A Passive Electrodeless Method for Determining the Interior Field of Biological Materials

A passive remote-sensing method is proposed for the measurement of the electric-field distribution within a biological sample. The instrument analyzes the intermodulation spectrum reradiated by a diode imbedded within the sample exposed to a dual-frequency illumination. Experimental results in a waveguide system indicate that the relative intensities of the intermodulation products are a unique function of the field intensity at the diode. These results are also used to determine the basic scattering properties of the diode. A low-power free-space apparatus is described which exposes the principal problem associated with this method: the inadvertent generation of intermodulation frequencies. Isolation techniques are described which eliminate this problem. Finally, an analysis of a complete system shows that at 910 MHz fields can be measured in 4.0 cm of soft tissue, tan delta = 0.5, without exceeding an incident power density of 10 mW/cm².