Image Number 3 for United States Patent #6445356.
The primary radiator comprises a circular waveguide having a cone-shaped horn portion at one end and an enclosing surface at the other end, and a first and second probes inserted into the waveguide through a wall thereof. A plurality of cutout portions are formed at an open end of the horn portion. Two or more pairs of cutout portions are disposed symmetrically with respect to an axis of the waveguide and a depth of each cutout portion is adjusted to be about one quarter of the wavelength of the radio wave .lambda.0 transmitted through the air. With such a configuration, the phase reversal of surface currents flowing through the cutout portions and an adjacent projecting portion (a portion without cutout portions) take place, and a side lobe of a radiation pattern can be reduced considerably.