Copyright 2003, ETS-Lindgren, L.P. An Introduction to RF Anechoic Chamber Technology Vicente...

Copyright 2003, ETS-Lindgren, L.P.

An Introduction to RF Anechoic Chamber

TechnologyVicente Rodriguez, Ph.D.

ETS-Lindgren

1301 Arrow Point Dr.

Cedar Park, TX, 78613

[email protected]


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 C63.4-1992/1998; EN50147-2 (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 frequenciesIt can be used for low frequencies if size (length) is increased



Ferrite Tile .Magnetic LossesPreferred technology for Low frequencies (up to 1GHz), it has low profile.

It cannot be used for high frequencies



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.



Flat laminate .

Electric Losses

Preferred technology for laboratory set ups. It is a sandwich of different foams.

About 20dB absorption as frequency increases.



Wedge and pyramid

Electric Losses

A variant of pyramidal absorber wedge does not show backscattering. Preferred technology for QZ treatment and for RCS chambers.


Popular types of absorber have constitutive parameters of:

12

1

jr

r

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.

This material is volumetrically loaded having the same constitutive parameters through the volume of the pyramid

Pyramidal Absorber Theory (Example)


At the tip of the absorberThe wave impedance is that of air

At the base of the pyramidThe wave impedance becomes

oZ 0377

o

jZ 3.13252

12

377

NO SUDDEN CHANGE IN WAVEIMPEDANCE = LOW REFLECTIVITY

Along the length of the pyramid the wave impedance falls between those two values.



dBee

jjjj

x

mNp

o

380123.0

2235.0

35.046.1"'

1.02222

1.02

1.022

Let’s assume a length of 30cmThe wavelength at 3GHz is10cmAnd at 10GHz is30mm

Let’s approximate by saying that the pyramid is equivalent to a solid medium of 1/3 the height

For 3GHz

Wavelength at 3GHz

Approximate thickness of equivalent solid material



dBee

jjjj

x

mNp

o

1271029.4

7335.0

35.046.1"'

71.07322

03.02

03.022

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 significantlySmaller 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

1m1,3,5,10m


EMC Chamber

A

B

D Tx1

Tx2

QzdQzs

abs

abs

abs

Rx2

Rx1

Dsin()

mQzsmRxwhere

absQzsRxQzdDA

1 ,21 :

212

Rx1 can be a minimum of 1.5m

It is desirable that > 0 so that reflections from the side wallsDo not arrive in phase to the test area.

There are some rules that can be applied when sizing an EMC Chamber once the test distance and the quiet zone size are known


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

abs

1 – 4 meter scan

1.5m

2m

EMC Chamber

• A typical standard will have wording similar to this:

– ANSI C 63.4-1992 § 5.4.2, 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 5.4.6 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.

Chamber Validation Requirements

Vertical Polarization VNSA

EMC Chamber

Horizontal Polarization HNSA

EMC Chamber

Testing the Uniform Area according EN 61000-4-3. 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.

0.5 m

0.5 m

0.8 m

This plane includes the uniform area, 12 from 16 points of E-Field are within +6/-0 dB.

EMC Chamber


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 inAugust 20 1999

It superseding bothMil Std 461D and Mil Std 462DWhich passed away (rest in peace)


Mil Std 461E

Mil Std 461EIs made of many parts(which one are YOU interested in)

Conducted

Radiated

Emissions

Susceptibility

Emissions

Susceptibility


Mil Std 461E Conducted

Conducted

Emissions

Susceptibility

CE 101

CE 102

CE 106

CS 101

CS 103

CS 104

CS 105

CS 109

CS 114

CS 115

CS 116


Mil Std 461E Conducted Emissions

CE

CE 101

CE 102

CE 106

Conducted Emissions, power leads 30HZ-10KHz

Conducted Emissions, power leads 10KHz-10MHz

Conducted Emissions, Antenna terminal, 10KHz to 40GHz

No chamber requiredShielded room recommended.

LISN, receivers, O’cope, data recorders, sig gens,Current probe, etc are part of the required Equipment


Mil Std 461E Conducted Susceptibility (Immunity)

CS

CS 101

CS 103

CS 104

CS 105

CS 109

CS 114

CS 115

CS 116

Conducted Susceptibility, Power leads, 30Hz, to 150KHz

Conducted Susceptibility, Antenna Port, Intermodulation, 15KHz to 10GHz

Conducted Susceptibility, Antenna Port, rejection of Undesired signals, 30Hz to 20GHz

Conducted Susceptibility, Antenna Port, Cross-modulation, 30Hz to 20GHzConducted Susceptibility, Structure Current, 60Hz to 100KHz

