Waveguide feedthroughs are used to create a penetration through a shielded enclosure without violating the shielding characteristics of an enclosure. These feedthroughs have a removable centre plug which allows pre-terminated fibre optic cables to be passed through the wall of a shielded enclosure. Once the cables are through, the pug can be reinserted to achieve the desired isolation. The bores are precision milled to allow a sepcific amount of attenuation at specific frequencies with the plug installed.