The paper describes the development of a tr switch mounted in coaxial line for use at 200Mc/s. Two designs of tr switch emerged, each representing a balance of the conflicting design features required and a solution to the problem of sputtering.In one design, the sputtering is minimised by using an extended electrode area, and this provides a simple and rugged construction. However, this is somewhat at the expense of breakthrough and recovery time. The other design uses an impregnated tungsten electrode with a low work function to further reduce sputtering, and this gives an improved performance, particularly in arc loss and firing power, but it does result in a more complicated and less rugged structure.The application of d.c. bias reduces the recovery time in both designs of switch, particularly for the simpler version.A simple pass/fail test is described, which can be applied in the field when complicated test equipment is not available.