Coupling-compensated 180° phase shift coupled-Line filters terminated in arbitrary impedances

Coupled-line filters (CLFs) with a 180deg phase shift are analyzed and coupling compensated design equations are derived. They can be applied for arbitrary termination impedances and arbitrary coupling coefficients. To verify the design equations reasonable, a microstrip CLF with a coupling coefficient of -7 dB is fabricated on a substrate (epsivr = 3.5 ,H = 30 mil, tan delta = 0.04) and tested for 100 Omega and 70 Omega termination impedances. The measured results show good agreement with the simulated ones.