Antenna device

1. An antenna device comprising:

a ground layer made of an electric conductor;

a plurality of array antennas provided in a layer above the ground layer so as to be spaced apart from the ground layer; and

a Rotman lens provided in a layer below the ground layer so as to be spaced apart from the ground layer,

each of the plurality of array antennas (i) including: a power feed line at a center of which a feedpoint is located; and a plurality of antenna elements connected to the power feed line and (ii) having a point symmetric shape with respect to the feedpoint as a center of symmetry,

the feedpoint of each of the plurality of array antennas being coupled to an end of any one of output ports of the Rotman lens via a slot provided in the ground layer,

wherein in a case where an effective wavelength, on the power feed line, of a center frequency of an operation band of the antenna device is defined as a center wavelength λ,

a branch section, which is a section at which each of the plurality of antenna elements is connected to the power feed line, is constituted by a plurality of unit sections which are continuously provided and each of which has a length of λ/4 along a direction in which the power feed line extends, and

the plurality of unit sections have respective widths each of which is determined so that characteristic impedances of each adjacent ones of the plurality of unit sections match each other.

2. The antenna device as set forth in claim 1 , wherein:

the branch section includes a first section, a second section, and a third section that are continuously provided from upstream to downstream along the power feed line;

each of the plurality of antenna elements is connected to the vicinity of a boundary between the first section and the second section;

the second section has a width with which a branching ratio at the branch section has a predetermined value;

the first section has a width with which a combined impedance between the second section and an antenna element branched from the branch section matches a characteristic impedance upstream of the branch section; and

the third section has a width with which a characteristic impedance of the second section matches a characteristic impedance downstream of the branch section.

3. The antenna device as set forth in claim 1 , wherein:

the number of the plurality of antenna elements is 4 or more; and

a branching ratio at a branch section at which each of the plurality of antenna elements is connected is lower as the branch section is provided more upstream along the power feed line and is higher as the branch section is provided more downstream along the power feed line.

4. The antenna device as set forth in claim 1 , wherein:

the power feed line includes (1) a power feed section including the feedpoint and extending along a first direction, (2) a first radiation section extending from one end of the power feed section along one of two directions of a second direction that intersects with the first direction, and (3) a second radiation section extending from the other end of the power feed section along the other of the two directions of the second direction;

one or more antenna elements connected to the first radiation section and one or more antenna elements connected to the second radiation section are arranged on the same straight line;

an end section including the end of the any one of the output ports of the Rotman lens, which end is coupled to the feedpoint, extends along the first direction; and

a section of the any one of the output ports which section is continuous with the end section extends along the second direction.

5. The antenna device as set forth in claim 1 , wherein the plurality of antenna elements are congruent.