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Description: An Introduction to RF Anechoic Chamber Technology

An Introduction to RF Anechoic Chamber Technology
Vicente Rodriguez, Ph.D. ETS-Lindgren 1301 Arrow Point Dr. Cedar Park, TX, 78613

SUMMARY The Chamber Family Absorber Materials The EMC Chamber
The antenna Chamber Tapered and Rectangular. RCS chambers References

Chamber Types: Two Main Families
EMC testing Properties: Semi-free space or half free space Absorber: 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 Software Std. 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 absorbers RCS: mostly military applications, is a chamber to measure radar cross section of a target Antenna: military and commercial, these are chambers to measure the radiation pattern of a radiator, could be an antenna or an antenna plus other system.

The Absorber Family 1 Microwave Pyramidal absorber. EMC and EHP series
Electric Losses Preferred technology for High frequencies It can be used for low frequencies if size (length) is increased

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

The Absorber Family 3 Hybrid Absorber . Electric and Magnetic Losses
Preferred technology for EMC Applications. 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.

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.

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.

Pyramidal Absorber Theory (Example)
This material is volumetrically loaded having the same constitutive parameters through the volume of the pyramid Popular types of absorber have constitutive parameters of: Non magnetic material Low permittivity with losses We will study how the electromagnetic wave behaves as is incident on to a wall of this type of absorber.

At the tip of the absorber The wave impedance is that of air NO SUDDEN CHANGE IN WAVE IMPEDANCE = LOW REFLECTIVITY Along the length of the pyramid the wave impedance falls between those two values. At the base of the pyramid The wave impedance becomes

Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the height Let’s assume a length of 30cm The wavelength at 3GHz is 10cm And at 10GHz is 30mm For 3GHz Wavelength at 3GHz Approximate thickness of equivalent solid material

Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the height For 10GHz Wavelength at 3GHz Approximate thickness of equivalent solid material In practice the reflection coefficient may not be as small as this but it will be significantly Smaller than at 3GHz

EMC Chamber Design is guided by the standards and the test that the customer is going to perform. Frequency range is from 30 to 1000MHz At 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 space NSA is a measurement of how close we are to this goal. Std.Docs call for NSA being within +- 4dB of the theoretical. 4m 1m 1,3,5,10m

EMC Chamber There are some rules that can be applied when sizing an EMC Chamber once the test distance and the quiet zone size are known A B D Tx1 Tx2 Qzd Qzs abs Rx2 Rx1 Dsin() a Rx1 can be a minimum of 1.5m It is desirable that  > 0 so that reflections from the side walls Do not arrive in phase to the test area.

EMC Chamber The 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 measurement A H D Tx1 1m abs 1 – 4 meter scan 1.5m 2m

Chamber Validation Requirements
EMC Chamber Chamber Validation Requirements A 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 5.4.3 What this means is that it must be shown that the chamber performs like an infinite ground plane with no obstructions anywhere.

EMC Chamber Vertical Polarization VNSA

EMC Chamber Horizontal Polarization HNSA

EMC Chamber Testing 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.

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 Bench 1m

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.

Mil Std 461E What 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

Mil Std 461E Mil Std 461 E came to be in It superseding both
August It superseding both Mil Std 461D and Mil Std 462D Which passed away (rest in peace)

Mil Std 461E Emissions Conducted Susceptibility Mil Std 461E
Is made of many parts (which one are YOU interested in) Emissions Radiated Susceptibility

Mil Std 461E Conducted CE 101 Emissions CE 102 CE 106 Conducted CS 101
Susceptibility CS 105 CS 116 CS 109

Mil Std 461E Conducted Emissions
CE 101 Conducted Emissions, power leads 30HZ-10KHz No chamber required Shielded room recommended. LISN, receivers, O’cope, data recorders, sig gens, Current probe, etc are part of the required Equipment CE CE 102 Conducted Emissions, power leads 10KHz-10MHz CE 106 Conducted Emissions, Antenna terminal, 10KHz to 40GHz

