The Microlab HR-25 series DC Blocks are used to prevent the flow of direct current and low frequency current surges along the inner conductor of a transmission line, while permitting the unimpeded flow of RF signals. Applications include the blocking of current surges in subway tunnels and antenna sites.The unit consists of a length of coaxial line with a distributed series capacitor in the center conductor to block the flow of DC and low frequencies, while passing RF with negligible loss or reflections.