A capacitive coupled extendable antenna for portable radio communication devices with improved operating characteristics is disclosed. The extendable antenna includes a quarter-wavelength helical antenna, a quarter-wavelength whip antenna movable between the extended position and the retracted position through the helical antenna, and an electrically conductive sleeve acting as the feed point of the antenna. The helical antenna is electrically insulated from the sleeve so that the helical antenna is capacitively coupled to the sleeve. The whip antenna is enclosed within an insulating tube, which is disposed within a guide for longitudinal movement therethrough. The whip antenna is disposed in electrically insulated relation to the sleeve in the retracted position and electrically connected to the sleeve in the extended position via a stopper. When the whip antenna is in the extended position, the whip antenna is operative through direct coupling with the sleeve and when the whip antenna is in the retracted position, the helical antenna is operative through capacitive coupling with the sleeve. In a preferred embodiment, the whip antenna is short-circuited at the point of 90.degree. in electrical phase so as to form a .lambda./4 balun. The resulting extendable antenna offers wider operating bandwidth and improved radiation efficiency when operating in the retracted or stand-by mode.