This paper describes a laboratory investigation of two types of dipolar RF admittance probes which could be of use in the ionosphere and inner magnetosphere. Simple theoretical models are presented, using the quasi-static approximation and vacuum sheath. Independent accurate measurements of both the electron density and temperature are made, using Langmuir probes and the dispersion properties of Bernstein waves, and the results are compared with the RF probe predictions. In general, good agreement is obtained, indicating both the usefulness of the probes as a way of measuring electron densities in space and the applicability of the model used. These results also give an estimate of the effective sheath thickness around the probe, which is smaller than previous measurements have suggested.