24 GHz miniaturised phase-selectable balun

The design, fabrication and test of a uniform microstrip and a slow-wave microstrip version of a two phase state balun based on an ultra compact 90deg quasi-lumped hybrid coupler design are presented for use in the 24 GHz Industrial, Scientific and Medical frequency band. The balun, realised using Gallium Arsenide integrated circuit technology, has the property that when fed from one of its input ports it provides a differential phase of 180deg at its output ports and when fed at its second input port it provides 0deg phase shift between output ports. The lambda/8 quasi- lumped coupler core reported occupies 0.15 of the area of a conventional quarter-wavelength coupler and has 0.5 dB insertion loss. The slow-wave version of the coupler is 40% shorter than the lambda/8 quasi-lumped coupler and exhibits 0.9 dB insertion loss.