PURPOSE: To optimize the gain of a dipole antenna with a reflecting plate, by taking an interval between a convex reflecting plate and the dipole antenna as ≤1/4 the wavelength of electromagnetic waves.

CONSTITUTION: A spherical reflecting plate 4 forms a convex reflecting plate, and after electromagnetic waves from a dipole antenna 1 are reflected at the plate 4, the phase of electric field produced on an aperture 5 is delayed compared to the case of a virtual plane type reflecting plate 6. When a distance (h) between the plate 4 and the antenna 1 is regulated to ≤λ/4 of employed electromagnetic wavelength, the direct wavs from the antenna 1 and the reflecting waves from the plate 4 are summed in inphase, allowing to optimize the gain toward the main radiation.