Radio-frequency identification (RFID) technology relies on the modulated scattering technique (MST) as a means to convey the information from the tag to the reader. The interaction mechanism between the reader and the tag is described in a physically meaningful way by the reciprocity theorem which is deeply rooted into Maxwell equations. This approach provides a simple and yet complete formulation that allows to fully describe the interaction between the reader and the tag, even in complex environments. From this formulation, a clear understanding is derived on how the different design tradeoffs affect system performance. Based on this approach, the paper is focused on several issues of practical relevance for antenna tag design such as maximum power transfer, maximum sensitivity, and nonlinearity effects.

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.

This paper presents microwave tomographic reconstructions of the complex permittivity of lossy dielectric objects immersed in water from experimental multiview near-field data obtained with a 2.45-GHz planar active microwave camera. An iterative reconstruction algorithm based on the Levenberg-Marquardt method was used to solve the nonlinear matrix equation which results when applying a moment method to the electric field integral representation. The effects of uncertainties in experimental parameters such as the exterior medium complex permittivity, the imaging system geometry and the incident field at the object location are illustrated by means of reconstructions from synthetic data. It appears that the uncertainties in the incident field have the strongest impact on the reconstructions. A receiver calibration procedure has been implemented and some ways to access to the incident field at the object location have been assessed.

For newly developed iterative Newton-Kantorovitch reconstruction techniques, the quality of the final image depends on both experimental and model noise. Experimental noise is inherent to any experimental acquisition scheme, while model noise refers to the accuracy of the numerical model, used in the reconstruction process, to reproduce the experimental setup. This paper provides a systematic assessment of the major sources of experimental and model noise on the quality of the final image. This assessment is conducted from experimental data obtained with a microwave circular scanner operating at 2.33 GHz. Targets to be imaged include realistic biological structures, such as a human forearm, as well as calibrated samples for the sake of accuracy evaluation. The results provide a quantitative estimation of the effect of experimental factors, such as temperature of the immersion medium, frequency, signal-to-noise ratio, and various numerical parameters.

A comparative study at 2.45 GHz concerning both measurement and reconstruction parameters for planar and cylindrical configurations is presented. For the sake of comparison, a numerical model consisting of two nonconcentric cylinders is considered and reconstructed using both geometries from simulated experimental data. The scattered fields and reconstructed images permit extraction of very useful information about dynamic range, sensitivity, resolution, and quantitative image accuracy for the choice of the configuration in a particular application. Both geometries can measure forward and backward scattered fields. The backscattering measurement improves the image resolution and reconstruction in lossy mediums, but, on the other hand, has several dynamic range difficulties. This tradeoff between forward only and forward-backward field measurement is analyzed. As differential temperature imaging is a weakly scattering problem, Born approximation algorithms can be used. The simplicity of Born reconstruction algorithms and the use of FFT make them very attractive for real-time biomedical imaging systems.