Mil Std 461E Conducted Susceptibility (Immunity)
CS 101 Conducted Susceptibility, Power leads, 30Hz, to 150KHz CS 103 Conducted Susceptibility, Antenna Port, Intermodulation, 15KHz to 10GHz CS 104 Conducted Susceptibility, Antenna Port, rejection of Undesired signals, 30Hz to 20GHz CS 105 Conducted Susceptibility, Antenna Port, Cross-modulation, 30Hz to 20GHz CS CS 109 Conducted Susceptibility, Structure Current, 60Hz to 100KHz CS 114 Conducted Susceptibility, Bulk Cable Injection, 10kHz to 200Mhz CS 115 Conducted Susceptibility, Bulk Cable Injection, Impulse excitation CS 116 Conducted Susceptibility,Damped Sinusoidal transients, cables and power leads, 10KHz to 100MHz

Mil Std 461E Radiated RE 101 Emissions RE 103 RE 105
More details about equipment and facilities will be given Radiated RS 101 RS 103 Susceptibility RS 105

Mil Std 461E Radiated Emissions
RE 101 Radiated Emissions, Magnetic Field, 30Hz to 100KHz RE RE 102 Radiated Emissions, Electric Field, 10KHz to 18GHz RE 105 Radiated Emissions, Antenna Spurious and harmonic Outputs, 10KHz to 40GHz

Mil Std 461E Radiated Susceptibility (Immunity)
RS 101 Radiated Susceptibility, Magnetic Field, 30Hz to 100KHz RE RS 103 Radiated Susceptibility, Electric Field, 2MHz to 18GHz RS 105 Radiated Susceptibility, Transient Electromagnetic field

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”

The Absorber Family EMC-24 Mil Std 461E requirements TABLE I page 10
6dB absorption 80Mhz to 250Mhz 10dB absorption 250Mhz and above

Mil Std Chamber side view

Mil Std Chamber Side View

Mil Std Chamber RE and RS Equipment
RECOMMENDED REQUIRED Current Probe EMCO M CHAMBER EMCO 7604 EMCO 4-TR

REQUIRED EMCO M CHAMBER EMCO 4-TR EMCO 7-TR EMCO 3115 EMCO 3110B EMCO 3301B EMCO 3106 MIL STD 461E does not longer accept Log Periodic and spiral Logs only double ridge horns above 200MHz

RECOMMENDED REQUIRED Current Probe EMCO M CHAMBER EMCO 7605/7606 EMCO 4-TR

REQUIRED EMCO M CHAMBER EMCO 4-TR EMCO 7-TR EMCO 3115 EMCO 3109 EMCO 3301B EMCO 3106 MIL STD 461E does not longer accept Log Periodic and spiral Logs only double ridge horns above 200MHz

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

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 systems SAE J and SAE J are equivalent standards

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.

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

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.

CISPR 25 Chamber Side View (Bicon)

CISPR 25 Chamber Monopole Testing

Apart from component testing the CISPR 25 rooms could accommodate some vehicle testing if the floor is reinforced.

CISPR 25 TEM Cells Additionally CISPR 25 allows for testing of equipment in TEM cells and other TEM like devices

Automotive Testing: A Short Introduction
50 Every 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 10 meters LPDA BICONICAL HORN The 10m emission testing locates the antenna 10m from the outer shell of the vehicle The 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 tested 10 meters

52 Plane of longitudinal symmetry 10 meters mid point of engine compartment Antenna in line with The antenna is to be in line with the middle point of the engine compartment. A two antenna position chamber makes the test much easier 10 meters The antenna not in use is set At a different polarization to reduce coupling between antennas 10 meters

EMC Chamber There 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.

Antenna Chamber: Requirements
The purpose of these chambers is to measure the radiation pattern and characteristics of a radiator Frequency range: Far field Requirement: Quiet Zone Size: This may determine if a tapered or rectangular chamber should be used Directly 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. Requirements d

3. Quiet Zone Size: 4. Source Antenna Selection: Test region where the level of reflected energy is equal or smaller than certain specified value Must 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: Spherical Cubical volume Cylindrical Requirements

4. Source Antenna Selection: 5. Back wall Considerations: 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. Requirements Absorber normal incidence must be at least equal to Qz level, avoid lights, cameras and doors, ( for both rectangular and tapered.)

