Helical antennas in 920-MHz band for wireless sensor nodes under the ground

In this study, the radiation characteristics of the 920-MHz band helical antennas in free space and in the ground were analyzed and compared. Some lobes and nulls appeared in the underground case and the radiation toward the air from the underground was weakened as a whole compared to the case in free space. Based on the result, a wall was added to the ground conductor plate of the antenna, and the space between the helix and the ground was filled with a dielectric material. As a result, the radiated waves were almost uniformly toward the air, and the radiation was strengthened. This antenna is useful when connecting sensors underground that measures environmental data to a wireless network. By installing the antenna in the ground together with sensor nodes, measurement is possible without interfering with farm work.