Thesis (PhD (Electric and Electronic Engineering))--University of Stellenbosch, 2005. / A tunable X-band PIN diode switch, implemented in evanescent mode waveguide, is presented. To allow in-situ tuning of resonances after construction, a novel PIN diode mounting structure is proposed and verified, offering substantial advantages in assembly costs. Accurate and time-effective modelling of filter and limiter states of the proposed switch is possible, using an evanescent mode PIN diode and mount model. The model is developed by optimizing an AWR Microwave Office model of a first order switch prototype with embedded PIN diode, to simultaneously fit filter and limiter measurements of four first order prototypes. The model is then used in the design of a third order switch prototype, achieving isolation of 62 dB over a 8.5 to 10.5 GHz bandwidth in the limiting state, as well as reflection of 15.73 dB and insertion loss of 1.23±0.155 dB in the filtering state over the same bandwidth.