PURPOSE: To uniformize the excitation of a radiation element over a broad band by extending plural microstrip line feeders radially from a center feeding point, branching the feeders in parallel toward the circumference resulting in increasing number of the feeders thereby reducing the length of the feeders.

CONSTITUTION: Strip line feeders 1, 3, 5, 8, 10 with a narrow width and strip line feeders 2, 4, 9, 11 with a wide width at an interval of 1/2 wavelength are installed. Then microstrip antenna arrays 14, 15 whose radiation is set in response to the difference from the width of the feeders are connected in parallel as two branches from the feeders' side at a phase difference of 180° with a deviation of 1/2 wavelength. Thus, the difference from the line width of each radiation unit is increased as the point is remoter than the feeding side. Then the travelling wave series feeding system is adopted and the feeders are arranged side by side at a phase difference of 180° so that the narrow width feeder part is opposite to the wide width feeder part. As a result, the length of the feeders is reduced, the spurious radiation of the narrow width feeders is suppressed and the radiation element is uniformly excited in the direction of the feeding end.