Half-Mode-Like Parasitic Transmission Found in Millimeter-Wave Bandpass Filters Realized in Post-Wall Waveguide

Measured isolation levels of bandpass filters (BPFs) built of post-wall waveguides (PWWs) with two microstrip-line based interfaces are found to depend on details of on-wafer measurement settings. The BPFs are fabricated into a low-loss 0.52-mm-thick silica substrate and designed to have a 5-stage Chebyshev response. The measured |S21| outside the pass-band shows a rise not expected from the Chebyshev response when the device under test (DUT) is placed on a metal chuck. On the other hand, it does not show such a rise when placed on a 5-mm-thick quartz chuck. Electromagnetic field simulation indicates that a parasitic propagating mode resembling the primary mode in a half-mode substrate integrated waveguide is excited outside the post-walls especially when the chuck is metallic. This could be affecting the levels of |S21| outside the pass-band.