Antenna device

An antenna device includes a ground conductor (1), a dielectric tube (2) containing an ionizing gas and passing through the ground conductor, whose folded-back portion (201) and both ends are disposed in different sides of the ground conductor, first electrodes (3, 4) disposed at both ends of the dielectric tube (2), a plasma excitation power supply connected to the first electrodes, bringing the ionizing gas into a plasma state; a second electrode (6) ring-shaped and disposed at the both ends side of dielectric tube (2) from the ground conductor (1), fitted to and in contact with the dielectric tube outer surface, a high frequency transmitter (7) supplying a high frequency signal to the second electrode (6), and a feed line (8) connecting the second electrode and the high frequency transmitter. Each end of the dielectric tube (2) is larger than the second electrode in the inner diameter.