Jean-Charles Bolomey

RFID Multiprobe Impedance-Based Sensors

In this paper, a passive multiprobe sensor using conventional UHF radio frequency identification (RFID) tags is presented. The RFID link between the reader and the tag is analyzed from a general point of view by means of reciprocity principle, to obtain a compact and explicit expression of the response of RFID tags that can be exploited in sensing measurements. A calibration scheme to remove nonlinearities in the response of RFID tags, which are due to the scavenging powering mechanism of passive tags, is also presented along with measurements. Finally, experimental results and discussion of a complete multiprobe sensing scenario are used to show the different challenges involved and how to overcome them.

Rapid microwave imaging of living tissues

Microwave imaging is a relatively new modality to perform non- invasive diagnostic of biological tissues. The potential of these techniques results from the dependence of the dielectric properties of these tissues with respect to quantities of practical and clinical relevance such as water content, blood flow rate, temperature, etc. Initiated in the 80s, these techniques have suffered from the complexity of the interaction mechanisms between microwave beams and biological structures. The compensation of strong diffraction effects by high dielectric contrast structures requires efficient reconstruction algorithms. Recently, major advances have been achieved for improving the time resolution and the quantification of microwave images. This paper is focused on the first aspect. It reports some unique results obtained, for the first time, with a microwave camera providing qualitative images of biological targets at the rate of 15 images per second. Image reconstruction is performed by means of spectral diffraction tomography algorithms. The camera consists of a 2D array of 32 X 32 sensors, covering a square area of approximately 22 cm X 22 cm. The camera is operated at 2.45 GHz, according to the Modulated Scattering Technology (MST). The biological target is immersed in water (or can be inserted between bolus) and illuminated by a lens-compensated horn antenna. From the amplitude/phase measurement of the field scattered by the target, microwave images can be reconstructed, thanks to numerical focusing, in any plane located between the transmitting antenna and the camera. Typically, the investigation depth is 25 cm, and the spatial resolution is about 5 mm. The capabilities of this microwave camera will be illustrated by means of a short VHS video tape showing quick motions of living structures. Expected improvements of the camera performances are discussed and possible clinical applications are analyzed.

Three decades of active microwave imaging achievements, difficulties and future challenges

This paper provides an overview of the microwave imaging techniques, which have been developed during the last three decades for ISM applications. This overview is not limited to the state of the art in terms of software and equipment issues. Taking profit of a significant time stand off, the real impact of these techniques with respect to their targeted applications is critically analyzed. Beyond undisputable achievements, specific persisting difficulties are identified as well as future challenges.

Passive RFID based sensing

The principle of communication of passive RFID, which relies on the Modulated Scattering Technique (MST) to transmit the information from the tag to the reader, opens a way to enable conventional RFID for sensing purposes, without any additional circuitry. In this paper a short description of the physical principle behind the sensing is presented along with measurements of RFID tags used for sensing in multiple applications, such as electromagnetic field measurement temperature or presence detection.

On the Statistics of Reverberation Chambers and Applications for Wireless Antenna Test

In this paper, statistical tools were applied to clarify the Rayleigh distribution of the fading signals in a reverberation chamber. Autocorrelation and cross-correlation functions, AFD and LCR are measured. The signals correlation coefficient versus spatial locations is assessed. With the above considered reverberation chamber, measurements reveal a good agreement for emulating mobile fading channels. The measurement procedure described in this paper can be used to verify the performance of any chamber for wireless characterizations

