On the measurement of microwave permittivity of biological samples using needle type coaxial probes

A new kind of coaxial probe for the measurement of radiofrequency and microwave permittivity of biological samples, the needle type coaxial probe, is studied both theoretically and experimentally. The authors' probe is obtained by a slanted cut and therefore the radiation aperture becomes 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 calculation data. Measurements on three biological samples-sonicated DNa macromolecules, water solutions of protein (the protamine), and ε-aminocaproic acid are made from 1 MHz to 1 GHz. All the measured results are in very good agreement with similar tests obtained from the same materials using a standard probe