The paper describes the methods and results of experiments made to map out the electromagnetic fields associated with a slot radiator. The radiator was a narrow slot about half a wavelength long cut in a large circular metal sheet of diameter about 9 wavelengths. The results include the relative magnitudes and phases of the electric vector over a quadrant of the metal sheet close to the slot, the relative magnitudes of the magnetic vector along the axes of the slot, and also some polar diagrams of radiation at relatively large distances from the slot, both on and off the sheet. The results obtained are correlated as well as possible with the theoretical expressions for the fields associated with a half-wave slot of infinitesimal width in a sheet of infinite extent. These theoretical expressions are derived from the usual formulae for the fields of a thin dipole by the use of an extension of Babinet's principle recently described by Booker.
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