Next-Generation Band Pass Filter using SSPP Structures for B5G Wireless Communication

Table of Contents

• Getting Started
• Abstract
• Introduction
• Motivation
• Design Methodology
• Results and Analysis
• Conclusion
• References
• Acknowledgements
• License
• Contact

Getting Started

If you're new to this repository, here's how you can get started:

1. Clone the repository to your local machine using git clone https://github.com/shashankarya9999/BTech-Project.
2. Navigate to the project you're interested in within the repository.
3. Run, and explore the code.

Abstract

The evolution of wireless communication beyond 5G (B5G) requires RF front-end components with high selectivity, compact size, low loss, and excellent stopband rejection. Conventional transmission-line-based filters face challenges in terms of size, bandwidth, and performance degradation at higher frequencies. This project presents the design, simulation, and analysis of a Next Generation Band Pass Filter (BPF) based on Spoof Surface Plasmon Polariton (SSPP) structures, capable of delivering wideband performance, sharp selectivity, and strong upper-stopband suppression.

Inspired by recent research in hybrid SSPP–Substrate Integrated Waveguide (SIW) filters, the proposed BPF employs periodic metallic corrugations on a planar substrate to realize engineered dispersion characteristics. The SSPP structure provides a tunable high-frequency cutoff and strong field confinement, while the SIW region provides low-loss propagation at lower frequencies. The hybrid design ensures both lower- and upper-cutoff frequencies can be independently controlled. Using ANSYS HFSS, dispersion curves, electric-field distributions, S-parameters, and stopband characteristics were analyzed. The designed filter achieves a wide passband in the X-band region suitable for B5G applications, with excellent stopband rejection, reduced physical size, and low insertion loss. Simulation results from CST Studio Suite confirm that the filter achieves a wide passband in the X-band (8-12 GHz) with deep stopband rejection exceeding 40 dB and significant size reduction, particularly when utilizing the dumbbell-shaped groove geometry. The results confirm that SSPP-based BPFs can serve as promising next-generation RF components for future wireless technologies.

Introduction

Designing RF and microwave components at high operating frequencies ($>1\text{ GHz}$) introduces severe realization challenges:

• Lumped RLC Non-Ideality: Discrete analog components fail to behave ideally due to dominating parasitics like lead inductance and pad capacitance.
• Self-Resonance Frequency (SRF): Most commercial discrete surface-mount inductors and capacitors are completely unusable above 1–2 GHz.
• Distributed Effects: At high frequencies, the operating wavelength becomes comparable to the physical device size, requiring distributed-element transmission line models.
• Radiation & Interference: Traditional planar open architectures easily couple or radiate unwanted electromagnetic energy out of the path.

Motivation

Beyond 5G (B5G) networks target sub-THz and millimeter-wave frequencies demanding wide bandwidths, low latencies, and high data rates. Standard microstrip or coplanar waveguide (CPW) filters suffer massive conductor and dielectric losses alongside poor out-of-band rejection at these scales.

To overcome this, this project customizes a hybrid transmission approach

1. Substrate Integrated Waveguide (SIW): Acts as an artificial waveguide embedded inside a planar dielectric bounded by metallic via walls. It operates natively as a high-pass filter with low insertion loss.
2. Spoof Surface Plasmon Polaritons (SSPPs): Artificial, subwavelength periodic corrugations engineered on a metal surface that mimic optical surface plasmons at microwave regions. They offer an exceptional slow-wave factor, sharp natural low-pass filtering properties, and ultra-tight field confinement.

By cascading or hybridizing these two structures onto a single substrate, we generate a highly selective Bandpass Filter where the lower and upper cutoff bands are independently tuned by independent geometric dimensions.

Design Methodology

Software Tools

• ANSYS HFSS 2024: Employed for structural physical modeling, full-wave 3D electromagnetic finite element simulations, S-parameter assessment, and tracking vector E-field distributions.
• CST Studio Suite 2019: Utilized via its Eigenmode Solver to extract phase constants and map exact unit-cell dispersion diagrams.
• MATLAB: Implemented for post-processing and generating customized mathematical data visualization plots.

Substrate Selection

• Material Material: Rogers RT/Duroid 5880 (Lossy)
• Dielectric Constant ($\varepsilon_r$): 2.2
• Substrate Thickness ($h$): 0.508 mm
• Loss Tangent ($\tan\delta$): 0.0009

Geometric Formulation & Equations

The fundamental $TE_{10}$ wave mode dispersion behavior within an SIW is governed by its effective width ($a$) using the equation:

$$\beta = \sqrt{\left(\frac{\omega}{c}\right)^2 \mu_r \varepsilon_r - \left(\frac{\pi}{a}\right)^2}$$

To translate this waveguide structure safely onto a planar printed circuit board without radiation leakages, the physical structural width ($W_e$) is bounded with precise pitch ($s$) and via-hole diameter ($d$) design equations:

$$W_e = a - \frac{d^2}{0.95s}$$

Where compliance parameters require $d < \frac{\lambda_g}{5}$ and $s \le 2d$[cite: 2]. For this project layout, the specific physical baseline definitions are mapped as follows:

• Via Diameter ($d$): 0.6 mm
• Via Pitch ($s$): 0.9 mm
• Effective Width ($a$): 14.85 mm

The low-pass filtering limit is configured using periodic metallic corrugations on the surface, whose surface wavenumber ($\beta$) is modeled via:

$$\beta = \frac{\omega}{c}\sqrt{1 + \frac{g^2}{p^2}\tan^2\left(\frac{\omega h}{c}\right)}$$

Where $g$ represents groove spacing, $p$ represents period, and $h$ defines groove depth. Two distinct groove models were evaluated and integrated:

• Rectangular Groove Unit: Baseline vertical straight slotting with an active slot height of $h = 7.1\text{ mm}$.

