CSRR-DGS Bandpass Filter Based on Half Mode Substrate Integrated Waveguide for X-Band Applications

In this research, a novel half mode substrate integrated waveguide (HMSIW) bandpass filter based on defected ground structure cells (DGS) is proposed. By using the periodic square Complementary Split Ring Resonator (CSRR) resonant properties of DGS according to design requirement, an X-band bandpass filter is designed and analyzed to meet compact size, low insertion loss, and high rejection. The simulation results obtained by CST and HFSS simulators in X-band show that the proposed filter is characterized by a large transmitted bandwidth of about 1.6 GHz from 13.2 to 14.8 GHz. The higher simulated insertion loss is about −2.6 dB and the lower return loss is about −39 dB. The proposed filter size is 9.5 × 38.0 mm2 which make it a compact component. The filter design is optimized using CST and to validate the proposed filter the simulation results is compared by HFSS. The HFSS simulation results are in decent promise with the CST results. INDEX TERMS Substrate integrated waveguide, band-pass filter, defected ground structure, CSRR.


I. INTRODUCTION
ubstrate integrated waveguide (SIW) was first proposed in 2003 [1]. Based on the TE 0 transmission mode, SIW replaces the side metallic walls of the classic rectangular waveguide with two rows of metallic via holes, which converts conventional waveguides into planar structures [2][3][4]. Therefore, SIW not only have the properties of high quality factor and low radiation loss which are similar to metallic waveguides but also have the prominent advantage of compact size due to their planar physical structure. SIW have been applied to the design of filters [5]. Couplers [6], oscillators [7], power dividers [8], and many other microwave components. Defected Ground Structure (DGS) is a geometrical periodic and/or aperiodic slots engraved on the ground plane of microwave circuits. The defects on the ground plane disturb the current distribution of the ground plane; this disturbance changes the characteristics of a structure by including some parameters (slot resistance, slot capacitance, and slot inductance) to the structure parameters [9]. The half mode substrate integrated waveguide (HMSIW) is used for reduces to SIW components size by the half or so and keeps the same performance of full SIW, it has been developed and widely used in varied applications. [10][11][12] Complementary split ring resonator (CSRR) was proposed in 2004 as a 3D metamaterial [13] that can exhibit negative permeability near its resonant frequency and therefore can be considered as a composite right/left-handed (CRLH) structure. When CSRRs are employed in SIW, pass bands based on the evanescent mode below the cutoff frequency of the SIW can be created [14,16], which can further miniaturize the size of SIW microwave devices [17,18]. This paper describes a compact BPF for the TE10 mode based on periodic CSRR as DGS cells and by using HMSIW technique we reduced the filter size by half. We have obtained a good results for X-band in term of losses, size and rejection.

II. SIW WAVEGUIDE DESIGN METHOD
The main parameters in SIW design are the SIW width "W", the diameter "d" of the metal vias and the spacing "p" between the centers of two successive vias [19]. The dielectric filled waveguide DFW is characterized by the dimension "ad" which is given by equation 1.
εr represent the relative permittivity of the dielectric that fills the waveguide [19].
For TE10 mode the cut-off frequency (fc) for a DFW is calculated by equation 2 [19].
Where c is the speed of light in a vacuum.
The optimal width of the SIW is calculated by equation 3 [20].
Two conditions are required for minimize the losses due to the field leakage from the gaps between vias: d < λg × 0.2 and p ≤ 2 × d where λg is the guided wavelength [20].

III. DGS SIW BAND-PASS FILTER DESIGN
Using the CST we designed a new SIW-DGS band-pass filter structure for X-band on Arlon Cu Clad 217 based substrate having a loss tangent approximately 0.018, a relative permittivity of 2.2, a thickness h of 0.508 mm and the thickness of the conductor is 0.05mm and for reduced the filter size we used filter half mode technique. The DGS CSRR inserted at the top plane are shown in the figure 2. We designed the filter based on two cascading CSRR in the top plane separated by a distance M of 5.5 mm and for impedance matching we used tapered transition.    From Figure 3 we observe that the insertion loss is about of 1.6 dB in band ranging from 13.1 GHz to 14.3 GHz also we observe the appearance of two peaks, the best reached -47 dB at the frequency of 14.1 GHz. The rejection is fair under 13 GHz and above 14.5. From this analysis it can be said that this SIW filter is functional in the X band with Fair performance. By using the half-mode technique, we reduced the filter size and kept the same performance of the SIW filter. The half mode SIW band pass filter structure and simulation are shown in the Figure 4.    It can also be noted that we have an improvement in the rejection bands that ranging from 11 to 13.2 GHz and from 14.8 to 17 GHz also the return loss is under -14 dB in all filtering band about insertion loss is about of -2.6 dB in the transmitted band. We obtained a good agreement between CST Studio and Ansoft HFSS results which demonstrates our proposed filter.

V. CONCLUSION
In this work a band-pass filter based on HMSIW and periodic CSRR is designed and simulated for X-band applications, the design method is discussed. CST and HFSS simulators are used for the simulation, and we have compared between the results obtained. The proposed filter is characterized by a good performance in term of insertion loss, return loss, size and rejection. This type of filter is easy for integration with other planar circuit compared with conventional waveguide. For future work we will work to reduce the size of the proposed filter and improve its losses and its rejection.