Effect of Defocusing on the Prompt Response of an IRA: I. Hyperboloidal Reflector

Abstract The reflector on an impulse radiating antennas (IRAs) is normally designed to be paraboloidal in shape, and are oriented so that the electrical feed point of the TEM feed line coincides with the focal point of the paraboloid. In many practical cases, the feed point and focal points are not exactly aligned, producing some defocus of the reflector. There are two types of defocus that can be analyzed exactly in the geometric optics approximation: converting the paraboloid into either a hyperboloid or a prolate spheroidal (elliptical cross-section) reflector. The paraboloidal reflector converts the spherical wave emanating from the feed point into a plane wave (i.e. a spherical wave focused at∞). The hyperboloid converts the expanding spherical wave into a second expanding wave which appears to emanate from the second focal point of the hyperboloid (which is behind the reflector); the prolate spheroid converts the expanding spherical wave into a second spherical wave that converges on the second focal point of the spheroid (which is in front of the reflector). The two cases are roughly equivalent to moving the electrical feed closer to the reflector (hyperboloid) or further from the reflector (prolate spheroid). In this paper, we examine the effect of hyperboloidal defocus. Previously [1], we derived the Eand H-plane responses from an in-focus IRA, and demonstrated that these responses are symmetric with respect to the center of the response. The results shown here demonstrate that the defocusing causes these responses to become asymmetric. These results explain the responses that have been measured from supposedly in-focus IRAs in the past.