These DC Blocks are used to prevent the flow of direct current and low frequency current surges along the inner and outer conductors of a transmission line, while permitting the unimpeded flow of RF signals. Applications include the blocking of current surges in subway tunnels and at antenna sites during lightening storms. HR-21 series is similar except only the outer conductor is blocked. The unit consists of a length of coaxial line with a series capacitor in both the center conductor and outer conductor to block the flow of low frequencies, while passing RF with negligible loss or reflections. Options for different polarity or alternate connectors are available on request.