Standing Waves and Notches in an EMP Simulator and Their Reduction

The high standing-wave ratio in a model simulator in the midfrequency range defined by kh = 27¿h/¿ ~ 2¿ (where h is the height of the parallel-plate region) is investigated experimentally. It is shown that the isolated deep minimum or "notch" is due to the mutual cancellation of the imaginary parts of the TM01 and TEM modes. The effects on the SWR and the deep minimum of changes in the magnitude and location of the terminating resistance are investigated. Also studied are the introduction of a bifurcating plate, resistive modal filters, and series aprons. It is shown that, with the proper adjustment of the last named, the "notch" can be eliminated and the SWR reduced to near two at discrete frequencies that span the entire midfrequency range. Since the low-frequency and high-frequency ranges already have low SWR\´s, the simulator has been made effective over the entire frequency band, in the sense that the SWR is low.