Substrate integrated waveguide bandpass filter for short range device application using rectangular open loop resonator

Received Mar 21, 2019 Revised Mar 25, 2021 Accepted Apr 10, 2021 The substrate integrated waveguide (SIW) structure is the candidate for many application in microwave, terahertz and millimeter wave application. It because of SIW structure can integrate with any component in one substrate than others structure. A kind components using SIW structure is a filter component, especially bandpass filter. This research recommended SIW bandpass filter using rectangular open loop resonator for giving more selectivity of filter. It can be implemented for short range device (SRD) application in frequency region 2.4-2.483 GHz. Two types of SIW bandpass filter are proposed. First, SIW bandpass filter is proposed using six rectangular open loop resonators while the second SIW bandpass filter used eight rectangular open loop resonators. The simulation results for two kinds of the recommended rectangular open loop resonators have insertion loss (S21 parameter) below 2 dB and return loss (S11 parameter) more than 10 dB. Fabrication of the recommended two kind filters was validated by Vector Network Analyzer. The measurement results for six rectangular open loop resonators have 1.32 dB for S21 parameter at 2.29 GHz while the S11 parameter more than 18 dB at 2.26 GHz – 2.32 GHz. While the measurement results has good agreement for eight rectangular open loop resonators. It has S21 below 2.2 dB at 2.41-2.47 GHz and S11 16.27 dB at 2.38 GHz and 11.5 dB at 2.47 GHz.


INTRODUCTION
The rapid growth of wireless communication systems leads to a crowded occupation of the electromagnetic spectrum. In order to avoid possible interferences, the availability of high selective filters is necessary. Lowpass and bandpass filters with high selectivity have been presented in [1]- [4]. By using Hilbert curve ring and sierpinski carpet defected ground structure (DGS) in paper [1], the compactness and selectivity were achieved. Selectivity of filter can also be achieved by coupling between two resonators as shown in [2]- [4]. Resonator coupling occurs between two parallel resonators closely. The parallel coupled resonator in [2] are quite long enough then the single resonator is made become rectangular open loop resonator as shown in [3]. By using material with low loss dielectric and high permittivity can improve S21, selectivity and compact of filter as shown in [4].
Compact filter can be achieved by using substrate integrated waveguide (SIW) structure because many components can be implemented in one substrate, known system on substrate (SoS). SIW also offers good performance such as the low loss, light weight and easy to construct. The first filter in SIW structure was inductive post filter by using three-pole Chebyshev filter to give low S21 and better S11 values [5]. For giving more compact filter, DGS has been successfully implemented in the SIW structure as presented in [6], [7]. The DGS is scratched at the upper metal cover with three cascaded cells to give more selective.
Complimentary split ring resonator (CSRR) can be categorized as metamaterial structure, has been analyzed in [8]- [10] to give compact and sharp rejection besides DGS [11]. In [8], several CSRR is used to form stopband with very sharp rejection in wideband. The internal and external coupling are extracted by Qfactor in the dominant mode at [9], [10]. Unlike the conventional SIWs, the passbass located below the dominant frequency.
This research proposed a compact and sharp selectivity bandpass filter by using rectangular open loop resonator. The rectangular open loop resonator will generate coupling between two resonators and selectivity will be archived. The filter design is for SRD application at 2.44 GHz as the frequency center. Overall, this research is presented as follow: research method for SIW and rectangular open loop for six and eight resonators in section 2. Meanwhile in section 3 gives the simulation and measurement results discussion. Some parametric studies are offered to give better understanding of the rectangular open loop resonator characteristic in SIW filter. Finally, this research is closed by conclusion in section 4.
SIW structure is applied as the basic of transmission line consists of the parallel metal holes as shown in Figure 1. It required the lowest frequency that can transmit to the SIW structure. The lowest frequency can be named as the dominant frequency, fmnl. The dominant frequency is obtained by the equation for the rectangular waveguide as (1), where m, n and l are the integer number of differences in the standing wave pattern for the rectangular coordinate respectively [26]. Parameter a, b, e are the equivalent broadness, thickness and extent of the cavity. Because of the ratio between broadness and thickness substrate are too high, only TEm0l modes can propagate in the SIW structure, n=0. The lowest mode for transversal electrical (TE) mode was TE101. It should be implemented for miniaturization design. The configuration for metallized holes have to be satisfied by 2p/d<5 and d/0≤0.1 where the metallized hole diameter d the center distance between two adjacent metallized holes p and 0 is the wavelength at the free to air condition [27]. The SIW transmission line is feed by tapper with the minimum length is quarter wavelength of the quasi-TE mode. Rogers RO3210 with substrate thickness 0.64 mm, tangent loss (δ) 0.003, the relative permittivity constant (εr) 10.2 is used in this design in order to get more compact bandpass filter. Figure 1 shows the dominant frequency design. Unlike usual, in this design used dominant frequency higher than the bandpass frequency. The dominant frequency simulation is achieved by using Ansys HFSS.

