The E-face insert waveguide filter has wide loss due to low loss, high Q value, low cost and convenient processing, and is suitable for mass production and thus has been widely used. Although there are many advantages, there are also shortcomings such as slow out-of-band attenuation, narrow stopband, and sometimes large size, which limits its application in many places. In order to overcome the shortcomings of the E-plane insert waveguide filter, this paper proposes a new type of highly selective filter structure, which combines the E-plane insert waveguide and the ridge waveguide, and the resonator of the E-side insert filter. The periodic ridge waveguide structure is embedded, which realizes the characteristics of fast out-of-band attenuation and small size. Different types of periodic structures have different characteristics and have always been hot research topics. Adding a periodic structure to the filter allows the filter to have a reduced size and improved out-of-band attenuation. This is due to the slow wave effect of the periodic structure: the phase velocity of the slow wave and the waveguide wavelength are significantly reduced relative to the wave propagating in the uniform transmission line, and thus the length of the half-wavelength resonator is correspondingly reduced. At the same time, due to the scattering relationship of slow waves, the improvement of the out-of-band attenuation characteristics is achieved. The periodic structure composed of the E-plane insert waveguide and the ridge waveguide is an easily realized structure. As shown in FIG. 1, a ridge waveguide having an equal gap is cascaded in the resonator of the E-face insert filter. A periodic structure replaces a resonator in a rectangular waveguide. The new structure proposed in this paper is very similar to the standard E-plane insert filter. In addition to the general advantages of the E-face insert filter, it also has the advantages of fast out-of-band attenuation and small size. Figure 1 E-plane filter loading the ridge waveguide 2 Theoretical analysis For the new filter structure of this paper, it is divided into two modules for analysis, E-plane insert module and rectangular waveguide-double-ridge waveguide module. The main analysis method is pattern matching method. The model of the E-face insert structure is shown in Fig. 2. The thickness of the diaphragm is t, the length is w, the cross section is a*b, and the plane of the insert is parallel to the narrow side of the waveguide. Figure 2 E-side insert structure Since the discontinuity is continuous in the x direction and in the y direction, the TEm0 mode is only excited at the discontinuity when excited by the TE10 mode. Taking into account all modes of propagation in the +Z and -Z directions in each region, the transverse electric field and magnetic field at the interface Z=0 of the I and II, I and III regions satisfy the boundary condition when the sum is Continuity, so as to solve the scattering parameters of the diaphragm structure by mode expansion, the specific calculation process can refer to the relevant literature. The model of the ridge waveguide structure is shown in Fig. 3. Both the TE and TM modes exist in the ridge waveguide, and the field mode of the lowest mode is similar to that of the rectangular waveguide, except that the electric field in the convex portion is more concentrated and angular. There is an uneven electric field, and the magnetic fields are more concentrated on both sides of the ridge. Since the function of the flange is equivalent to increasing the broad side of the rectangular waveguide, the cutoff wavelength of the lowest mode is longer, and the cutoff wavelength of the second mode is larger, so that the frequency band of the single mode operation is wider, that is, In the case of the same frequency, the ridge waveguide is smaller in size. Figure 3 ridge waveguide structure In the analysis of the ridge waveguide, the transverse wave resonance method is used to obtain the cutoff wave number Kc and the eigenfunction, and the field distribution of the fundamental mode and the higher order mode is determined. Then the pattern matching method is used to calculate the scattering parameters of the ridge waveguide structure. These methods are classic methods, and no specific expressions are given here. The analysis result of the ridge waveguide is applied to the rectangular waveguide-ridge waveguide structure. The structure of the rectangular waveguide-double ridge waveguide is as shown in FIG. 4, the region of Z "0 is a rectangular waveguide, and the region of Z"0 is a ridge waveguide region, The analysis of the discontinuous structure can refer to the analysis method of the previous E-plane insert model, and the scattering parameter of the structure is calculated by the pattern matching method of the dual port network. Figure 4 Rectangular Waveguide - Double Ridge Waveguide Model The periodic structure is composed of a rectangular waveguide-double ridge waveguide module and the like, and the entire filter structure can be regarded as an E-plane insert module and a periodic structure cascade. In order to overcome the shortcomings of the E-plane insert waveguide filter, this paper proposes a new type of highly selective filter structure, which combines the E-plane insert waveguide and the ridge waveguide, and the resonator of the E-side insert filter. The periodic ridge waveguide structure is embedded, which realizes the characteristics of fast out-of-band attenuation and small size. 3 Numerical results and comparison In this paper, several sets of 3rd-order new filter structure design examples are given and compared with the corresponding standard E-plane insert waveguide filters. When the E-plane insert waveguide filter is analyzed by the pattern matching method, 20 modes are taken. For the new filter with periodic structure loading, 25 TE and TM odd modes are taken, and the Matlab programming is used for analysis and calculation. . Table 1 shows the data of the K-band wideband waveguide bandpass filter, and Table 2 shows the data of the X-band wideband waveguide bandpass filter. The thickness of all inserts and ridges is 0.25 mm, and in the new structural filter, lr1 = lr2. The S-parameter curves of the two sets of filters of Table 1 and Table 2 are obtained by the pattern matching method, as shown in Fig. 5 and Fig. 6. Figure 5 K-band wideband filter It can be seen from Fig. 5 and Fig. 6 that the E-plane filter loaded with the ridge waveguide proposed in this paper can improve the stopband characteristics of the standard E-plane insert waveguide filter, greatly improve the attenuation characteristics outside the band, and the size ratio is higher than the standard. The E-side insert filter is much smaller and achieves miniaturization. Figure 6 X-band broadband filter 4 Conclusion In this paper, based on the E-plane insert waveguide filter, an improved structure is proposed. The periodic ridge structure is added to the E-plane waveguide resonator, which preserves the advantages of the standard E-plane insert waveguide filter while overcoming the advantages. The E-plane waveguide filter has a narrow stopband and slower out-of-band attenuation, which greatly reduces the size of the filter. The experimental results verify the correctness of the proposed method.
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