Suppression of spurious responses in microstrip filters using a reduced number of resistors

The stop band of a microstrip filter was extended by suppressing its harmonics, using additional stubs together with a smaller number of resistors than in the previous work. The possibility of suppressing the second to the fifth harmonics with one resistor per resonator is investigated in a simulation. Equations for initial estimates for losses and required coupling coefficients for the new stub design are given. In a simulation of a fourth-order filter, a total of only two resistors were required to suppress the second, third and fourth harmonics to -35-dB; -29-dB was achieved in a fabricated filter. The pass band loss was only increased by 0.2-dB.