Research Article  Open Access
Novel Hexagonal DualMode Substrate Integrated Waveguide Filter with SourceLoad Coupling
Abstract
Hexagonal dualmode cavity and its application to substrate integrated waveguide (SIW) filter are presented. The hexagonal SIW resonator which can combine flexibility of rectangular cavity and performance of circular cavity is convenient for dualmode bandpass filters design. By introducing coupling between source and load, the filter not only has good selectivity due to two controllable transmission zeros, but also has a small size by the virtue of its singlecavity structure. A demonstration filter with a center frequency of 10 GHz and a 3 dB fractional bandwidth of 4% is designed and fabricated to validate the proposed structure. Measured results are in good agreement with simulated ones.
1. Introduction
Dualmode cavity bandpass filters have been widely used in the development of various wireless communication systems. The metal waveguide dualmode filters have excellent performance owing to their high factor and powerhandling capability. However, they cannot be easily integrated with microwave planar circuits [1, 2]. Recently the substrate integrated waveguide (SIW), which is synthesized in a planar substrate with arrays of metallic via, provides a lowprofile, lowcost, and lowweight scheme while maintaining high performance [3–5]. Particularly, the application of SIW technology makes the implement of dualmode cavity filters with compact size and easy integration possible [6].
On the other hand, filters with multiple transmission zeros (s) are required to meet the increasing demands of modern communication systems in regards to compact size and high selectivity. Commonly, no more than one can be obtained in a conventional singlecavity dualmode filter. In order to generate more s, many approaches have been proposed to design dualmode SIW filters. One approach is cascading two adjacent dualmode rectangular cavities to generate up to two s in stop band [7]. Similarly, a dualmode filter using two connecting circular cavities with two s is introduced in [8]. However, their physical sizes will become larger because of cascaded structures. Another approach can be fulfilled by marshaling the effect of sourceload coupling in single cavity. By adding a direct signal path between the source and the load, finite transmission zeros can be generated. In [9], a dualmode filter using a nonresonating node with indirect sourceload coupling is proposed, and two s are obtained in such a singlecavity filter.
Ordinarily, conventional dualmode SIW filers are always built based on rectangular and circular cavities. In our previous work, a novel hexagonal resonator using SIW technology and its applications to trisection filters are proposed in [10]. The hexagonal SIW cavity can combine flexibility of rectangular cavities and performances of circular cavities. Meanwhile, as any of the six sides of a hexagonal resonator can be utilized for coupling, the filter configuration is very flexible to design. In this paper, we present a SIW filter with dualmode hexagonal cavity. By introducing coupling between source and load, the filter not only has two s to improve frequency selectivity, but also has a small size by profit from its singlecavity structure.
2. Filter Analysis and Design
Figure 1 shows the coupling topology of the proposed dualmode filter. By adding the coupling between the source and the load, one additional can be obtained. In other words, the source and the load are directly coupled which can add an extra transmission path. Under this circumstance, the topology can generate up to two s. The coupling matrix of the proposed topology can be written as
The conventional doublet without sourceload coupling has a in the stopband, and an explicit expression relating the coupling elements and the transmission zero Ω is provided in a lowpass prototype as follows [11]:
Here, since the topology exhibits symmetrically, the relationships and can be hold.
When introducing the sourceload coupling into this doublet, an additional can be obtained. To get more insight of location of two s in this topology, an explicit expression relating and the s is given by where where is normalized angular frequency.
To achieve the proposed topology, a SIW filter with hexagonal dualmode cavity is designed and embedded in a PCB substrate as shown in Figure 2. The single cavity operates with mode which consists of two intersectant modes illustrated in Figure 3. Bypass crosscouplings between the modes and source/load are introduced through symmetrical feeding structure, while sourceload coupling is introduced by the upclose input and output ports.