. Simulation
The simulation variables of j dan n1 are shown in Figure 4. Some parametric studies was given to give more understanding about coupling between two or more rectangular open loop resonator. Figure 4 variable n1. If n1 variable increases from 1 mm to 3 mm than narrow bandwidth will be achieved and S11 will be low as shown in Figure 4(b). Figure 4(c) shows if the distance between n1 and j are small than bandwidth will wider than distance between n1 and j are far away. Table 2 to Table 4 Figure 5 gives the simulation results for variables j and n1 for eight rectangular open loop resonator. If variable j increases from 0.3 mm to 0.7 mm the bandwidth will be wide, the S11 are quite same and still below at 10 dB as presented in Figure 5(a). While in Figure 5 3751 the passband will change to the lower frequency and the S11 gets low until 8.52 dB. Figure 5(c) shows if the distance between n1 and j are far away than the narrow bandwidth will be achieve and the passband will change to the higher frequency. It is vice versa for the condition if the distance between variables n1 and j are small. The detail summary of parametric studies are shown in Table 5 to Table 7.  Comparison between six and eight rectangular open loop resonator is given in Figure 6. It can be analyzed that eight rectangular open loop resonators give narrow bandwidth than six rectangular open loop resonators. The S21 at six rectangular open loop resonator is better than the S21 at eight rectangular open loop resonator. Likewise, the S11 value will be higher if the S21 value is small. For six rectangular open loop resonators, simulation results are presented in Figure 9(a) for narrow band and Figure 9(b) for wideband. The simulation for S parameter gives the S21 values 1.11 dB at 2.4 GHz and 1.19 dB at 2.483 GHz. The 83 MHz bandwidth range is achieved by simulation result which it can be applied for SRD application with 2.44 GHz frequency center. While the S11 values show more than 20 dB for frequency region at 2.38 GHz until 2.48 GHz. By using VNA, the fabrication filter is validated. The measurement displays the frequency changing through a low frequency where the frequency center become 2.3 GHz. It means that the difference is around 140 MHz. The discrepancy occurs because of inaccuracy fabrication process in mm-scales and ports connection. The S21 value becomes increase to a 1.32 dB at 2.29 GHz. The S11 values are still over than 18 dB at frequency region 2.26-2.32 GHz. The discrepancy usually occurs in the measurement results but overall the simulation results and measurement results give an acceptable values. In Figure 9(b) shows a wide bandstop until 5 GHz. Relationship between simulation and measurement results is compared for eight rectangular open loop resonators as shown in Figure 10. Figure 10(a) is for the narrow band while Figure 10(b)  parameter still have acceptable values because S21 still below than 3 dB even S11 still more than 7 dB. The second bandpass are come up to the first bandpass.

CONCLUSION
A substrate integrated waveguide (SIW) bandpass filter using complementary split rectangular resonator has been designed, fabricated and validated for short range device (SRD) application at 2.44 GHz frequency center. The simulation and measurement results give good values for S parameter event though the discrepancy occurs. Mostly, it always happens due to soldering connector or fabrication process.