Hi All:
We sometimes want our filter to have better selectivity so we will try to add transmission zeros at higher or lower bands. When I read the paper, they would use additional open stub or parallel-resonance to locate zeros at specific frequency; however, it usually accompanies with radiation loss. (We may see |S11|^2+|S21|^2<1; for instance the |S11| = -7 dB and |S21|=-30 dB at that frequency.) I'm wondering how to eliminate it? Could we add some metal in the top of circuit?
Thank you