(a)
(b)
As far as we know, there is no exacted equation for calculating the resonant frequencies through geometrical parameters in a dualmode hexagonal cavity. According to conventional resonant frequency formulas of metallic circular waveguide resonators, the corresponding resonant frequency of in the hexagonal cavity can be determined by modified formulas as follows: where is the speed of light, is the relative dielectric constant of dielectric substrate, is the modified root coefficient based on the Bessel function, is the resonant frequencies of mode in the hexagonal SIW cavity.
Figure 4 shows the relationship between the fitted size and the resonant frequency of the hexagonal cavity. As can be seen, the resonant frequency of mode decreases when the geometrical parameter increases.
A feeding technique named current probe is adopted in the I/O SIW design to achieve the transition from SIW to microstrip. As the symmetrical input/output dominates the bypass crosscoupling, offset between center line and feeding structure has obvious influence on the frequency response. Figure 5 shows the frequency responses for different values of . Donate the s at the lower and upper stopbands as and , respectively. is produced through bypass cross coupling; thus it move towards the lower frequencies with increasing values of . As is dominated by sourceload coupling, its location changes slightly when varying the values of . The feeding structure is right upon mode, hence positions of poles change while varying . As shown in Figure 5(b), and shift towards each other when the value of increases.
(a)
(b)
On the other hand, the length () of input/output current probes in feeding structure determines not only the quality factor ( factor) of the filter, but also the strength of sourceload coupling. Frequency responses for different values of are illustrated in Figure 6. As shown in Figure 6(a), only at upper stopband is obtained when the value of is too small to introduce sourceload coupling (e.g., mm). By increasing to implement sourceload coupling, at the lower stopband can be realized. Then the increase of will result in increasing of sourceload coupling; hence moves towards the passband. In fact, is a parameter which also influences bypass cross coupling, so shifts away from the center of the passband when the value of increases. has impact on parameter similar to . As described in Figure 6(b), shifts away from and the passband broadens towards the lower frequencies when augments. To achieve demanded frequency responses during design process of the proposed dualmode hexagonal SIW filter, parameter of feed probes should be carefully tuned.
(a)
(b)
3. Experimental Results
To validate the abovementioned concept, a 10 GHz hexagonal SIW filter with a 3 dB fractional bandwidth of 4% is fabricated on a PCB substrate with dielectric constant of 2.2. The complete parameters are finely tuned by using commercial full wave electromagnetic (EM) simulation software HFSS. Detailed dimensions of the proposed filter are illustrated in Table 1. The photograph of the fabricated filter is shown in Figure 7. By virtue of the single hexagonal cavity and flexible sourceload coupling manner, the overall size of the filter is 28.6 mm × 28.6 mm × 0.508 mm.

An Agilent E8363B vector network analyzer is used for measurement. The measured and simulated frequency responses are plotted in Figure 8. The measured result shows a central frequency of 9.99 GHz with a fractional bandwidth of 3.9%, minimum passband insertion loss of 1.66 dB, and inband return loss greater than 17 dB. In addition, there are two transmission zeros located at 9.6 GHz with 35.6 dB rejection and 10.75 GHz with 42.5 dB rejection, respectively. The measured results are in good agreement with the simulated ones except a small frequency shift of s and a little discrepancy in the inband insertion loss. The degeneration of the inband insertion loss may be caused by the test fixture as well as the abrasion on the surface. Overall, the measured results validate the feasibility of the proposed design, with its high selectivity being demonstrated.
4. Conclusion
A novel compact hexagonal dualmode SIW filter with high selectivity is proposed. Two s are produced to improve the frequency selectivity by introducing sourceload coupling to the proposed singlecavity filter. A filter sample is fabricated, and the measurement results agree well with EM full wave simulation. Its compact size and high selectivity make it suitable for microwave communication application.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grants Nos. 61101030, 61201001, and 61301052) and Fundamental Research Funds for the Central Universities of China (Grant no. ZYGX2011J132).
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Copyright
Copyright © 2014 Ziqiang Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.