The Microlab HR-20 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, at antenna sites during lightning storms and to route DC to tower mounted amplifiers. 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.