Conducted Susceptibility, Bulk Cable Injection, 10kHz to 200Mhz

Conducted Susceptibility, Bulk Cable Injection, Impulse excitation

Conducted Susceptibility,Damped Sinusoidal transients, cables and power leads, 10KHz to 100MHz


Mil Std 461E Radiated

Radiated

Emissions

Susceptibility

RE 101

RE 103

RE 105

RS 101

RS 103

RS 105

Mor

e deta

ils ab

out

equi

pmen

t and

facil

ities

will

be

give

n


Mil Std 461E Radiated Emissions

RE

RE 101

RE 102

RE 105

Radiated Emissions, Magnetic Field, 30Hz to 100KHz

Radiated Emissions, Electric Field, 10KHz to 18GHz

Radiated Emissions, Antenna Spurious and harmonic Outputs, 10KHz to 40GHz


Mil Std 461E Radiated Susceptibility (Immunity)

RE

RS 101

RS 103

RS 105

Radiated Susceptibility, Magnetic Field, 30Hz to 100KHz

Radiated Susceptibility, Electric Field, 2MHz to 18GHz

Radiated Susceptibility, Transient Electromagnetic field


Mil Std 461E Where do we test?

Paragraph 4.3.2

Paragraph 4.3.2.1

“To prevent interaction between the EUT and the outside environment, SHIELDED ENCLOSURES will be usually required for testing”

“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 requirementsTABLE I page 10

6dB absorption 80Mhz to 250Mhz10dB absorption 250Mhz and above


Mil Std Chamber side view• Mil Std 461 Defines the size of the chamber in terms if the EUT being measured,



Mil Std Chamber Side View• Mil Std 461 Defines the size of the chamber in terms if the EUT being measured,



Mil Std Chamber RE and RS Equipment

RE101

RECOMMENDED REQUIRED

CHAMBER

EMCO 3725-2MCurrent Probe

EMCO 4-TREMCO 7604



RE103

REQUIRED

CHAMBER

EMCO 3725-2M

EMCO 4-TREMCO 7-TR

EMCO 3301BEMCO 3110B

EMCO 3106

EMCO 3115

MIL STD 461E does not longer accept Log Periodic and spiral Logs only double ridge horns above 200MHz



RS101

RECOMMENDED REQUIRED

CHAMBER

EMCO 3725-2MCurrent Probe

EMCO 4-TREMCO 7605/7606



RS103

REQUIRED

CHAMBER

EMCO 3725-2M

EMCO 4-TREMCO 7-TR

EMCO 3301BEMCO 3109

EMCO 3106

EMCO 3115

MIL STD 461E does not longer accept Log Periodic and spiral Logs only double ridge horns above 200MHz



RS103

paragraph 5.19.4, 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 551-4 and SAE J 1113-41 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


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.


CISPR 25 Chamber Side View (Bicon)


CISPR 25 Chamber Monopole Testing


EMC Chamber: CISPR 25 Chamber

• 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


Automotive Testing: A Short Introduction

51

• 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

LPDA

BICONICAL

HORN

10 meters

10 meters


Automotive Testing: A Short Introduction 52

• 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

Plane of longitudinal symmetry

10 meters

mid point of engine compartment

Antenna in line withmid point of engine compartment

10 meters

The antenna not in use is setAt 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.


The purpose of these chambers is to measure the radiation pattern and characteristics of a radiator

Antenna Chamber: Requirements

Requirements

1. Frequency range:

2. Far field Requirement:

3. 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.

freq.lowest

22

dS d



Requirements

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.

Absorber normal incidence must be at least equal to Qz level, avoid lights, cameras and doors, ( for both rectangular and tapered.)


• Rectangular• 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


Antenna Chambers: Rectangular and Tapered

CRITERIACRITERIA RECTANGULARRECTANGULAR TAPEREDTAPERED

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) Freq. independent Freq. Dependent

Boresite error low Potentially high


Antenna Chamber Rectangular I

Pyramid

Top (or side view)

Pyramid

Pyramid

A

B

Pyramid

Qz

2

Path length

depthabsorber 24

y accuratell moreor 3

depthabsorber 2

freq.lowest

2freq.lowest

2

QzB

QzB

PL

QzPLAd


Antenna Chamber Rectangular II

QzPath length

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 becomputed.

Assume a chamber with: width “B”; path length “L”; Qz radius “r”, then

rBdLd

dL

)(tan

)(tan1

1

d

Br

It is desirable to have <45º


Antenna Chamber Rectangular III

QzPath length

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 direction

The reflection at the edge of the quiet zoneIs given by:

Where R is the absorber reflectivity and G is the gain of the source antenna

atat GRQz tyreflectivi


Antenna Chamber: The Absorber Treatment

Back wall(receive end wall)

Side wall

Normal Reflectivity better than QZ level

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



1. 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 [*]

2. 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

3. [*] 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.

