Compact microstrip bandpass filter with tunable notch

Two different designs combining a bandpass and a notch filter are developed to operate in the receiving band from 350-470 MHz. The bandpass filter is designed from a simple structure, by use of only four short circuited stubs and a half wavelength transmission line connecting the stubs. The tunable notch filter ensures an attenuation level of 19.3 dB to 27.3 dB in the frequency range from 360-480 MHz. The measured passband ripple of the combined filter is less than 0.5 dB, while the insertion loss for the simplest design is less than 1.7 dB only 10 MHz from the notch frequency. Even though the wavelength on the selected substrate (ε_r = 3.55) is approximately 45 cm, the outer dimensions of the final filter only measure 10×10 cm².