A simple millimeter-wave transformer between microstrip line and post-wall waveguide

We propose a new transformer between microstrip line and post-wall waveguide fabricated on liquid-crystal polymer substrates. Matching to 50 Ω impedance is accomplished by a combination of a through-hole with an anti-pad and open-ended stubs. The bandwidth of the transformer for a reflection smaller than -15 dB is 8 GHz. The estimated loss of the transformer associated with the mode conversion and the measured loss of the PWW is 0.45 dB and 0.1 dB/mm at 60 GHz, respectively.