Compact Wideband SIW Based Bandpass Filter for X, Ku and K Band Applications

This paper presents a miniaturized bandpass filter (BPF) with characteristics of wider passband and stopband rejection using substrate integrated waveguide (SIW) technology. Slot loading mechanisms deployed on the upper layer of SIW and defective ground structure (DGS) on the ground plane are utilized to achieve wider passband and stopband respectively. The slots on the top layer along with DGS significantly enhances the selectivity of the filter by generating three transmission zeros (TZs) on the upper side of the passband. The proposed filter is simulated using fullwave simulators and the performance is validated through fabrication and testing of the prototype. The proposed SIW filter exhibits a low insertion loss of 1.52 dB over a wider passband from 9.17 GHz to 20.31 GHz with a 3 dB fractional bandwidth (FBW) of 76%. Further, a wider upper stopband is achieved with the rejection of more than 16 dB in the frequency range of 23 GHz to 40 GHz. The filter provides a flat group delay response of approximately 0.19 ns over the wider passband. The electrical size of the fabricated prototype is 1.05_g × 0.67_g, where _g denotes the guided wavelength in the dielectric substrate for the designed center frequency of 16 GHz.


Introduction
Recent developments in wireless communication systems demand the requirement of a Bandpass filter (BPF) with compact size, wideband, low insertion loss, high selectivity and wide out of band rejection. Substrate integrated waveguide (SIW) is a suitable technology to design BPF which yields the benefits of both the microstrip and rectangular waveguides. This enables to attain a cost effective solution, reducing losses, increasing the Q (quality) factor and power handling capacity, providing ease of integration with other planar components in the microwave communication system [1]. Microstrip BPF is implemented by combining low pass filter and high pass filter using the low pass to bandpass transformation, insertion loss (IL) technique or by employing multiple resonators. In [2], [3] the authors have designed a via less compact BPF using metamaterials (MTMs) to obtain large fractional bandwidth (FBW) and small insertion loss (IL). The resonant frequency is attained through split circular rings and rectangular stub in [2] and composite right-left handed transmission line (CRLH-TL) in [3] respectively.
Several SIW based filters were existing in the literature for realizing compact BPF with enhanced performance. BPFs are developed in SIW technology by using coupling resonators or introducing low pass structures such as electromagnetic bandgap (EBG) structures into SIW high pass structures, MTMs, multimode resonators (MMRs) etc. MTMs are used in the design of SIW filters to achieve miniaturization and enhanced selectivity as in [4], [5]. EBG structures are incorporated in SIW to design millimeter-wave [6] and super-wide [7] BPF with merits of wide out of band rejection (OBR). EBGs along with folded SIW (FSIW) [8] and half-mode T septum SIW (HMTSSIW) [9] are used to achieve a significant size-reduction and larger bandwidth but sacrificing the requisite of low IL. Quasi-elliptic filters are designed using SIW by utilizing slot-coupling [10] and cross coupling [11] with benefits of low IL.
Different coupling topologies such as source-load coupling [12], cascaded trisection [13], stepped impedance nonresonating node (NRN) [14], modified feeding and coupling [15], mixed electric and magnetic coupling [16], negative coupling [17] and magnetic coupling [18] are used in the design of SIW BPF. These topologies achieve high selectivity, but with the demerits of high IL [12], [15] due to their inverse relationship between bandwidth and has a larger circuit size [12,13,15]. SIW BPF is realized using planar resonators suitable for System-on-package (SoP) [19], where it lacks selectivity at the upper side of the passband. A compact SIW BPF useful for millimeter (mm) wave is demonstrated using 3-D ICs technology [20].
Compact wideband SIW filters are designed using MMRs such as U-shaped slots and comb-shaped slots together with defective ground structure (DGS) in [21], [22] respectively to gain good upper stop band performance. The non-fundamental mode is utilized in [23] to design a K-band SIW filter for enhancing stopband performance where a multilayered structure is deployed. Stepped impedance face-toface E-shaped slots that act as DGS is employed to design SIW filter for the forthcoming 5G applications at the expense of large IL [24]. Multimode BPF such as triple mode filter [25] and quintuple mode filters [26] are designed using SIW and quarter mode SIW (QMSIW) structures respectively. They have improved out of band rejection in both the edges of pass bands. To achieve significant size reduction, QMSIW and Eighth Mode SIW (EMSIW) structures [27] are used to design balanced filters. Most of the SIW filters reported in in the literature [12-15, 19, 23-25] suffer from narrow FBW, high IL and larger circuit size. As global technology progresses towards miniaturization, the requirement for wideband filters with a smaller footprint, small insertion loss and good selectivity has gained much interest among the researchers.
In this paper, the SIW bandpass filter with enhanced performance useful for X, Ku and K band applications is proposed.  The wider passband is generated due to multimode resonance by a combination of longitudinal and transverse slots engraved on the top layer of SIW. Band rejection characteristics of bowtie-shaped DGS improves the stopband performance and further due to the combined action of slots and DGS the selectivity of the proposed filter is enhanced by creating 3 TZs. The size-reduction of the designed filter is attributed to the loading of multiple slots that eventually leads to an increase in electrical length. For validation purposes, the filter prototype has been fabricated and good agreement is achieved between the simulated and measured results. The key features of the proposed work are: • The size of the proposed SIW based wideband filter is highly miniaturized than the previously reported filters [6, 7, 9, 11-16, 19, 21-26].
• Compared to the existing filters, the proposed filter structure has low IL [4, 7- • The proposed wideband filter provides wider stopband rejection and flat group delay response.
• To investigate the multimode resonant behavior of the slot-loaded filter, the variation of input impedance w.r.t the number of slots is analyzed.
• Parametric analysis is explored to verify the dependence of the filter performance on the slot dimensions, their position and bowtie-shaped DGS dimensions. The rest of the paper is organized as follows. Section 2 elaborates the design evolution of the proposed filter configuration along with parametric study. Section 3 shows the fabrication and experimental validation of the proposed filter and finally, Section 4 presents the conclusions.

