a dielectric substrate having two opposing surfaces;

a radiation conductor plate formed on one of the surfaces of the dielectric substrate;

a feed point provided on the radiation conductor plate;

a ground conductor plate formed on the other surface of the dielectric substrate;

at least one directivity switching element provided on the ground conductor plate side of the dielectric substrate; and

at least two polarization switching elements provided on the ground conductor plate side of the dielectric substrate, wherein,

the radiation conductor plate is shaped so as to be axisymmetrical with respect to a line extending through a center of gravity of the radiation conductor plate and through the feed point, the feed point being a point where feeding means is in contact with the radiation conductor plate;

the at least one directivity switching element includes

a first slot which is formed by removing a loop-like portion from the ground conductor plate, and

at least two directivity switching switches which are connected so as to bridge between an internal conductor surrounded by the first slot and the ground conductor plate surrounding the first slot;

the first slot resonates at a frequency which is substantially equal to a resonant frequency of the radiation conductor plate;

the peripheral length of the first slot corresponds to one effective wavelength at an operating frequency;

the directivity switching switches are positioned so that, when the first slot is split into a plurality of slots in high-frequency terms by allowing all of the at least two directivity switching switches to conduct, the length of each slot having been split at both ends which are the at least two directivity switching switches is less than half the effective wavelength, or is greater than half the effective wavelength and yet less than one effective wavelength;

the at least two polarization switching elements each include

a second slot which is formed by removing a loop-like portion from the ground conductor plate, and

at least one polarization switching switch which is connected so as to bridge between an internal conductor surrounded by the second slot and the ground conductor plate surrounding the second slot;

a portion of the second slot is in a position overlapping the radiation conductor plate;

the circular polarization index Q0(Δs/s) has a value of no less than 0.8 and no more than 1.6, where Δs is an area of an overlap between the radiation conductor plate and a region surrounded by each second slot; s is an area of the radiation conductor plate; and Q0 is an unloaded Q of the radiation conductor plate; and

with respect to an angle ξ between a line extending through the center of gravity of the radiation conductor plate and through the feed point and a line extending through the center of gravity of the radiation conductor plate and through a center of gravity of the second slot,

one second slot of the at least two polarization switching elements is provided so as to satisfy either a range of 0°<ξ<90° or a range of 180°<ξ21 270°; and

another second slot of the at least two polarization switching elements is provided so as to satisfy either a range of 90°<ξ<180° or a range of 270°<ξ360°.