ON THE MEASUREMENT OF MICROWAVE PERMITTIVITY OF BIOLOGICAL SAMPLES USING NEEDLE-TYPE COAXIAL PROBES

In this paper, a new kind of coaxial probe for the measurement of radi o frequency and microwave permittivity of biological samples-a needle- type coaxial probe-is studied both theoretically and experimentally in detail. The standard coaxial probe, made by a right angle cut from a circular coaxial line, produces a circular radiator. The present probe is obtained by a slanted cut, and therefore the radiation aperture be comes elliptical. Both theoretical analysis and experiments show that the measurement sensitivity is improved with the increase of the bevel angle of the probe. Calibration results by using water and air at low frequencies are in agreement with theoretical calculation. Measuremen ts on three biological samples, sonicated DNA macromolecules, water so lutions of protein (the protamine), and epsilon-aminocaproic acid, are made from 1 MHz to 1 GHz. All the measured results are in very good a greement with similar tests obtained from the same materials using a s tandard probe. Theoretical and experimental results show that this typ e of probe has the following advantages: it improves the measurement s ensitivity, and it facilitates the insertion of the probe into some te st samples (e.g., biological tissues, a living body, etc.).