Recessed ground microstrip line at 60 GHz and its application of band‐pass filter

The effect of finite‐size recessed ground on characteristic features of a microstrip transmission line is investigated and verified experimentally on alumina substrate of height 0.127 mm with ε_r = 9.8 at 60 GHz. A measured characteristic impedance of 238 Ω, effective dielectric constant of 3.09, and attenuation constant of 3.4 Np/m is achieved by using a recessed ground of dimensions (width × depth) 4.5 mm × 0.95 mm, below a 50‐Ω (on conventional ground plane) microstrip line. The effect of recessed ground on lumped equivalent circuit elements of microstrip line discontinuities including series‐gap, open‐end, and step discontinuities is also studied. To show the usefulness of recessed ground microstrip line, a prototype of fifth‐order Chebyshev‐type recessed ground end‐coupled band‐pass filter is designed and fabricated at 60 GHz. The filter exhibits measured insertion loss lower than 2.2 dB and return loss better than 13 dB over 3‐dB passband of 6% centered at 60 GHz. The measured results show good consistency with simulated results and confirm the usefulness of recessed ground plane microstrip line.