A dual band back coupled meanderline antenna for wireless LAN applications

We present a novel dual band printed meander line antenna with a shaped ground plane and a back coupled rectangular patch. The aim was to achieve 2 GHz and 5 GHz covering the two main frequency bands of commercially available WLANs. To obtain a dual band with wide bandwidth operation, a meander line microstrip printed antenna with a matched feed and a shaped ground plane was first designed at 2.45 GHz. Then, a rectangular patch was added at the back of the meandered antenna substrate, with a small gap between the rectangular patch and the shaped ground plane. Changing the length of the rectangular patch allows the resonance frequency to be tuned, for instance 5.25 GHz. In fact, the resonance frequency can be continuously adjusted between 5.2 GHz and 7 GHz by changing the rectangular patch length. Furthermore, the tuning of the second resonance frequency has very little effect on the first one. The obtained bandwidth is 11% at 2.45 GHz and 6% at the second frequency for a return loss less than -10 dB. Consequently, this antenna offers sufficient flexibility for all types of wireless local area networks in the available frequency bands. This small antenna structure (31/spl times/8/spl times/1.6 mm/sup 3/) and its achievable low price with an SMA type connector, make it appropriate for Notebook, PC, PDA and other commercial WLAN communications applications.

New concepts for microwave sensing

For a long time, microwaves have been considered as a possible sensing agent for nondestructive testing/evaluation purposes. This trend has still been reinforced these last years with the advent of new microwave penetrable materials, such as composites. Inspection of materials via a mechanically scanned probe has proven to offer a convenient, but time consuming, way to measure local reflexion or transmission coefficients and, hence, to evaluate defects, faults, etc... High speed measurements are now possible by using arrays of fixed probes, resulting in attractive imaging equipments. Indeed, the availability of amplitude/phase data allows us to consider different processing techniques, the complexity of which can be selected according to the required performances in terms of contrast, spatial and time resolutions. This paper reviews some of the most promising approaches, such as non-linear inverse scattering techniques and neural networks. Prospective considerations are devoted to the future of such sophisticated microwave sensing techniques.

High-Sensitivity Optically Modulated Scatterer for Electromagnetic-Field Measurement

The optically modulated-scatterer (OMS) technique is developed for electromagnetic-field-distribution measurement with minimum disturbance to the field under test. In this paper, an OMS with newly designed photoconductive-switching structure is proposed. The performances of the new OMS are evaluated with a monostatic-field-measurement system. Measurement results show that an improvement of 6 to 8 dB in sensitivity is achieved compared to previous OMS devices. The developed OMS was used in an electromagnetic-field-distribution mapping system to measure the field distribution in a cubic phantom radiated by a mobile phone. The results show the suitability of this OMS for specific-absorption-rate measurement application

Influence of source - phantom multiple interactions on the field transmitted in a flat phantom

For many applications involving antenna systems in the vicinity of lossy scatterers, such as phantoms used for Specific Absorption Rate (SAR) assessment, it would be of great interest to know how local absorption can be related to the free-space radiated electromagnetic field. However, when the source closely approaches e. g. a phantom, complex interactions mechanisms begin to occur, leading to modifications of the nearfield effectively impinging on the phantom. Based on plane-wave expansion, this paper proposes an analysis of the contribution of the free-space field to the field transmitted in a phantom, as the antenna-phantom distance is varied. The particular cases of λ/2 dipoles at 900 and 1800 MHz close to a flat phantom are studied numerically. The results surprisingly show that the electric field topography inside the phantom is poorly distorted by multiple interactions, even for distances as short as 2.5 mm. It appears that this distortion is generally even weaker, for source-phantom distances where the modulus of the reflection coefficient at the antenna input port is high.

Diversity Techniques with Dipole Antennas in Indoor Multipath Propagation

In this paper, we investigate the correlation coefficient and the diversity gains of two parallel dipole antennas in various configurations. The correlation coefficient in an indoor environment is first computed from the 3D farfield radiation patterns of each antenna; then, it is measured inside a mode stirred reverberation chamber used as a model of indoor multipath. The diversity gains for switch combining and maximum ratio combining versus the dipole separation are derived from the measurements. Depending on the dipoles separation, three types of diversity techniques are defined: space, pattern and phase diversity. At large distances, the correlation coefficient is highly controlled by the space factor, but when the antennas are approached (up to 0.05 wavelength), phase diversity is the most dominant factor which greatly reduces the correlation coefficient and allows an efficient diversity