• Dumbbell-Shaped Groove Unit: Circularly capped slot arrays with an active inner height of $h = 6.5\text{ mm}$ and cap tip radius of $r = 0.41\text{ mm}$.

Linear microstrip tapers and graded slot heights are utilized to ensure flawless structural matching from the feedlines down into the main filter body.

Results and Analysis

1. Parametric S-Parameter Response (Transmission Plots)

Full-wave HFSS execution verifies a robust passband centered cleanly in the X-band (8–12 GHz) range:

• SIW Width ($a$) Sweeps: Increasing $a$ from 14.56 mm up to 15.14 mm systematically shifts the lower cutoff edge downward, while leaving the upper band limit entirely stationary.
• Groove Height ($h$) Sweeps: Modifying $h$ scales the upper cutoff frequency limit, highlighting the modularity of the design.
• Dumbbell Radius ($r$) Sweeps: Altering $r$ offers a precise fine-tuning mechanism over the high-frequency slope due to capacitive modification at the slot ends.

2. Dispersion Analysis

Extracted CST Eigenmode plots show that as structural parameters increase capacitive loading (e.g., deeper groove depths $h$ or wider dumbbell end radii $r$), the phase curves pull down from the light line toward an asymptote. This indicates a profound slow-wave propagation velocity shift, verifying why the dumbbell structure achieves identical cutoff responses at lower physical heights ($6.5\text{ mm}$ vs $7.1\text{ mm}$ rectangular).

3. Vector Electric Field Profiles

Field profiles plotted using ANSYS HFSS visually demonstrate the operation of the filter across different bands:

• Lower Stopband ($f = 5.0\text{ GHz}$): Complete transmission blocks are recorded; the operational wave wavelength falls below the cutoff boundaries of the SIW $TE_{10}$ fundamental mode profile.
• Passband ($f = 10.5\text{ GHz}$): Smooth, uninterrupted wave propagation through the device, where electromagnetic energy clings strongly along the internal metallic corrugation walls.
• Upper Stopband ($f = 15.0\text{ GHz}$): Strong transmission rejection exceeding 40 dB. Energy decays exponentially within the initial slots as the wave encounters the asymptotic "stop-light" boundary condition.

NOTE: Transmission Plots, Dispersion Diagram & Vector Electric Field Profile can be found in the detailed Project Report, uploaded in this repository.

Conclusion

This B.Tech final year project successfully demonstrates a high-selectivity, miniaturized, planar bandpass filter using a hybrid SIW-SSPP architecture. By confirming independent control over both the upper and lower transmission band boundaries through parametric optimization, this design presents an efficient solution for reconfigurable RF front-ends in B5G setups. The dumbbell-shaped architecture offers superior performance in terms of spatial constraints and strong energy concentration over traditional rectangular geometries.

Future research tracks include fabricating the layout for physical Vector Network Analyzer (VNA) validation, upgrading to dynamic tuning grids using varactors or RF-MEMS switches, and scaling the architectural parameters to Ka-band frequencies.

References

1. R. S. Sangam and R. S. Kshetrimayum, "Hybrid spoof surface plasmon polariton and substrate integrated waveguide bandpass filter with high out-of-band rejection for X-band applications," IET Microw. Antennas Propag., vol. 15, pp. 289–299, 2021.
2. R. S. Sangam and R. S. Kshetrimayum, "Approximate design equation for iris width calculation of iris substrate integrated waveguide (SIW) bandpass filters," in 2017 Twenty-Third National Conference on Communications (NCC), IEEE, pp. 1–3, 2017.
3. R. S. Sangam and R. S. Kshetrimayum, "Linear tapers: analysis, design and applications," in 2018 IEEE MTT-S International Microwave and RF Conference (IMaRC), IEEE, pp. 1–4, 2018.
4. R. S. Sangam and R. S. Kshetrimayum, "Comment on hybrid spoof surface plasmon polariton and substrate integrated waveguide broadband bandpass filter with wide out-of-band rejection," IEEE Microw. Wireless Compon. Lett., vol. 30, no. 2, p. 222, 2020.

(For a comprehensive review of software execution loops, raw simulation tracking sheets, and source data distributions, consult btech_project_report.pdf verbatim inside this repository)

Acknowledgements

I would like to extend my sincere gratitude to Dr. Ramanand Sagar Sangam, whose expert guidance, constant support, and valuable technical insights were pivotal to the successful progression of this project. His mentorship not only enhanced my understanding of the vast RF & Microwave applications but also encouraged a deeper curiosity in RF filter design. His willingness to share knowledge and offer help at every step significantly enriched my learning experience. I am grateful to my supervisor for his support, patience, and encouragement throughout the duration of the project.

License

This project is licensed under the MIT License. You are free to use the code and resources for educational or personal purposes with citation or reference to the original code and resources used.

Contact

I welcome any feedback, discussion, suggestion or question regarding any of the projects in the repository. Feel free to reach out to me via email at shashankarya9831@gmail.com