Statistical Investigation of Frequency-Stirred Reverberation Chambers

1. Statistical Investigation of Frequency-Stirred Reverberation Chambers Shih-Pin Yu Charles F. Bunting Founded by NASA Grant NCC-1-01032

2. Statistical Investigation of Frequency-Stirred Reverberation Chambers What is the Reverberation Chamber ? • Reverberation chamber is an electrically large cavity consisting of metal walls with a perturbation mechanism inside to obtain an uniform, isotropic and randomly polarized electromagnetic environment throughout the working volume. • Conventional way to perturb the field inside the chamber is by using metallic stirrer/tuner(s) to continuously change the boundary conditions in the chamber. • A novel way to perturb the field is by using a amplitude and frequency modulated source instead of a CW source.

3. Mechanical-Stirred Reverberation Chambers *RTCA/DO-160D, "Environmental Conditions and Test Procedures for Airborne Equipment (Draft #9 for the Change Notice to Section 20)," October 6,2000.

4. 0 Frequency-Stirred Reverberation Chambers • The rotation of the mechanical stirrer has equivalent effect to frequency and amplitude modulation. Frequency domain Time domain *D. I. Wu and D. C. Chang, "The Effect of an Electrically Large Stirrer in a Mode-Stirred Chamber,“1989

5. Frequency-Stirred Reverberation Chambers *M. L. Crawford, T. A. Loughry, M. O. Hatfield, and G. J. Freyer, "Band-limited, White Gaussian Noise Excitation for Reverberation Chambers and Applications to Radiated Susceptibility Testing," National Institute of Standards and Technology Technical Note 1375, 1996.

6. Why Frequency-Stirred Reverberation Chambers ? • Real-time homogeneous field can be achieved. • Test time reduced up to a factor of 200 compare with mechanical stirring.

7. How to achieve it ? • There are many ways to achieve frequency stirring by using different sources, one of them is Band Limited White Gaussian Noise (BLWGN) source. • By given a designated source bandwidth, the frequency stirring can be applied to reverberation chambers.

8. Y b I0 (x0,y0) a X Hill’s 2-D simulation (1994) • D. A. Hill performed a 2-D simulation in 1994 using a electric line source for frequency stirring and obtain a good field uniformity result. Line source

9. Good uniformity! Is Field Uniformity Good Enough ?

10. Theoretical curve Theoretical curve Statistics of the E-field (2-D)

11. The Effects on the Statistics • Mode structure

12. The Effects on the Statistics • Mode structure High number of possible modes !

13. The Effects on the Statistics • Line source bandwidth Several modes exist even in 1 MHz BW !

14. More modes exist ! Few modes exist ! The Effects on the Statistics • Chamber Q

15. How to Improve the Result ? • Lower chamber Q • Increase the line source bandwidth Standard derivation lower than 1 dB!

16. Statistics with lower Chamber Q Worse Standard derivation due to lower Q !

17. Theoretical curve Theoretical curve Statistics with increased line source band width Better curve fitting !

18. Y Aperture X 3-D Simulation using Modal_MoM • Modal_MoM was developed by M.D. Deshpande for the simulation of the shielding effectiveness of the lossless cavity with apertures on it. • A 3-D simulation of frequency stirring is performed by using the aperture as the excitation source to the chamber.

19. 3-D Simulation using Modal_MoM

20. Theoretical curve Theoretical curve Statistics of the E-field (3-D)

21. Conclusion and Future Work • In 2-D simulation the field uniformity dose not indicate good statistics of the field itself. • In 2-D simulation, in order to be analogous to mechanical stirring, the chamber Q and the line source bandwidth should be carefully choose.

22. Conclusion and Future Work • The 3-D simulation of a lossless cavity shows poor field uniformity and field statistics indicate not enough modes existing in the frequency stirring simulation. • A thorough investigation should be performed for further use of the real frequency stirring in reverberation chambers.