Microwaves can be emitted by an antenna arrangement – a variation on a bare wire, or transferred by a waveguide. An antenna arrangement was judged to be an ineffective approach for endometrial ablation, and waveguide technology was the favored approach. An air-filled waveguide would need to be about 30 mm in diameter to transfer microwaves at 9.2 GHz. 3 The preferred diameter for the microwave device was 8 mm – a compromise between feasible cervical dilation, but adequate ‘bluntness’ to avoid uterine perforation. To ‘pipe’ microwaves down an 8 mm waveguide, it was necessary to find an appropriate dielectric filler for the waveguide. Initially, a ceramic rod was used as the dielectric. This was effective, but liable to fracture with rough handling, and further refinements produced a more robust, easily manufactured applicator.