⚡ electrical engineering

Compact Microstrip Triplexer Using Square Open-Loop Resonators with High Isolation for 5G Sub-6 GHz Applications

This paper presents a compact microstrip triplexer utilizing square open-loop resonators to achieve high inter-channel isolation exceeding 45 dB across 2.2, 2.6, and 3.0 GHz bands for 5G sub-6 GHz applications.

Original authors: Bhaskarareddy N C, Rashmi Ravi, Augustine O. Nwajana

Published 2026-08-17

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CC BY 4.0

Original authors: Bhaskarareddy N C, Rashmi Ravi, Augustine O. Nwajana

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the air around us is a bustling, invisible highway filled with radio waves. These waves carry our texts, videos, and calls, but they all travel at different speeds and on different "lanes" called frequencies. In the past, your phone might have had a separate, tiny radio for every single lane it needed to use. But as technology races forward, packing more and more lanes into the same space, engineers face a tricky problem: how do you build a single, tiny device that can listen to three different lanes at once without them crashing into each other? This is the world of radio frequency engineering, where the goal is to create "traffic controllers" that sort signals perfectly. If these controllers are too messy, your video call freezes, or your music skips. The key to solving this is building a device that can split one signal into three distinct paths, keeping them so far apart that they never accidentally talk to one another.

This paper introduces a new design for such a traffic controller, specifically for the upcoming 5G networks that will power our future devices. The researchers, working at the University of Greenwich, have built a "triplexer"—a fancy word for a device that splits one signal into three. Think of it like a magical three-way fork in a road where a single car (the signal) arrives and is instantly sorted into three different lanes (2.2 GHz, 2.6 GHz, and 3.0 GHz) without ever getting stuck or mixing with the other cars. To do this, they used a clever trick involving "square open-loop resonators." Imagine taking a long, straight piece of wire and folding it into a tight square loop; this makes the wire act like a spring that vibrates at a specific note, but in a much smaller space. By arranging these loops into three separate groups, the team created a device that can handle three different 5G frequencies simultaneously.

The most exciting part of their work is how well these lanes stay separated. In many previous designs, the signals would sometimes "leak" from one lane to another, causing interference, much like hearing a neighbor's radio through your wall. The researchers simulated their design on a computer and found that their new device keeps the lanes incredibly quiet. The signals in one lane are blocked from the others by more than 45 decibels of isolation. To put that in perspective, if the signal in one lane were a shout, the signal leaking into the next lane would be quieter than a whisper. This is a significant improvement over most other designs, which usually only manage to keep the noise down to about 20 or 25 decibels.

The team didn't just guess; they ran detailed computer simulations to prove their idea works. They built a virtual model of their triplexer on a special type of circuit board material and tested how it performed. The results showed that the device is very good at its job: it lets the right signals through with very little loss (only about 1 decibel of signal strength lost), and it reflects almost none of the signal back to the source. While this is currently a computer simulation and not a physical device they have built and tested in a lab yet, the numbers suggest that if they were to build it, it would be a highly efficient, compact component perfect for the next generation of wireless technology. The paper argues that by using this specific square-loop design and a special "T-junction" connection, they have solved the problem of signal leakage better than most other methods currently available, offering a cleaner, more compact way to manage the crowded airwaves of the future.

Technical Summary: Compact Microstrip Triplexer Using Square Open-Loop Resonators for 5G Sub-6 GHz Applications

Problem Statement
The rapid deployment of 5G New Radio (NR) sub-6 GHz systems, alongside the coexistence of LTE and Wi-Fi networks, has created a critical demand for compact, high-isolation passive multiplexing components. Modern transceivers require the ability to route multiple frequency channels through a single shared antenna port while minimizing inter-channel interference and signal degradation. While existing microstrip triplexer designs utilizing technologies such as Stepped-Impedance Resonators (SIRs), Complementary Split-Ring Resonators (CSRRs), and coupled-line cells offer miniaturization, they frequently suffer from limited inter-channel isolation, typically ranging between 20 dB and 25 dB. This level of isolation is often insufficient for demanding multi-standard front-ends where rigorous control of channel crosstalk is required. Furthermore, cascaded diplexer arrangements often accumulate insertion loss and matching-network complexity.