Antenna Chambers: Rectangular and Tapered
Free Space condition What Antennas can be measured? Omni-directional and directional. Tapered Quasi-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 measured It can be used for High frequency testing but positioning of the source antenna is critical

Antenna Chambers: Rectangular and Tapered
CRITERIA RECTANGULAR TAPERED Antenna Patterns Poor at LF, good at mid and HF Good at Low Mid and High F Source placement Not critical Critical at HF Source antenna Limited by Far field 15dB directivity Axial ratio <0.1dB <0.5dB Cross polarization > 35dB >25dB Swept frequency measurements Ideal configuration Not recommended Amplitude taper (Qz) Freq. independent Freq. Dependent Phase deviation (Qz) Boresite error low Potentially high

Antenna Chamber Rectangular I
Top (or side view) Pyramid A B Qz 2 Path length Pyramid

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 chamber Determining the specular performance: Based on the thickness of absorber the behavior at different incident angles can be computed. Qz Assume a chamber with: width “B”; path length “L”; Qz radius “r”, then Path length r B   d It is desirable to have  <45º

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 and The knowledge of  it is possible to compute The gain of the antenna in that direction The reflection at the edge of the quiet zone Is given by: Where R is the absorber reflectivity and G is the gain of the source antenna Qz Path length  

Antenna Chamber: The Absorber Treatment
Back wall (receive end wall) Normal Reflectivity better than QZ level Side wall Oblique incidence Reflectivity with off main beam gain better than QZ level

Side wall absorber is only needed on those areas where a specular reflection exists between the source and the QZ Everywhere else shorter absorber can be used

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

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.

At high frequencies the antenna under test may re-scattered the backscattered energy from the pyramidal absorber surrounding it

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

Top (or side view) Pyramid Wedge 2 Qz Pyramid B Pyramid Wedge Pyramid A

QZ FF Far Field and QZ Absorber reflectivity, chamber size,
Gain of source antenna QZ FF Antenna size and frequency of operation

QZ reflectivity can be found. For any frequency.
Far Field and QZ Knowing the chamber size, absorber layout, Gain of source antenna, and QZ diameter QZ reflectivity can be found. For any frequency. But that does not imply that the whole QZ is in the FF

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

Far Field and QZ So for a given size
So for a given size Chamber 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.

Far Field and QZ But of the given 2ft of QZ and for a fixed 13ft path length only smaller spheres are in the FF of the source antenna

Antenna Chamber Tapered I
Tapered chamber concept was develop to avoid the deficiencies of the Rectangular chambers at low frequencies At 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 amplitude and phase tapers approach those Expected in free-space hence the term QUASI-FREE-SPACE Top (or side view) Pyramid Qz 2 Wedge

Antenna Chamber Tapered II
This area absorber is critical Pyramid Top (or side view) This area absorber is less than ¾  thickness 2 Wedge Qz Wedge Pyramid Pyramid Wedge Pyramid Testing antenna Std Horn or ridge horn dipole minimum, No Log Periodic (phase center moves away from the side walls)

Antenna Chamber Tapered III
Qz needs to be 1/3w clear from the sides half wavelength clear from absorber tips Apex angle less than 28 deg. Hence length Top (or side view) Pyramid Qz 2 Wedge 

RCS Chambers I To measure Radar cross section
We only want reflection from the target Wedge 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) Wedge Pyramid A/3 A B Pyramid Pyramid

The RCS Chamber To 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 back Wedge is used on part of the walls, ceiling and floor to reduce reflections from incidence on the flat part of the pyramids.

RCS Chambers II Reflection 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 wall Minus the normal incidence reflectivity of the absorber placed on the back wall. D

RCS Chamber: Target Illumination
Top (or side view) Pyramid Wedge A/3 A B Top (or side view) Pyramid Wedge A/3 A B Always try to reduce the any energy going to the side walls. Two antennas (one receive one transmit) One antenna (receive and transmit) Compact range reflector Top (or side view) Pyramid Wedge A/3 A B

References Brownell 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

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