We studied the improved effects a diffuser had on the output power of small wind turbine systems, aiming to introduce these systems to radio relay stations as an independent power supply system. A frustum-shaped diffuser was chosen from an economical standpoint and wind speed distribution. The effect the diffuser's shape had on the wind speed was analyzed by simulation and showed that the wind speed in the diffuser was greatly influenced by the length and expansion angle of the diffuser, and maximum wind speed increased 1.7 times with the selection of the appropriate diffuser shape. The wind speed in the diffuser was fastest near the diffuser's entrance. We conducted field tests using a real examination device with a diffuser and confirmed that the output power of the wind power generator increased by up to 2.4 times compared to that of a conventional turbine. Moreover, it was confirmed that the diffuser was especially useful where the wind direction was constant. (c) 2005 Elsevier Ltd. All rights reserved.