A Capacitive Microwave Sensor With Guard Electrodes for Biological Cell Characterization

Parallel plate capacitor configurations are common for permittivity measurements. Depending on the exact geometry of the plates, the contribution of the usually unwanted fringing fields can be significant. At low frequencies, this effect can be simply avoided by means of properly driven guard electrodes. This way, only the approximately homogeneous field beneath the main electrode is measured. This paper proposes a microwave implementation of such a sensor and a simple method based on scattering parameter measurements for permittivity extraction. The intended application is the size independent assessment of biological cell properties in a microfluidic setup. The paper presents simulation results for a parallel plate cell sensor without and with a guard electrode. Experimental verification is provided with a scaled version realized in printed circuit board technology. The measurements are performed on a dielectric slab at different positions.