A Compact 28GHz Bandpass Filter Using Quartz Folded Waveguide

For massive MIMO antenna units in 5G, a compact quartz folded waveguide bandpass filter in 28 GHz band with wide passband has been developed. To design the filter, some analyses using EM simulation were carried out and a unique construction has been proposed. An air gap coupling-window has been found to be insensitive against gap distance variations in fabrication. An asymmetrical Y-shaped terminal for TEM-TE mode transition has shown lower external Q less than 10. The fabricated folded waveguide filter has a size of 8.9 × 14.85 × 1.6 mm³ and realizes a compact I/O terminal layout. The measured insertion loss is very low such as 0.9 dB which is better than conventional straight type by 0.3 dB, keeping with 50 dB min. attenuations.