The types of antenna array commonly used in the UK in conjunction with high-power h.f. transmitters and their principles of operation are outlined. Methods of predicting field strengths and power densities in both the near and far fields of such arrays are discussed. Particular reference is made to curtain arrays, log-periodic antennas, and rhombic antennas. Typical field strength patterns are illustrated for each type.An application is the prediction of the external radio frequency environment in which aircraft and flight weapons systems have to operate.