Effect of defocus on the prompt response of a reflector IRA

J. Scott Tyo, Everett G. Farr, Dean I. Lawry

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


Impulse radiating antennas (IRAs) are an emerging class of antenna that are designed to radiate extremely short electromagnetic pulses with multiple decades of instantaneous bandwidth. The most common IRAs are made with a transverse electromagnetic (TEM) transmission line feeding a paraboloidal reflector. The IRA is usually constructed so that the electrical feed point of the TEM transmission line coincides with the focal point of the paraboloid. The paraboloidal reflector converts the spherical wave emanating from the feed point into a plane wave (i.e., a spherical wave centered at -∞). In many practical cases, the feed point and focal points are not exactly aligned, producing some defocus of the reflector. In this paper, we model the case of hyperbolic defocus because of its analytic form and compare this model with experimental measurements. A hyperboloidal reflector fed from one focus 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). Hyperboloidal defocus is roughly equivalent to moving the electrical feed closer to the reflector than the optical focal point. Previous theoretical results from in-focus IRAs predicted that the E- and H-plane temporal responses should be symmetric with respect to the temporal center of the response. The results shown here demonstrate that the defocusing causes these responses to become asymmetric. The new results are in better agreement with experimental measurements of IRAs and provide a physical explanation for experimental results that differ from the original theory.

Original languageEnglish (US)
Pages (from-to)3247-3254
Number of pages8
JournalIEEE Transactions on Antennas and Propagation
Issue number10
StatePublished - Oct 2005


  • Impulse radiating antennas (IRAs)
  • Physical optics
  • Time-domain electromagnetics
  • UWB electromagnetics
  • Ultrawide-band (UWB) antennas

ASJC Scopus subject areas

  • Electrical and Electronic Engineering


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