Our work on this appears in SSN 430. A Half IRA consists of a half parabolic reflector fed through the ground plane by an electrically large coaxial feed. Due to high voltages, the coax must be electrically large, so a lens is required at the feed point to convert the plane-wave TEM mode to a spherical wave launched onto the feed arms. We have demonstrated such an antenna, building the lens from a combination of epoxy and titanium dioxide. These sensors overcome the problem of making fast impulse field measurements with derivative sensors, which have very low sensitivity and small effective areas at high frequencies.
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