A directional slot is used in aerials for the very high frequency hands, providing a high efficiency beam width of less than 30 deg. and providing minimum phase distortion. One single slot (F) is used in a rectangular wave-guide which is fed in the te01 mode. It is terminated in a matched load, Phase distortion is corrected by including in the waveguide a suitable dielectric material whose thickeness varies along the guide length. The slot itself, set in the width, ie the greatest cross section dimensioned face, is uniform over part of its length and then has a discontinuity such that it curves laterally away from the centre line of the wall and then returns to it.
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