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channel sounder which is a measuring system for the radio propagation characteristics of the subject undersea envi-ronment.2Development of underseawater channel sounder2.1Configuration[3]Figure 2 shows the appearance of the underseawater channel sounder. e size is 2.7 m (depth) × 2.5 m (width) × 2.2 m (height), and the weight is around 500 kg. It is designed to be able to resist the hydraulic pressure at a depth of 500 m from the surface. e channel sounder is mainly composed of the undersea antennas for measure-ment that send and receive, a switch to switch a connected antenna (Antenna switching device), and a vector network analyzer to measure the radio propagation characteristics between antennas.Concerning the undersea antennas, one is placed for sending and three for receiving on the stands, -arrayed at a 60 cm distance between each antenna. e distance be-tween antennas for sending and receiving can be changed to up to about 2 m. is makes it possible to measure the propagation loss and phase rotation amount, which are necessary for the design of an undersea wireless transmis-sion method, between antennas for sending and receiving. In addition, as the sending antenna is placed on the single dimensional positioner (Y stage) and it is possible to emit radio waves by changing the position for a sending source, the undersea direction estimate experiment for a wave source can be conducted using three receiving antennas. is experiment result is important to establish the signal processing technology on the radar under the seabed and so on. e underseawater channel sounder is equipped with a CTD meter to measure the electric characteristics of the sea, a triaxial inclination sensor to measure the tilt of the channel sounder, an underwater camera to monitor the measuring system, and a battery to drive them, as well as a radio wave propagation measuring system. ese de-vices all connect to the shipboard controller through the optical cable in order to be controlled and transmit data remotely.Figure 3 shows the appearance of the undersea antenna used by the underseawater channel sounder. e undersea antenna is composed of a cylindrical container (size: 320 mm dia., 253 mm high) which is lled with fresh water and in which a magnetic loop antenna (black element in Fig. 3) is arranged. e fresh water layer is provided with an aim to improve the electrical matching and to have a role to absorb the eect of the hydraulic pressure exerted on the element. e resonance frequency band of this antenna (underwater) is 10 MHz. is is selected as the frequency band making it possible to measure the radiated electromagnetic eld even taking into account the distance between the sending antenna and receiving antenna of the underseawater channel sounder. e passband width for these antennas is around 45 kHz, if the frequency band-width is VSWR 2.0 or less.2.2Example of measured result [4]-[7]e measurement in waters was made in the large-scale water tank facilities (40 m long, 4 m wide and 2 m deep), which JAMESTEC possesses, with the aim of the func-tional verication of the developed undersea radio wave propagation measuring system. Figure 4 shows the descrip-tion at the time of the functional verication. As this verication is the measurement underwater, the measure-ment was made aer removing the sending and receiving antennas from the frame of the underseawater channel FiF3　Appearance of undersea antenna developedFiF4Function verification of underseawater channel sounder using large-scale water tank facilities2 Terrestrial Communication Technology Research and Development54　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 2 (2017)