2

22

22k

D

nk

D

mk

x

ix

y

iy

DD

22



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



1. 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



Pyramid

Top (or side view)

Pyramid

Pyramid

A

B

Pyramid

Qz

2Wedge

Wedge


Far Field and QZ

QZ )(xf Absorber reflectivity, chamber size,Gain of source antenna

FF )(xf Antenna size and frequency of operation


Far Field and QZ

QZ reflectivity can be found. For any frequency.But that does not imply that the whole QZ is in the FF

)(xf

Knowing the chamber size, absorber layout,Gain of source antenna, and QZ diameter


Far Field and QZ

If the path length is fixedThat 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

ANECHOIC CHAMBER DESIGN SUMMARY

Job # : 0 Customer: XXXXXXXXXXXXXX

19-Aug-03 Location: XXXXXXXXXXXXXX

SPHERICAL Quiet Zone 2 ft. dia.

Receive Endwall Abs Size = EHP-24 EHP- 24 CL

Specular Region Abs Size = EHP-18 EHP- 18 CL

Dimensions are (LxWxH)= 20 x 10 x 10

Path Length (ft) = 13

RCV Ant. Discrimination added

Angle of Incidence = 57.3 Degrees

SIDE WALLS, FLOOR/CEILING CALCULATIONS FOR

ANECHOIC CHAMBER QUIET ZONE PERFORMANCE

WORST CASE

Frequency Wavelength Thickness/ Absorber Spec 5 10 15 18

(MHz) (inches) Reflectivity (-DB)

1000 11.82 1.52 24 25 25 27 35 38

2000 5.91 3.05 33 34 36 43 45

2500 4.73 3.81 35 35 37 39 47 51

7000 1.69 10.66 48 50 49 51 55 55

8000 1.48 12.18 50 50 52 55 55

9000 1.31 13.71 52 50 51 53 55 55

14000 0.84 21.32 53 52 53 55 55

18000 0.66 27.41 53 50 52 53 55 55

Source Antenna Gain (DB)

GUARANTEED PERFORMANCE

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.


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

freq (MHz) c lambda QZ ft QZ m PL m PL ft1000 3.00E+08 0.3 2.5 0.762 3.87096 132000 3.00E+08 0.15 1.8 0.54864 4.013411 133000 3.00E+08 0.1 1.45 0.44196 3.906573 134000 3.00E+08 0.075 1.25 0.381 3.87096 136000 3.00E+08 0.05 1.05 0.32004 4.097024 138000 3.00E+08 0.0375 0.9 0.27432 4.013411 13

10000 3.00E+08 0.03 0.8 0.24384 3.963863 1318000 3.00E+08 0.016667 0.6 0.18288 4.013411 1340000 3.00E+08 0.0075 0.4 0.12192 3.963863 13


Antenna Chamber Tapered I

Top (or side view)

Pyramid

Pyramid

Pyramid

Qz

2

Wedge

Wedge

Wedge

Pyramid

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


Antenna Chamber Tapered II

Top (or side view)

Pyramid

Pyramid

Pyramid

Qz

2

Testing antennaStd Horn or ridge horn dipole minimum, No Log Periodic (phase center moves away from the side walls)

Wedge

Wedge

Wedge

Pyramid

This area absorber is critical

This area absorber is less than ¾ thickness


Antenna Chamber Tapered III

Top (or side view)

Pyramid

Pyramid

Pyramid

Qz

2

Wedge

Wedge

Wedge

Pyramid

1. Qz needs to be 1/3w clear from the sides2. half wavelength clear from absorber tips

3. Apex angle less than 28 deg. Hence length

depthabsorber 24

y accuratell moreor 3

freq.lowest

QzB

QzB

o

LF

B

Length

28

2width)(absorber 2

tan

2


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.

Pyramid

Pyramid

Top (or side view)

Wedge

Wedge

Pyramid

Pyramid

A/3A

B


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.

2DRCS

D


RCS Chamber: Target Illumination

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

Wedge

Pyramid

Pyramid

A/3A

B

Pyramid

Top (or side view)

Pyramid

Wedge

Wedge

Pyramid

Pyramid

A/3A

B

Pyramid

Top (or side view)

Pyramid

Wedge

Wedge

Pyramid

Pyramid

A/3A

B

Pyramid


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

Copyright 2003, ETS-Lindgren, L.P. An Introduction to RF Anechoic Chamber Technology Vicente...

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Transcript of Copyright 2003, ETS-Lindgren, L.P. An Introduction to RF Anechoic Chamber Technology Vicente...