1. Project 139: Developing Eco-friendly Radio Absorbing Materials (RAM) for Anechoic Chambers
Qian Xu

2. Outline
Background Information
Research Questions
Systematic Solutions

3. Background Information
Micro-level Macro-level
RAM Type?
RAM Thickness?
How to design the chamber ?
Field Uniformity?
Chamber Shape?
RAM Arrangement?
Pictures from: ETS LINDGREN
Pictures from:

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

5. S. M. J. Razavi, M. Khalaj-Amirhosseini, and A. Cheldavi, PIER B, Vol

6. Outline
Background Information
Research Questions
Systematic Solutions

7. Research Questions 1. How to choose/analyse/optimise RAM?
Ferrite tiles
Hybrid
Pyramid
Pictures from:

8. Research Questions
2. How to design/analyse/optimise the whole chamber?
Tapered Chamber
Compact Chamber
Aircraft Chamber
Picture from:
Picture from:
Picture from:

9. Outline
Background Information
Research Questions
Systematic Solutions

10. 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

11. Trial and error
Empirically based, mostly determined by lower frequency limit.
Thickness~Reflectivity
Extraneous energy level~Reflectivity (Free Space VSWR Method)
W=1/2L
Picture from: Electromagnetic Anechoic Chambers A Fundamental Design and Specification Guide, Leland H. Hemming, Chapter 9.3.2

12. Free Space VSWR Method
Repeated again for horizontal and vertical polarisation.

13. Free Space VSWR Method If a 0.3dB ripple
Extraneous signal :
If a 0.3dB ripple
is observed 20dB down from the peak, the extraneous signal level is -55dB

14. Full Wave Method Algorithm limit
Consider lossy(inhomogeneous) material
FEM, FDTD, GO
N PO/SBR/GO
N*log(N) MLFMM
N1.1~1.2 FDTD
N2 FEM
N3 MoM
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

15. Ray Tracing/GO(Geometric Optics)
High frequency approximation
2nd order
1st order
T=R+G+P(dB)
normalized to direct ray in dB
Ref: Electromagnetic Wave Theory, J. A. Kong, p.722.
T=Total strength R=reflection coefficient
G=Antenna pattern gain P=Propagation parameter
Ref: Electromagnetic Anechoic Chambers A Fundamental Design and Specification Guide, Leland H. Hemming, Chapter 5.2.3

16. Ray Tracing/GO Workflow
Preprocessing: Modeling (.stl)
Modified rectangular, Compact Range, Tapered
Mesh Generation,
Material/Boundary Assignment
GND, side-wall, back-wall….
different area with different reflectivity
Ray Tracing Code
Field Distribution
1st order, 2nd order, 3rd order reflection
(Propagation direction, Amplitude, Phase)
GPU,
Parallel
Performance & Cost

17. Expected Results Performance Field uniformity (±?dB) Cost
Debug/Optimize
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, , 2010.

18. Thank you!