Compact SIW dual-band bandpass fi lter using novel dual-resonance quasi-SIW-transmission-line-structure resonators

A compact substrate integrated waveguide (SIW) dual-band bandpass filter (DBSF) using novel quasi-SIW-transmission-line-structure resonators is proposed in this study. This dual-resonance SIW resonator which works in TE100 mode and TE201 mode can be evolved from a conventional SIW-transmission line or a two-conductor transmission line. Multiple signals transmission paths bring extra transmission zeros (TZs) and improve the selectivity and out-band rejection of the DBSF. This filter works at 4.75 GHz/4.82 GHz, 5.38 GHz/5.51 GHz with 3 dB fractional bandwidths of 3.8 and 3.9%, respectively, and has six TZs. The measured results agree with the simulation results well.


Introduction
Dual-band bandpass filters (DBSFs) are playing an important role in modern multi-band communication systems [1] and lots of works have been done to design DBSFs [1][2][3][4][5].In [1], a substrate integrated waveguide (SIW) DBSF using novel back-to-back E-shaped defected ground structure (DGS) and two-side loading scheme is designed while it has a large insertion loss because of the DGS radiation.Wu et al. [2] propose the novel symmetric double-spiral resonators to design DBSF which has a frequency shift downwards.A DBSF designed in [3] has independently controlled passbands with a complex topology, whereas an SIW DBSF with controllable centre frequencies and bandwidths is proposed in [4] and it has a large size and a high insertion loss.A DBSF using triple-mode ring resonator is designed in [5] with seven transmission zeros (TZs).All the discussed DBSFs above use the extra resonators with a relative complex topology, whereas the proposed filter in this paper uses a new simple resonator, which is a quasi-SIW-transmission-line structure.A new dual-resonance SIW resonator is proposed with a simple configuration.The conventional SIW-transmission line interrupted by two magnetic walls at its both-ends forms the proposed SIW resonator.The conventional dual-conductor transmission line interrupted by two electric walls at its both-ends can also form the proposed SIW resonator.The two procedures can be explained by Fig. 1.Then a DBSF with high out-band rejection and high selectivity using novel dual-resonance SIW resonators is proposed and fabricated which has six TZs.All the simulated results are given by high-frequency structure simulator 13.A substrate with a height of h = 0.5 mm and ε r = 3.48 is used to fabricate the filter.
2 Design of SIW DBSF   at the low out-band, two TZs locate at the high out-band while the remaining TZs locate at between the two passbands.This filter consisted of two SIW resonators and each SIW resonator works in TE 100 mode and TE 201 mode, forming a dual-resonance.Slots are introduced to tune the TE 100 mode resonant frequency and TE 201 mode resonant frequency independently.This SIW DBSF has lots of elements to tune passband bandwidths, passband positions, out-band rejection, and return loss of the two passbands.Fig. 3 gives some tune examples.This dual-resonance SIW resonator shown in Fig. 1 is equivalent to a conventional rectangular waveguide [6] where D is the diameter of via holes, b is the distance between metal vias, W SIW is the width of SIW resonator, and W RE is the width of rectangular waveguide.
Following the first forming way of SIW resonator, the transmission line is SIW structure and it is a single-conductor, so it can work in TE mode.Following the second forming way of SIW resonator, the transmission line works in TEM (TE 00 ) mode as it is a twoconductor line.By above analysis, this SIW resonator works in TE 00n mode (TE 100 in this work) and TE mop (TE 201 in this work).

Conclusion
A compact SIW DBSF is designed using proposed dual-resonance SIW resonator for the first time.The two passband bandwidths are nearly equal to each other as showing the 3 dB fractional bandwidths of 3.8 and 3.9%.The measured results show a good performance and are in agreement with the simulated results.

Fig. 1
Fig. 1 Procedure of the transmission lines changing into SIW resonator a First forming way b Second forming way

Fig. 2
Fig. 2 shows the proposed SIW DBSF.Multiple signal transmission paths and the source-load coupling bring six TZs.Two TZs locate

Fig. 2
Fig. 2 Configuration of the proposed SIW DBSFs: based on SIW resonator with two electronic walls and two magnetic walls

Fig. 3
Fig. 3 Tuning of a First passband position b Second passband position c Out-band rejection d Second passband bandwidth e First passband return loss f Second passband return loss This is an open access article published by the IET under the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/)

Fig. 4
Fig. 4 Photographs of the proposed dual-band SIW bandpass filter

Table 1
Dimension of the proposed dual-band SIW BSF