Comment on “Compact UWB Bandpass Filter Using Stub-Loaded Multiple-Mode Resonator”

For original paper by R. Li and L. Zhu, see ibid., vol. 17, no. 1, p. 40-42, (2007). An unloaded MMR must have a sufficiently large impedance ratio (R >5) so that the leading three resonances have a relatively close proximity to form a broad passband. Therefore, at least two points are worth investigating from circuit design point of view. One is that the B value of the unloaded MMR is only 1.5, and the other is where the fourth resonance and the zero come from.