Upgrade of the EM transient range monocone antenna

Lawrence Livermore National Laboratory has operated an electromagnetic transient range facility (EMTRF) since 1968 using a succession of antennas. This report describes the latest upgrade involving a precision monocone antenna and the use of a shroud of EM absorbing material strategically placed around the antenna to significantly enhance the clear time. This upgrade has resulted in highly repeatable measurements of exterior and interior responses of test objects for frequencies up to approximately 3 GHz for times on the order of hundreds of nanoseconds. Highly resonant interior responses can now be resolved experimentally. The radiated signal is nearly as clean as that from a simple vertical wire radiator and the field intensity is 12 dB larger. The cone is very nearly a true conic with very small azimuthal variations. As expected, the field behaves as 1/r for periods on the order of the time required for the transient pulse to each the cone edge. This perturbation is minimized by the use of strategically placed EM absorber which also serves to increase the clear time by minimizing room reflections. With improve pulsers it may be possible to extend the range of operation of this facility to 10 to 2 GHz.