Antenna device including helical antenna

An antenna device comprising:

• a ground plane having a surface;
• a dielectric body disposed on the surface of the ground plane;
• a first antenna including a first helical part disposed in the dielectric body, the first helical part helically wound up in an axial direction perpendicular to the surface of the ground plane, the first helical part including a plurality of one-turn portions each having a peripheral length of M times a wavelength λ of use, where M is a positive natural number;
• a second antenna including a second helical part disposed in the dielectric body, the second helical part disposed outside the first helical part in a direction perpendicular to the axial direction so as to be away from the first helical part, the first helical part wound up along the axial direction;
• a feeding circuit including an oscillator, a divider coupled with the oscillator, a first phase shifter coupled with the divider and a feeding point of the first antenna, and a second phase shifter coupled with the divider and a feeding point of the second antenna, wherein
• the second helical part includes a plurality of one-turn portions,
• one of the plurality of one-turn portions of the second helical part closest to the surface of the ground plane has a peripheral length Ks that is N times the wavelength λ of use, where N is a positive natural number greater than M,
• in consecutive two of the plurality of one-turn portions of the second helical part, a first one closer to the surface of the ground plane has a peripheral length K1, a second one farther away from the surface of the ground plane has a peripheral length K2, and the peripheral lengths K1 and K2 satisfy a relationship of K1≧K2, and
• one of the plurality of one-turn portions of the second helical part farthest away from the surface of the ground plane has a peripheral length Ke that satisfies a relationship of (M·λ)<Ke<Ks(=N·λ)