Electric length of half-wavelength resonator is [3]:

3 Results and discussion

3.1 Metal insert thickness impact on the filter performance

We applied the standard waveguide R220 to produce a five-resonator single metal insert band-pass filter, size as follows: w1=2.05mm, w2=7.15mm, w3=7.85mm, l1=7.47mm, l2=7.5mm, l3=7.5mm,t=0.5mm. The optimization results which are demonstrated in Fig. 2, are calculated from certain metal insert thickness. When the metal is symmetrically placed, thickness of the metal inserts, respectively, are t=0.2mm, t=0.3mm, t=0.5mm,t= 0.8mm,t=1mm,t=2mm, the S21 curve was plotted in Fig. 3, and the S11 curve was plotted in Fig. 4. The S-parameter curve which is shown in Figs. 3 and 4, from left to right order corresponds to the metal insert thickness are t=0.2mm,t=0.3mm,t=0.5mm,t=0.8mm,t= 1mm,t=2mm.

From Figs. 3 and 4, we found that, taking S-parameter curve for the thickness t=0.5mm as a reference curve, with the metal insert becoming thin, passband shifts to the left, that is, passband shifts to the low frequency; and with the diaphragm thickness thickening, passband shifts to the right, that is, passband shifts to the high frequency. This set of curves shows that, when both the metal width and the length of the resonator are fixed, the

Fig. 2 Filter characteristic for the given w, l, t value.

Fig. 3 S21 for metal symmetric placed and different thickness.

metal insert can change the passband center frequency by its thickness at a certain extent. However, in Fig. 4, we have also found that, from the curve for t=0.5mm, with the metal thickness increased, filter performance deteriorates rapidly. So when the metal thickness increases to a certain value, the original filter size should not have been used.

Fig. 4 S11 for metal symmetric placed and different thickness.

As is referred in[4], by changing the thickness of a single metal insert, when the thickness of the single metal is comparable to the interval of the double metal insert strip, the stop-band characteristic is improved. But according to our analysis, the result we got is slightly different from the result which is mentioned in[4]. Therefore, it can only improve the stop-band by changing metal insert thickness in an appropriate range.

3.2 The impact of resonator length on center frequency

With given thickness and width of the metal insert, a group of curves in Fig. 5 to Fig. 7 are obtained by changing the resonator length. For Figs. 5, 6 and 7, pass-band center frequency were 21.96GHz, 21.07GH and 20.92GHz respectively.

From Fig. 5 to Fig. 7, we can find that, to take the curve in Fig. 6 as a reference, if Li increases, f0 shifts to the left, that is, the center frequency shifts to the low frequency; and if Li decreases, f0 shifts to the right, that is, the center frequency shifts to the high frequency; After a large number of analyses, we find out that it affects the center frequency mostly by changing L1, but if L1 is close to L2~Li, the best performance of the filter is achieved.

3.3 The impact of metal width on bandwidth

After a large number of simulations, the results verify the conclusion mentioned in[5], that is, metal insert width wider, narrower bandwidth; metal insert width narrower, wider bandwidth. So according to above conclusion, it can provide a reference for simulating and optimizing E-plane waveguide filters.

Fig. 5 S21 for l1=6.8mm, l2=6.9mm, l3=6.9mm.

Fig. 6 S21 for l1=7.47mm, l2=7.5mm, l3=7.5mm.

Fig. 7 S21 for l1=7.6mm,l2=7.6mm,l3=7.6mm.