PURPOSE: To operate an antenna over a wide frequency range by using an antenna of kind based upon angle principle, and holding the ratio of the phase center- to-center distance between a couple of antenna element and the wavelength of an incident electromagnetic wave invariably constant regardless of variation in frequency.

CONSTITUTION: Antenna elements 61 and 62 are conical spiral antennas based upon the angle principle. The origins of the antenna elements 61 and 62 which determine their structures, i.e. vortexes of cones of the elements 61 and 62 coincide with each other at a point 0, and their center lines contain an angle . The distance between the phase centers 64 and 65 of the elements 61 and 62 is denoted as D and the distance between the point 0 and phase centers 64 and 65 of the elements is denoted as L. A main radiation area moves on the elements 61 and 62 according to the frequency of the incident electromagnetic wave, and the distance L between equivalent phase centers 64 and 65 and the origin 0 of the structures is invariably constant when represented by a unit of wavelength λ.