Abstract: It is pointed out theoretically that the prototype drop in microstrip junction circulators (or isolators) without excellent operating characteristics are due to unsatisfaction of the second circulation condition.It is proposed that the impedance matching condition can be improved by changing the height of the centre junction and length of matching sections,which lead to the improvement of the operating characteristics of the devices.A drop in isolator that operates in Ka band is obtained,which exhibits less than 1 dB insertion loss and greater than 20 dB isolation in 2.0 GHz absolute bandwidth(30.5~32.5 GHz).