PURPOSE: To prevent intrusion of rain water or dust by fitting main clutch board and inverted clutch board rotatably then coupling through a pin such that upon application of heavy load on the inverted clutch, the pin is disengaged to block both clutch boards.

CONSTITUTION: An inverted clutch 12 of drum case and main clutch board 11 of disc shape are fitted rotatably. Pressure members 17A, 17B are placed in the recess 11c of the clutch board 11 and energized radially by means of coil springs 18A, 18B. It is arranged such that the pin 16 is engaged in the engaging groove 12e of the clutch board 12 by means of the pressure member 17A. Consequently upon rotation of main clutch board 11 by means of motor, the rotation is transmitted through the inverted clutch board 12 to the rotary shaft 13 thus to telescope the antenna. Upon completion of antenna, heavy load is applied on the inverted clutch board 12 to remove the pin 16 from the groove 12e thus to rotate only the main clutch board 11.