Broadband millimetre-wave passive spatial combiner based on coaxial waveguide

A broadband millimetre-wave passive spatial combiner using a microstrip probe array and an oversized coaxial waveguide is successfully designed and tested. The equivalent circuit approach of transverse electromagnetic (TEM) lines is adopted to synthesise the coaxial stepped impedance transformer from a K connector with the oversized coaxial waveguide and to analyse the microstrip probe array. The equivalent susceptances of the step discontinuities for the power divider are calculated. A four-way coaxial waveguide passive power combining circuit operating at the entire Ka-band is designed, fabricated and measured. Experiments on the four-way passive combiner show that a minimum insertion loss of 0.7 dB has been achieved at about 29.5 GHz. The combiner has shown a bandwidth of 26.5-40 GHz with 10 dB return loss and less than 2.5 dB insertion loss.