Prepulse Associated with the TEM Feed of an Impulse Radiating Antenna

The field radiated fit)m an impulse radiating antenna (HL4) utilizing a reflector and driver by a step-function source consists of three distinct parts. First, there is a prepulse, which looks ikc a step function, and lasts for a time equal to twice the electrical length of the feed. Following this comes the actual impulse-like waveform. Finally, there is a small tail that appears after the impulse. Although the impulse portion of the waveform has been analyzed elsewhere, the prepulse has been only approximate i. The purpose of this note is to provide simple anal ytical formulas for calculating the prepul, e for the two most common feed types, two bent circular cones, and two conical coplanar plates. By ~enerating simple formulas for the forward and backward radiation on axis, one can compare the area of the prepulse to that of the impulse. After doing so, it is easy to identify cases where the areas of the impulse and prepulse are equal, to within a very small tolerance. Since the total area under the waveform must be zero, this suggests that the tail after the impulse has zero area, a necessary condition for making the tail small.