Meanwhile, the length of the two fe

Meanwhile, the length of the two feed probes (parameter c) and the off-center distance of the disturbed metal cylinder (parameter k) are the key parameters controlling the coupling of the three resonant modes. The mode chart of four normalized resonant modes against the varied of the off-center distance k is depicted in fig.4(a). As the offset values k of the disturbed metalcylinder approach -2 mm, the two degenerate modes (TE11+ mode and TE11- mode) tend to merge, in this case the higher mode is also relatively low, in order to the degenerate modes separate from each other and to keep the higher modes away from the degeneracy modes, we choose 1 < k < 3. Then we analyze the effect of length c of feed probes on normalized resonant frequency, see fig. 4(b). Obviously, when the length of feed probes c >10 mm, the two degenerate modes rapidly separate, so we can use the random distribution of the first three resonant modes in the desired passband to design a three-mode broadband bandpass filter.