Radio-frequency resonant cavity measurements for rapid, accurate assessment of body composition and human exposure to electromagnetic fields

Body composition measurements play an important role in nutritional studies, renal medicine and sports science, while human exposure to electromagnetic fields (EMF) is an area of growing concern owing to the implementation of Directive 2013/35/EU on EMF. Resonant cavity techniques offer an attractive alternative to traditional methods in these fields, as they allow rapid, non-invasive measurements without using ionising radiation. At frequencies of a few tens of MHz, a large screened room can act as a cavity resonator. A human subject inside the room perturbs its low-order resonances, and the resulting shift in frequency depends on the tissue dielectric properties, which correlate strongly with water content. This approach has been well tested and shows good agreement with current methods of measuring total body water. The number of resonant modes increases rapidly with frequency, so if we instead use microwaves at 1GHz and above, many modes can be excited simultaneously. Adding a rotating paddle to the room creates a ‘stirred mode’ environment, where the body is effectively illuminated by microwave radiation from all directions. For EMF exposure studies this a more realistic scenario than considering only a single direction and polarisation. The average absorption cross section (ACS), which is closely related to specific absorption rate (SAR), can be rapidly obtained over a very broad band (1GHz to 15GHz and beyond), whereas the alternative is detailed computer simulations that take many hours for just a single frequency. At these microwave frequencies, the field penetration into tissues is a few cm, so the ACS gives useful information about the composition of tissues near the body surface. Normalising the ACS to body surface area gives us an ‘absorption efficiency’ that is independent of body size. Results will be presented of the relationship between this parameter and the thickness of subcutaneous body fat. Both techniques are comfortable for the subject, use safe levels (around 1mW) of non-ionising radiation, and allow measurements to be made in less than 10min

Time Domain Technique for Rapid, Broadband Measurement of Human Absorption Cross Section in a Reverberation Chamber

Absorption cross section (ACS) of an object is used in stochastic power balance models, while human ACS is closely related to microwave dosimetry parameters such as specific absorption rate (SAR) and thus characterises exposure as well as effect of human bodies on multipath propagation. ACS, averaged over all directions of incidence, can be obtained in the frequency domain from the S-parameters of two antennas in a stirred-mode reverberation chamber; however, our new time domain method is faster, avoids the need to determine antenna efficiency, and has been validated with a test object of calculable ACS. We can now measure human ACS from 1 to 18GHz, to within 3%, in under 10 minutes. We have done this for 48 subjects, and explored correlations between ACS and body parameters including mass, height, surface area and subcutaneous fat thickness

Structured Mesh Generation : Open-source automatic nonuniform mesh generation for FDTD simulation

This article describes a cuboid structured mesh generator suitable for 3D numerical modelling using techniques such as finite-difference time-domain (FDTD) and transmission-line matrix (TLM). The mesh generator takes as its input an unstructured triangular surface mesh such as is available from many CAD systems, determines a suitable variable mesh discretisation and generates solid and surface meshes in a format suitable for import by the numerical solver. The mesher is implemented in the MATLAB language and is available as open source software

On Measurement of Reverberation Chamber Time Constant and Related Curve Fitting Techniques

The reverberation chamber time constant quantifies how fast a reverberation chamber loses its stored energy at different frequencies, which makes it a very important parameter in many power related tests, such as the measurement of antenna efficiency, the measurement of absorption cross section, and the electromagnetic immunity test of electronic devices. The chamber time constant is usually obtained by doing regressions of power delay profile and calculating its gradient. But the shape of power delay profile can sometimes be distorted by the band limited window function applied in the frequency domain. A non-linear curve fitting technique which can cancel the effect of window function was developed, aiming to give a robust determination of the chamber time constant. With the help of this technique, window functions with much smaller bandwidth can be applied without introducing error in the evaluation of chamber time constant. In this paper, a 1 MHz wide window function in which only 10 samples of S21 are available was put under test and it was found a robust answer of chamber time constant can still be given by non-linear curve fitting techniques. Therefore the measurement time can be reduced and the frequency resolution of the chamber time constant can be increased at the same time

Statistical distributions of the re-radiated spectrum from two correlated non-linear devices in a reverberation chamber

