Antenna device

1. An antenna device installable in a vehicle, said antenna device comprising:

a stacked dipole antenna unit including a plurality of dipole antennas arranged parallel to a plane perpendicular to a front-back direction of the vehicle,

wherein:

the stacked dipole antenna unit further comprises a flat antenna substrate that has a front surface and a back surface, each of the front and back surfaces being provided with an antenna element unit;

each of the antenna element units comprises antenna elements; and

one of the antenna elements on the front surface and one of the antenna elements on the back surface function as one of the dipole antennas.

2. The antenna device according to claim 1 , further comprising:

a second antenna unit,

wherein the stacked dipole antenna unit and the second antenna unit perform wireless communication different from each other.

3. The antenna device according to claim 1 , wherein:

the stacked dipole antenna unit performs wireless communication between vehicles.

4. The antenna device according to claim 1 , wherein:

a height of a feeding point of each of the dipole antennas from a ground surface is 0.25λ or more and 1.0λ or less, where λ represents a wavelength of a radio wave that is received by the stacked dipole antenna unit.

5. The antenna device according to claim 1 , wherein:

a distance between the dipole antennas is 0.4λ or less, where λ represents a wavelength of a radio wave that is received by the stacked dipole antenna unit.

6. The antenna device according to claim 1 , further comprising:

a wave director that is arranged at a front position or at a back position of the stacked dipole antenna unit.

7. The antenna device according to claim 6 , wherein:

the wave director has a flat wave director substrate and a conducting unit, and

the conducting unit is formed on the wave director substrate and extends in a direction that is parallel to an extending direction of the dipole antennas.