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

A needle-type coaxial probe for the measurement of radio frequency and microwave permittivity of biological samples is studied both theoretically and experimentally. The probe is obtained by a slanted cut, and therefore the radiation aperture is elliptical. Both theoretical analysis and experiments show that the measurement sensitivity is improved with the increased bevel angle of the probe. Calibration results using water and air at low frequencies are in agreement with theoretical calculations. Measurements on three biological samples (sonicated DNA macromolecules, water solutions of protein (the protamine), and ∈-aminocaproic acid) were 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. The theoretical and experimental results show that the probe improves measurement sensitivity and it facilitates insertion into some test samples