Qian Xu

Project 139: Developing Eco-friendly Radio Absorbing Materials (RAM) for

Anechoic Chambers

http://www.cgeinnovation.org/http://www.rainfordemc.com/

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Outline

• Background Information

• Research Questions

• Systematic Solutions

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Background Information

Pictures from: ETS LINDGRENhttp://www.ets-lindgren.com/

Micro-level Macro-level

How to design the chamber?

Pictures from: www.cst-china.cn

Γ , , ?

RAM Type?

RAM

Thickness?

Field Uniformity?

Chamber Shape?RAM Arrangement?

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William H. Emerson, IEEE Trans. on Antenn. And Propag., Vol. AP-21, No. 4, July, 1973.

• Mid 1930’s: Theoretical and experimental work.• 1936: First patented absorber.• WWII(1939-1945): Germany:

Schornsteinfeger(Wesch material & Jauman absorber ), US: HARP(Halpern-anti-radar-paint) by Halpern at MIT Radiation Laboratory, Salisbury screen.

• 1945-1950: Broad-band absorber.• The early 1950’s: First “dark rooms”.• The late 1950’s: New generation of broad-band.

absorbers(-40dB normal), antenna pattern comparison method, new chamber shape, shielded anechoic chambers.

• 1960’s: Low frequency absorber(-40dB 100MHz), 3rd generation RAM (-60dB normal), improved absorber measurement(Free-space VSWR), tapered chamber by Emerson.

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S. M. J. Razavi, M. Khalaj-Amirhosseini, and A. Cheldavi, PIER B, Vol. 19, 367-383, 2010.

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Outline

• Background Information

• Research Questions

• Systematic Solutions

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Research Questions

Pictures from: http://www.rainfordemc.com/anechoic-materials.html

1. How to choose/analyse/optimise RAM?

Ferrite tiles Hybrid Pyramid

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Research Questions

Picture from: http://www.rainfordemc.com/aircraft-chamber.html

2. How to design/analyse/optimise the whole chamber?

Picture from: http://www.mobilemag.com/2010/07/16/apples-100-million-test-chamber-droid-eris-and-blackberry-bold-9700-suffer-the-same/

Tapered Chamber Aircraft ChamberCompact Chamber

Picture from: http://gtresearchnews.gatech.edu/gtri-compact-range/

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Outline

• Background Information

• Research Questions

• Systematic Solutions

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Systematic Solutions: Macro-level

• Key problem: Whole chamber design

• Commercial Software: CST (FEM, FITD), HFSS (FEM), Feko (MoM)

• Friendly GUI, General simulation software, Pricy, High performance computers(cluster), Time consuming

Trial and error/cut and try Ray Tracing Full wave simulation

Accuracy

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Trial and error• Empirically based, mostly determined by lower frequency limit.• Thickness~Reflectivity

• Extraneous energy level~Reflectivity (Free Space VSWR

Method)

Picture from: Electromagnetic Anechoic Chambers A Fundamental Design and Specification Guide, Leland H. Hemming, Chapter 9.3.2

22DL

1~3

TestArea WW=1/2L

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Free Space VSWR Method

Repeated again for horizontal and vertical polarisation.

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Free Space VSWR Method

If a 0.3dB ripple is observed 20dB down from the peak, the extraneous signal level is -55dB

Ripple: max

min

20log d r

d r

E E E

E E E

/20

/20

10 120log 20log

10 1r

d

E

E

Extraneous signal :

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Full Wave Method• Algorithm limit

• Consider lossy(inhomogeneous) material

FEM, FDTD, GO

Frequency Electrical Size/Physical Size

Electrical Volume

10MHz 0.5λ/15m 0.125λ3

100MHz 5λ/15m 125λ3

1GHz 50λ/15m 125E3λ3

10GHz 500λ/15m 125E6λ3

40GHz 2000λ/15m 8E9λ3

N3 MoM N2 FEM

N1.1~1.2 FDTDN*log(N) MLFMM

N PO/SBR/GO

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Ray Tracing/GO(Geometric Optics)

• High frequency approximation

T=R+G+P(dB) normalized to direct ray in dB

T=Total strength R=reflection coefficientG=Antenna pattern gain P=Propagation parameter

0ik L rE r Ee 0ik L rH r He

0

n iL r H E H

k

0k c

0

iL r E n H E

k

n c

0

limk

2 2L r n

Ref: Electromagnetic Wave Theory, J. A. Kong, p.722.

Ref: Electromagnetic Anechoic Chambers A Fundamental Design and Specification Guide, Leland H. Hemming, Chapter 5.2.3

1st

orde

r

2 nd order

0

iL r E E

k g g

0

iL r H H

k g g

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Ray Tracing/GO Workflow

Mesh Generation,Material/Boundary Assignment

Ray Tracing Code

Field Distribution

Performance & Cost

Preprocessing: Modeling (.stl)

, ,

GND, side-wall, back-wall….different area with different reflectivity

GPU,Parallel

1st order, 2nd order, 3rd order reflection(Propagation direction, Amplitude, Phase)

Modified rectangular, Compact Range, Tapered

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Expected Results• Performance

Field uniformity (±?dB)

• Cost

• Debug/Optimize

Area Price

1st order, 2nd order, 3rd order reflection can be used to find the source of extraneous signal.

Ref: Minimum usage of ferrite tiles in anechoic chambers, S. M. J. Razavi, M. Khalaj-Amirhosseini, and A. Cheldavi. PIER B, Vol. 19, 367-387, 2010.

Thank you!