Design of Wideband SIW Bandpass Filter
The design of the proposed SIW wideband bandpass filter (WBBPF) is based on the evolution of steps that are illustrated in Fig. 1 along with its corresponding simulated S-parameters in Fig. 2. The design procedure is as follows: • In step-1, a conventional SIW filter is constructed using a rectangular SIW cavity by following the design equations in [1].
• Multimode resonance is created utilizing transverse slots in the top layer of the SIW filter as illustrated in step-2.
• In step-3, in-band return loss performance is enhanced by incorporating longitudinal slots together in cross-Section with transverse slots. The size-reduction of the proposed filter is contributed by the loading of multiple slots.
• Finally, stopband characteristics of the filter is improved by employing bowtie-shaped DGS in the ground plane as step-4.
The basic structure of the proposed filter consists of a rectangular SIW cavity that is fed by two 50 Ω microstrip lines. To ensure confinement of the electric field inside SIW, the pitch of the via-holes is chosen less than or equal to twice its diameter. The SIW filter depicted in step-1 of Fig. 1 exhibit high pass characteristics as shown in Fig. 2(a) whose cut-off frequency is in coincidence with the calculated cut-off frequency of SIW [1]. This filter structure exhibits 101 electric field distribution which is the dominant mode as shown in step-1 of Fig. 3. The following equation is used to compute the resonant frequency of the dominant mode of SIW cavity: where = 1, 2,..., = 1, 2,..., and denotes the permitivitty and permeability of the substrate. The effective length and width of the cavity are determined using: where s and s are specified as width and length of the cavity. The diameter and the center-to-center spacing between the via-holes are denoted by and , respectively. To meet the constraint of minimum leakage loss, the diameter and pitch need to be chosen as / g ≤ 0.1 and / ≥ 0.5 ( g denotes the guided wavelength at the resonating frequency).

The Impact of Transverse and Longitudinal Slots
To enable bandpass filtering, periodically spaced four transverse slots of length c and d < g /2 (where = 16 GHz) is etched from the top metallic layer of SIW. The slot-loading mechanism modifies the resonant mode of rectangular SIW since the slots are oriented in such a way to cut the current lines of unperturbed mode. Hence higher-order mode 201 gets excited that creates additional resonance as illustrated in step-2 of Fig. 3. The gap between the adjacent slots is assumed to be equal and considered as b = b = 0.58 g . These slots create a wider passband from 7.66 GHz to 17.54 GHz due to multimode resonance in spite of poor return loss performance in passband as shown in Fig. 2(b).
Two longitudinal slots of length t = g /2 separated by a distance of b = 0.58 g are etched from the top layer of the SIW filter. These slots create additional resonance due to perturbation of 101 mode that significantly improves inband filtering especially the return loss performance within the passband as depicted in Fig. 2(c).

Band Rejection Property of DGS
To achieve better stopband performance, bowtie-shaped DGS with e = 0.36 g and e = 0.47 g is incorporated in the middle of the ground layer as shown in step-4 of Fig. 1. After incorporating the DGS in the filter structure, the passband frequency is shifted from 8.96 GHz to 10.03 GHz due to the change in effective inductance and capacitance of the DGS. It is noticed that the bandwidth of the filter is widened from 11 GHz (8.96 to 19.96 GHz) to 12.43 GHz (10.03 to 22.4 GHz) because of the increased fringing field resulting in parasitic capacitance created by the DGS. This capacitance strengthens the coupling between the top and bottom metallic layer resulting in an improved bandwidth [22].
The schematic of the proposed wideband SIW BPF is depicted in Fig. 4. The physical parameters of the proposed filter are optimized to achieve the desired frequency response using the full-wave simulator and the final optimized dimensions of the proposed WBBPF are given in Tab. 1. Simulation results shown in Fig. 5 reveals that the proposed filter exhibits a low insertion loss of 0.64 dB with a 3 dB FBW of 89 % and a return loss of more than 17 dB. Stopband rejection at 24.22 GHz is found to be 44.13 dB with a wideout of band rejection of greater than 19.4 dB from 23 GHz to 40 GHz. The band stop behavior of DGS suppresses the higher-order harmonics thereby wide stopband is made possible. It can be seen from Fig. 6, group delay is approximately 0.19 ns over the entire desired passband.