The statistics of the re-radiated spectrum from two correlated non-linear devices are investigated in a reverberation chamber. The distribution of the mean-value normalized statistics is interpreted using a double-Weibull statistical model. Comparisons are made with the re-radiation spectrum of a single non-linear device showing the statistical distributions to be different. Furthermore, experiments indicate the spatial correlation between the two non-linear devices changes the statistical distributions. This work enhances the understanding about the statistical aspects of the re-radiated spectrum from complex digital equipment

A rectangular waveguide cell for measurement of the shielding effectiveness of anisotropic materials

A method for measuring the shielding effectiveness of planar anisotropic materials using a rectangular waveguide cell is proposed. Computational models are used to verify the behaviour of the cell and validate its ability to measure shielding effectiveness. Results of measurements on a control sample and three carbon-fibre composite laminates in a GTEM cell are presented. A dynamic range of 70 dB with a capability to discriminate 20 dB of anisotropy is achieved in the frequency range 400-1600 MHz using cubic cells of side length 200 mm and 300 mm

Virtual HIRF tests in CST STUDIO SUITE™ : A Reverberant environment application

The prediction of the shielding performances of reverberant environments such as vehicle bodiesand metallic enclosure racks is an important part of the global design process of an electromagnetic system in order to ensure predefined performance specifications and fulfill the EMC standards. The desire for a reduced time-to-market means that there is often no time or budget for performing extensive prototyping and test measurements. These limitations make the reliable computation and prediction of the shielding properties of a given structure critical. To speed-up this time-consuming process numerical electromagnetic methods can be successfully adopted to analyze and characterize the system under test in terms of shielding performance. The data analysis and validation of results can be challenging due to rapid variation of data over a small frequency range and due to the high sensitivity to small geometric variations in such a strongly resonant environment. In this paper the studied test case consists of a metal enclosure, which is illuminated externally, and the internal response observed for different configurations of the internal structure including: coupling to straight and curved single wire lines; coupling through different types of aperture and material in the box walls and loading of the cavity by lossy materials to vary the Q-factor. The results are all presented as a reception aperture, a figure described by the ratio between the power received at a probe antenna and the power density of incident field. CST STUDIO SUITE™ was used to analyze the electromagnetic behavior of the aforementioned configurations. The different geometries have been constructed inside the frontend thanks to its powerful CAD capabilities. A plane wave has been used as excitation and receiving probes connected to a matched load (discrete ports) have been defined in order to calculate the reception aperture as a post processing step. Two different numerical EM methods have been applied to the case-study in order to cross-check the accuracy of the simulated results in the frequency range 1 - 6 GHz: 1) CST MICROWAVE STUDIO® Transient solver based on the Finite Integration technique (FIT). 2) CST MICROWAVE STUDIO® Frequency Domain solver based on the Finite Element method (FEM). The computed results showed a good agreement with the measured curves over the whole frequency range for all the examined configurations

Circuit and Electromagnetic Modelling of a low cost IEMI Sensor

The design of a low cost broadband Intentional Electromagnetic Interference (IEMI) detector and antenna to achieve a flat frequency response over a broad range is considered. SPICE simulation of the antenna, detector and low power log-amplifier circuit is used to predict the detector performance. The SPICE Antenna model is derived from numerical electromagnetic simulation. Simulations are compared with measured performance

Broadband Measurement of Absorption Cross-Section of the Human Body in a Reverberation Chamber

We present broadband reverberation chamber measurements of the absorption cross section (ACS) of the hu- man body averaged over all directions of incidence and angles of polarization. This frequency-dependent parameter characterizes the interactions between the body and the enclosures of rever- berant environments such as aircraft cabins, and is therefore important for the determination of the overall Q-factor and hence the field strength illuminating equipment inside such en- closures. It also correlates directly with the electromagnetic expo- sure of occupants of reverberant environments. The average absorption cross section of nine subjects was measured at fre- quencies over the range 1-8.5 GHz. For a 75 kg male the ACS varied between 0.18 and 0.45 square meters over this range. ACS also correlated with body surface area for the subjects tested. The results agree well with computational electromagnetic simu- lations, but are obtained much more rapidly. We have used the obtained values of absorption cross section to estimate the effect of passengers on the Q-factor of a typical airliner cabin. Index Terms—reverberation chamber, absorption cross