An antenna device comprises a case and an antenna element. The antenna element is arranged in the case and radiates a radio wave with a predetermined frequency which has a plane of polarization in parallel to a predetermined plane. The case is formed with a predetermined opening. The predetermined opening has two main openings and a coupling opening. The two main openings are apart from each other in a first direction perpendicular to the predetermined plane. The coupling opening couples the two main openings together in the first direction. The coupling opening has a size smaller than another size of each of the two main openings in a second direction perpendicular to the first direction. The predetermined opening forms a split ring resonator which resonates at the predetermined frequency and radiates a radio wave corresponding to the radio wave radiated from the antenna element.
1 . An antenna device comprising:
a case made of conductor;
a printed circuit board arranged in the case and extending along a predetermined plane; and
an antenna element provided on a surface of the printed circuit board,
wherein:
the antenna element is arranged in the case and is configured to radiate a radio wave with a predetermined frequency which has a plane of polarization in parallel to the predetermined plane;
the case is formed with a predetermined opening;
the predetermined opening has two main openings and a coupling opening;
the two main openings are apart from each other in a first direction perpendicular to the predetermined plane;
the coupling opening couples the two main openings together in the first direction;
the coupling opening has a size smaller than another size of each of the main openings in a second direction perpendicular to the first direction;
the predetermined opening forms a split-ring resonator which resonates at the predetermined frequency; and
the predetermined opening is configured to radiate a radio wave, which corresponds to the radio wave radiated from the antenna element, outward from the case.
2 . The antenna device as recited in claim 1 , wherein:
each of the main openings has a rectangular shape;
the coupling opening linearly extends along the first direction;
the coupling opening has a first size in the first direction and a second size in the second direction; and
the first size is larger than the second size.
3 . The antenna device as recited in claim 2 , wherein the two main openings have shapes same as each other.
4 . The antenna device as recited in claim 2 , wherein:
the case is formed of a metal plate; and
a size of the coupling opening in the second direction is equal to or less than four times a thickness of the metal plate.
5 . The antenna device as recited in claim 2 , wherein a size of the coupling opening in the second direction is equal to or less than one-fiftieth a length of an inner edge of each of the main openings.
6 . The antenna device as recited in claim 1 , wherein the two main openings have shapes same as each other.
7 . The antenna device as recited in claim 1 , wherein:
the case is formed of a metal plate; and
a size of the coupling opening in the second direction is equal to or less than four times a thickness of the metal plate.
8 . The antenna device as recited in claim 1 , wherein a size of the coupling opening in the second direction is equal to or less than one-fiftieth a length of an inner edge of each of the main openings.