Parametric Analysis
A parametric study is carried out to investigate the effect of filter parameters on the performance of the proposed filter. The number of slots and their position determines the bandwidth of the filter. From Fig. 7, it is clear that as the number of transverse slots increases, significant variation is observed in the resonant modes of the filter in the upper X-band, Kuband and lower K-band. The slots yield impedance matching between the closely occurring resonance in X-band, Ku-band and K-band, due to which wideband is achieved.
From Fig. 8, it is observed that as the number of transverse slots is increased from 1 to 2, the bandwidth of the filter increases affecting the return loss (RL). Further, the addition of two more transverse slots, reduces the bandwidth; due to the excitation of higher-order modes. However, the desired bandwidth is obtained by the addition of two more longitudinal slots resulting in 6 slots. The effect of the position of slots on the in-band return loss performance of the proposed filter is depicted in Fig. 9. It can be seen that better return loss is achieved in the desired passband for the case where the transverse slots are placed at a spacing of b = 1.8 mm. Hence maximum of four transverse slots can be accommodated on the longitudinal slot of length t = 6.3 mm.    As the stub lengths c and d are increased from 5.4 mm to 5.8 mm and 5.6 mm to 6 mm respectively, return loss values slightly increases within the passband, though 1 location moves downwards towards the lower frequency side, there is a small effect on 2 location and no effect on 3 location that can be observed from Fig. 10. Since the length of stub affects the electrical length and finally impedance of stub which affects the frequency at which TZ occurs [22]. For c = 5.8 mm and d = 6 mm, there is a sharp transition from passband to stopband. As the width of the slots is increased, no significant influence is observed in the filter response. Also, to confirm that the important transmission zero is not created by the DGS itself a study is performed by varying the dimensions of DGS as illustrated through Fig. 11. It is clear that 1 is unperturbed even if the DGS dimensions are varied.
The effect of bow-tie-shaped DGS in the ground plane is analyzed by altering the width of bow e from 4.83 mm to 6.83 mm that simultaneously alters the parameter k from 1.26 mm to 1.78 mm. As the width is increased, the lower and upper band edge frequencies shift towards lower frequencies due to an increase in capacitive loading which can be clearly seen from the S-parameter response shown in Fig. 11.
Actually the width of SIW determines the higher cutoff frequency and the parameters such as dimensions of slots determines the position of 1 , the spacing between the adjacent slots have an effect on in-band return loss and the number of slots determines the desired bandwidth range of the proposed filter. On the other hand, DGS enhances the stopband performance of the filter by enhancing the stopband rejection.

Fabrication and Measurement
The proposed wideband SIW BPF is fabricated using 0.787 mm thick Rogers RT/Duroid 5870 substrate material having a relative permittivity r = 2.33 and loss tangent tan = 0.0012. Figure 12 shows the photograph of the fabricated prototype. The size of the proposed filter excluding the feeding lines is 15.5 mm × 9 mm. In order to study the performance of the fabricated filter, the input and output ports are provided via SMA F edge mount (40 GHz) connectors. Transmission and reflection characteristics of the fabricated prototype are measured using Anritsu MS46122B (43.5 GHz) vector network analyzer (VNA). Figure 13. plots the comparison of S-parameters of the fabricated filter obtained from simulations and measurements. The fabricated filter has a wider passband ranging from 9.17 GHz to 20.31 GHz with a 3-dB fractional bandwidth of 76 %. The filter has a measured insertion loss of 1.52 dB within passband and return loss of more than 17.6 dB. Transmission zero ( 1 ) location is shifted towards a higher frequency of 25.68 GHz with 44.4 dB rejection. In addition to this, the proposed filter achieves a wider upper stopband having an attenuation of 16 dB up to 40 GHz. Measured results match very well with those obtained from simulation, except for the slight discrepancies owing to connector loss and fabrication tolerances. Table 2 gives a comparison between the proposed wideband SIW filter and other reported works in literature. It is evident that the proposed filter has a larger 3 dB FBW than those reported in literature except in wideband SIW BPF proposed in [3], [8] and [9], where the filter design is complicated. The proposed filter has a comparatively smaller size, reasonable IL and better stop band performance than other reported SIW filters.

Conclusions
In this paper, a miniaturized wideband SIW BPF suitable for RADAR and satellites operating in the X band, Ku band and K band is designed, fabricated and experimentally verified. Wider passband is generated due to multimode resonance of slots loaded on the top layer of SIW whereas, the bowtie-shaped DGS incorporated on the ground plane is responsible for wider stopband characteristics of the filter. Further, slots along with DGS enhances the selectivity of the filter by generating 3 TZs on the upper side of the passband. The impact of slot and DGS on the filter performance is analyzed by varying its physical parameters. Compact size with simple structure, low insertion loss, flat group delay response, wider passband and stopband are the desirable features of the proposed filter.