We have developed a technique for measuring from boreholes the radiation pattern of a high frequency electromagnetic antenna system. Three coplanar holes are required with a single transmitter in one and two identical receiving antennas in the other holes. Signal attenuation is measured along several ray paths at different angles from the source. The method does not depend on the subsurface to be electrically homogeneous but does require reflections and refractions to be negligible. Measurements in fractured granite demonstrate the utility of the technique to obtain antenna radiation patterns. A modification of this same method could be used for characterization of downhole acoustic sources.