From material absorption to dosimetry for exposure of animals in reverberation chambers

Philippe Besnier (1)

1 IETR - Institut d'Électronique et des Technologies du numéRique

This chapter discusses the role of energy absorption in a reverberation chamber and some related applications. The composite Q-factor is the key indicator of the total amount of losses in a reverberation chamber since it quantifies the energy stored (per unit of time) per unit of transmitted/dissipated power. In a first part of this chapter, we settled the definition of this composite quality factor and described a method to estimate it in different ways with a set of two antennas. The scattering parameters at antenna ports enable to estimate the composite Q-factor in three different ways and enables to check for the enhanced backscattered coefficient. Obviously, an accurate enough estimation requires a high number of RC states. We suggest that averaging over small frequency bands is a solution to minimize intrinsic statistical variation of Q-factors estimated from a limited set of stirrer positions. The first application consists of evaluating the average absorption cross section of any piece of material within the RC. This absorption coefficient is retrieved from the contrast of quality factors measurement before and after putting the material in the chamber. For simple geometries of the inserted absorber, it is possible to estimate its efficiency. Moreover, from the knowledge of the intrinsic EM properties of homogeneous and thick (with regard to the skin depth) materials, we showed that the measured AACS computed from the modification of the Q-factor is consistent with the theoretical one. This last fact was at the origin of a specific calibration procedure for mm-wave RC dedicated to animal exposure. This is the second application of this chapter. In order to calibrate the EM dose during exposure, the power density in the RC must be controlled and proved to be consistent with the expected temperature rise. Using a rectangular homogeneous phantom, mimicking the properties of animal’s skin, we show that the temperature rise at its surface and measured with an infrared camera was indeed predictable. On a theoretical point of view, the measurement of the Q-factor, the knowledge of intrinsic parameters of the phantom and the properties of the diffuse field in the RC fully determine the heat deposition on the phantom surface. Solving the heat-equation gives then the correct temperature rise prediction with a reasonable accuracy. Chapter Contents: • 6.1 Losses and Q-factor • 6.1.1 Role of losses in a RC • 6.1.2 Definition of the Q-factor • 6.1.3 Origin of losses and their contribution to the Q-factor • 6.1.4 Q-Factor measurements • 6.1.4.1 Test setup for Q-factor measurements • 6.1.4.2 Q-Factor measurements using the full scattering matrix • 6.1.4.3 Q-Factor measurements using the reflection coefficient from each antenna • 6.1.4.4 QTx and QRx measurements • 6.1.4.5 Experimental results of Q-factor estimation • 6.2 The average absorption effective area of an object in a RC • 6.2.1 The Q-factor of an object in a RC and its average absorption cross section • 6.2.2 Theoretical absorbing cross section of a rectangular piece of absorbing material • 6.2.3 Measurement examples • 6.3 Dosimetry for animals • 6.3.1 Using RC for animal exposure • 6.3.2 Theory about heat transfer in a RC • 6.3.2.1 The phantom • 6.3.2.2 The heat equation • 6.3.2.3 The electrothermal coupling • 6.3.2.4 The 1-D approximation • 6.3.2.5 Solution of the 1-D heat equation • 6.3.3 Experimental validation • 6.4 Discussion • References. © The Institution of Engineering and Technology 2021.

Mots clés: Absorption coefficient; Absorption cross section; Acoustic wave absorption; Acoustic wave scattering; Animal exposure; Animal skin; Antenna ports; Backscattered coefficient; Biomimetics; Biothermics; Calibration; Calibration procedure; Composite Q-factor; Diffuse field; Dissipated power; Dosimetry; Energy absorption; Heat deposition; Heat-equation; Infrared camera; Inserted absorber; Intrinsic EM properties; Intrinsic statistical variation; Material absorption; MM-wave reverberation chamber; Phantom surface; Phantoms; Power density; Q-factor; Rectangular homogeneous phantom; Reverberation; Reverberation chambers; Scattering parameters; Skin; Skin depth; Stirrer positions; Transmitted power

Domaines: Sciences de l'ingénieur [physics]

Philippe Besnier. From material absorption to dosimetry for exposure of animals in reverberation chambers. Electromagnetic Reverberation Chambers: Recent advances and innovative applications, Institution of Engineering and Technology, pp.175-204, 2021, 9781785619311. ⟨10.1049/SBEW544E_ch6⟩. ⟨hal-03370659⟩