PURPOSE: To obtain an antenna device with an increased directivity and gain for observing the ionized layers by constituting ≥2 triangle antennas where the base having a feeding point is shared in common.

CONSTITUTION: In a travelling wave feeding antenna where a triangle is constituted by using conductors 1, 2, 3, 4 longer than the wavelength, its vertex is provided with a terminating load 5 and its base is provided with a feeding point 6, its bases 3, 4 are provided in parallel with a ground 7, a triangle antenna is constituted by sharing the bases 3, 4 in common with conductors 8, 9 longer than the wavelength and a terminating load 10. Each main beam from the conductors 1, 2, 8, 9 is matched to a normal taken above the ground 7 by selecting suitably the length of the conductors 1, 2, 8, 9, an angle to the ground 7, and phase adjusters 11, 12, allowing to increase the directivity and the gain.