A device for protecting a radio frequency transmission line from transient voltages includes an inner conductor for transmitting communication signals of a desired frequency band and a grounded, coaxial outer conductor electrically insulated from the inner conductor by a pair of annular insulators. As one feature of the invention, a tap conductor for discharging transient voltages carried by the inner conductor that fall outside the desired frequency band is coupled at one end to the inner conductor through a press-fit connection. As another feature of the invention, a pair of high-quality contacts are mounted onto opposite ends of the outer conductor and serve, together with the inner conductor, as the only electrical contact surfaces for the protective device that transmit the desired communication signals. As another feature of the invention, each insulator has a constant outer diameter along the entirety of its length and a variable inner diameter.