The finite-difference-time-domain (FDTD) method is used in an interactive mode to improve the performance of a TEM horn antenna for pulse radiation. The new design is shown to yield a small reflected voltage in the feeding transmission line, to radiate a far-zone field in the axial direction that is proportional to the derivative of the incident voltage, and to be fairly directive. Experimental results are in very good agreement with those predicted by the theory. © 1996 John Wiley & Sons, Inc.