High selective SIW bandpass filter with flexible bandwidth and transmission zero for 5G application

Why this tiny part matters for big 5G signals

As mobile networks race toward higher speeds and more connected devices, the hardware that cleans and shapes radio signals becomes critically important but remains largely invisible. This paper focuses on a small but essential building block called a bandpass filter that helps 5G systems pick out just the right slice of the radio spectrum while rejecting unwanted noise and interference. By rethinking how energy flows inside a compact metal-lined channel on a circuit board, the authors show how to build filters that are precise, flexible, and practical for mass-produced 5G equipment.

Guiding waves on a flat circuit board

Traditional high-frequency radio hardware faces a trade-off. Bulky metal waveguides carry signals with low loss and high power, but they are expensive and hard to integrate. Flat transmission lines printed on circuit boards are cheap and compact, but they suffer higher losses and perform poorly as frequencies climb into millimeter-wave bands used by 5G. A technology called a substrate-integrated waveguide (SIW) offers a compromise: rows of metal pins embedded in a circuit board imitate the walls of a hollow metal tube, forming a low-loss pathway for radio waves while keeping everything in a flat, manufacturable format. This makes SIW an attractive platform for filters that must work reliably around 27 GHz and above.

Shaping a narrow passband with smart geometry

The authors propose a new way to couple energy between SIW cavities using a combination of a narrow channel, a rectangular slot cut in the top metal, and a single metal post placed near that slot. Together, these features act like a carefully tuned mix of capacitance and inductance that drives how different frequencies pass or are blocked. The filter is designed to operate in a specific internal vibration pattern of the waveguide, and the geometry is arranged so that the strongest electric field lines intersect the slot and the post. This arrangement not only sets the width of the useful frequency band but also creates sharp notches, called transmission zeros, that carve deep holes in the unwanted regions just outside the band.

Tuning knobs for engineers

One strength of the design is that it gives engineers clear, independent “knobs” to tune different aspects of the filter without rebuilding it from scratch. The width of the slot mainly adjusts the capacitive part of the coupling: by widening or narrowing it, the passband can be broadened or tightened, and the high-side notch can be shifted, while the lower edge of the band stays almost fixed. The position of the metal post inside the narrow path controls the inductive part, which moves the lower band edge and changes the bandwidth but leaves the notch frequency nearly unchanged. A third geometric parameter changes how the post sits relative to the slot; this allows simultaneous tuning of the notch and bandwidth while keeping the band centered at the same frequency. Through simulations, the authors map out how each dimension affects key performance measures, giving a practical recipe for custom filter design.

From simulation to working 5G hardware

To show that the concept works in real hardware, the team builds and measures two different filters on a standard low-loss circuit-board material. The first uses a straightforward “inline” layout in which energy flows directly from input to output through two main cavities and the central coupling section. This version is centered around 27.12 GHz, passes a narrow band of about 5 percent relative width, and introduces a strong notch just above the passband, leading to a steep roll-off and high rejection of higher-frequency interference. The second filter rearranges the same building blocks into a cross-coupled layout, where the signal can travel along multiple paths that cancel at specific frequencies. This design adds a second notch below the passband, giving sharp edges on both sides while keeping losses low and bandwidth similar.

What this means for future 5G gear

In simple terms, this work shows how a carefully sculpted piece of metal and dielectric on a single-layer circuit board can act as a precise gatekeeper for 5G signals. By combining a narrow channel, a slot, and a post in a compact SIW cavity, the authors achieve filters that are easy to fabricate, highly selective, and tunable to different specifications. Such filters are well suited for millimeter-wave 5G front ends, where they can help radio units choose channels more flexibly, reject interference efficiently, and still fit within the tight space and cost limits of modern wireless infrastructure and devices.

Citation: Mishra, G.K., Pandey, H.K. & Pathak, N.P. High selective SIW bandpass filter with flexible bandwidth and transmission zero for 5G application. Sci Rep 16, 9639 (2026). https://doi.org/10.1038/s41598-025-34655-3

Keywords: 5G millimeter-wave, bandpass filter, substrate-integrated waveguide, transmission zero, RF front-end design