Methodology
The authors propose a compact microstrip triplexer designed to operate simultaneously at 2.2 GHz, 2.6 GHz, and 3.0 GHz, targeting specific 5G NR sub-6 GHz allocations. The design methodology follows a systematic approach:

Resonator Selection: The core building block is the Square Open-Loop Resonator (SOLR). By folding a half-wavelength ( $\lambda_g/2$ ) straight microstrip resonator into a square loop with a coupling gap, the linear dimension is reduced to $\lambda_g/8$ . This geometry is implemented on a high-permittivity Rogers RT/Duroid 6010LM substrate ( $\epsilon_r = 10.7$ , thickness = 1.27 mm) to achieve physical compactness.
Filter Synthesis: The triplexer comprises three independent three-pole Chebyshev bandpass filter (BPF) channels, totaling nine resonator poles. Each channel is synthesized from a standard normalized Chebyshev lowpass prototype with element values $g_0 = g_4 = 1.0$ , $g_1 = g_3 = 0.8516$ , and $g_2 = 1.1032$ . Each channel is assigned a fractional bandwidth (FBW) of 3% and a system impedance of 50 $\Omega$ .
Integration and Isolation Strategy: The three filter channels are integrated at a common input port via a T-junction. A key design feature is the dimensioning of the connecting transmission line stubs (TL $_1$ , TL $_2$ , TL $_3$ ) to approximately a quarter-wavelength ( $\lambda_g/4$ ) at their respective center frequencies. This configuration presents a high-impedance condition at the junction node for non-intended channels, thereby enforcing physical separation of signal paths and maximizing inter-channel isolation without auxiliary resistors.
Simulation: The design was modeled using Keysight PathWave Advanced Design System (ADS) Momentum for full-wave electromagnetic (EM) simulation, assuming copper conductors with 35 $\mu$ m thickness.

Key Results
Full-wave EM simulation results demonstrate the following performance metrics for the proposed triplexer:

Return Loss: The device achieves return losses of 21.1 dB (2.2 GHz), 23.1 dB (2.6 GHz), and 22.8 dB (3.0 GHz), all exceeding the 20 dB benchmark for well-matched microwave devices.
Insertion Loss: The insertion losses are recorded at 1.08 dB, 1.01 dB, and 0.98 dB for the respective channels, indicating low-loss performance for a nine-pole device.
Inter-Channel Isolation: The most significant result is the inter-channel isolation, which exceeds 45 dB across all port pairs: 45.7 dB (S $_{32}$ ), 45.2 dB (S $_{42}$ ), and 45.7 dB (S $_{43}$ ).
Physical Footprint: The complete active circuit occupies a bounding box of 69.45 mm $\times$ 39.7 mm on the substrate.

Significance and Claims
The paper claims that the achieved inter-channel isolation of over 45 dB represents a significant improvement over the majority of recently reported microstrip triplexer designs, which typically struggle to exceed 20–25 dB in the sub-6 GHz range. The authors attribute this high isolation directly to the T-junction integration scheme and the use of SOLRs on a high-permittivity substrate, rather than relying on post-design filtering or complex auxiliary structures.

By comparing the design against state-of-the-art literature (referencing works [5]–[10]), the paper positions this triplexer as offering the highest reported inter-channel isolation for microstrip triplexers operating in the 2–4 GHz range, with the exception of specialized designs targeting much higher frequencies. The design is presented as a symmetric, controllable, and reproducible solution that consolidates filtering and routing functions into a single monolithic device, thereby reducing circuit complexity and cumulative insertion loss compared to cascaded diplexer arrangements. The paper concludes that the strong agreement between the theoretical circuit model and the EM simulation validates the proposed Chebyshev synthesis and T-junction integration methodology.

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