A system was developed for accurately and repeatedly measuring the relative dielectric constant and conductivity of phantom modeling research materials. The frequency range of interest was 10 to 100MHz. The system is based on an antenna modeling theorem relating the impedance of a short monopole antenna in air to its impedence in dielectric media with high radiofrequency power absorption such as phantom modeling materials and biological tissues. System implementation resulted in an equipment configuration consisting of a signal source, network analyzer, and small in-vivo probe. By comparing measured electrical property data with corresponding reference data published in the open literature system, accuracy and repeatability were demonstrated. Water, methanol (67561), ethylene- glycol (107211), saline solutions, and phantom modeling materials were used in this data comparison. Accuracies within plus or minus 5% were demonstrated for both discrete and swept frequency measurements.