⚡ electrical engineering

Systematic Design of Wideband Single-Layer Microstrip 180° Hybrid Couplers Using a Novel Topology

This paper presents two compact, wideband (4–9 GHz) single-layer microstrip 180° hybrid couplers utilizing a novel topology that combines Wilkinson-based and five-port power dividers with phase-shifting networks to achieve excellent broadband performance, high isolation, and low amplitude and phase imbalances.

Original authors: Parisa Safaei, Masoud Movahhedi

Published 2026-07-01

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CC BY 4.0

Original authors: Parisa Safaei, Masoud Movahhedi

Original paper licensed under CC BY 4.0 (https://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 you are running a busy kitchen where you need to split a single stream of ingredients into two separate bowls. Sometimes, you want the ingredients to land in both bowls at the exact same time, moving in perfect sync. Other times, you want one bowl to get the ingredients while the other gets them exactly one step later, creating a "push-pull" rhythm.

In the world of radio waves and electronics, this job is done by a device called a 180° Hybrid Coupler. Think of it as a sophisticated traffic director for invisible signals. It takes a signal coming in and splits it into two paths: one where the signals march in step (in-phase) and one where they march in opposite directions (out-of-phase).

This paper, written by researchers Parisa Safaei and Masoud Movahhedi, introduces a new, smarter way to build these traffic directors. Here is the breakdown of their work in simple terms:

The Problem with the Old Tools

For a long time, engineers used a design called a "rat-race coupler." Imagine a racetrack that is shaped like a giant ring. To make the signals arrive at the right time, the track has to be very long—specifically, it needs to be a quarter or three-quarters of a wavelength long.

The Issue: These tracks are huge (taking up a lot of space on a circuit board) and only work well for a narrow range of radio frequencies. If you try to use them for too many different channels, they get confused and stop working well.

The New Solution: A Modular "Lego" Approach

The researchers proposed a new "topology" (a fancy word for the layout or blueprint). Instead of building one giant, rigid ring, they built a system using smaller, interchangeable building blocks.

Think of their design like a modular plumbing system:

The Splitter: They use a special "power divider" that acts like a Y-shaped pipe, splitting the water (signal) into two equal streams.
The Twist: They add a "phase shifter," which is like a twist in the pipe that delays one stream just enough so it arrives 180 degrees out of sync with the other.
The Mixer: They combine these pieces so that depending on which door you open (which port you use), the signals either march together or march apart.

Two New Designs

The team built two versions of this new system to prove it works:

1. The "Wilkinson" Version (Work I)

How it works: They used a standard, reliable type of splitter (called a Wilkinson power divider) combined with their new phase-shifting tricks.
The Result: It works great across a wide range of frequencies (from 4 GHz to 9 GHz). It's like a high-quality, all-terrain vehicle that handles bumpy roads (different frequencies) very well.
Key Feature: It keeps the signals very clean and prevents them from leaking back into the wrong pipes (high isolation).

2. The "Five-Port" Version (Work II)

How it works: To make the device even smaller, they replaced the standard splitters with a brand-new, compact design they invented called a "five-port power divider." Imagine shrinking a large, complex machine into a sleek, pocket-sized gadget.
The Result: This version is about 34% smaller than the first one and works on an even wider range of frequencies. It's like upgrading from a sedan to a compact sports car—same engine power, but much tighter and faster.

Why This Matters

The researchers achieved three major things:

Size: They made the device much smaller by putting everything on a single flat layer (like a sandwich with only one slice of bread), rather than stacking multiple layers. This makes it easier and cheaper to manufacture.
Speed: They made it work over a "wideband" range. Instead of working on just one radio station, this device can handle a whole radio station and its neighbors without losing quality.
Precision: They proved that the signals stay perfectly synchronized (or perfectly opposite) across the whole range, with very little error.

The Bottom Line

The paper claims that by using this new "Lego-like" blueprint, they created two compact, single-layer devices that are excellent at splitting radio signals into "in-sync" and "out-of-sync" versions. They tested these devices in a lab, measured them, and found that the real-world results matched their computer simulations perfectly.

In short, they built a smaller, faster, and more versatile traffic director for radio waves that is easier to build and fits into modern electronic systems much better than the old, bulky designs.

Technical Summary: Systematic Design of Wideband Single-Layer Microstrip 180° Hybrid Couplers Using a Novel Topology

Problem Statement
180° hybrid couplers are essential components in microwave and RF systems, utilized in balanced amplifiers, reflectometers, and beamforming networks to provide in-phase ( $\Sigma$ ) and out-of-phase ( $\Delta$ ) signal combinations. While the conventional rat-race coupler is widely used, it suffers from inherent limitations: narrow bandwidth due to fixed quarter-wavelength transmission lines and large physical size. Although various alternatives have been proposed—including multi-section structures, fractal geometries, and multilayer implementations—many existing solutions still face trade-offs regarding bandwidth, fabrication complexity, non-planar geometries, or the need for extensive via interconnections. There remains a need for compact, broadband 180° hybrids that utilize simple, single-layer planar microstrip technology while maintaining high isolation and low amplitude/phase imbalance.

Methodology
The authors propose a novel, systematic topology for realizing wideband 180° hybrid couplers based on the combination of in-phase power dividers, out-of-phase power dividers, and phase-shifting networks. The theoretical framework is established using a generalized block diagram comprising a four-port 180° phase shifter and four in-phase power dividers. The operating principle relies on signal flow analysis where:

Excitation at the $\Delta$ port results in signals that are split, phase-shifted by 180°, and recombined to produce out-of-phase outputs at ports 2 and 3, while destructive interference isolates the $\Sigma$ port.
Excitation at the $\Sigma$ port creates an in-phase power division network, delivering equal amplitude and phase signals to ports 2 and 3, while destructive interference isolates the $\Delta$ port.

To validate this topology, two distinct single-layer microstrip implementations were designed, fabricated, and tested on a Rogers RO4003C substrate ( $\epsilon_r = 3.38$ , $h = 0.508$ mm):

Work I (Wilkinson-Based): This design integrates a compact ultra-wideband (UWB) out-of-phase power divider (utilizing microstrip-to-slotline transitions), a novel compact UWB in-phase power divider (using elliptical patches and rectangular slots), and two Wilkinson Power Dividers (WPDs). This configuration eliminates the need for additional substrate layers found in previous iterations, achieving a fully planar structure.
Work II (Five-Port Based): To further enhance compactness and bandwidth, the Wilkinson dividers in the first design were replaced with a novel five-port power-divider configuration based on parallel microstrip lines with rectangular stubs. This structure utilizes a ground-plane slot for electromagnetic coupling among the $\Sigma$ , 2, and 3 ports.

Both designs were optimized using Ansoft HFSS, and prototypes were fabricated and measured to verify simulation results.

Key Contributions

Novel Topology: Introduction of a generalized framework that systematically combines specific power dividers and phase-shifters to create a family of 180° hybrids with flexible bandwidth and performance characteristics.
Single-Layer Implementation: The realization of fully planar, single-layer microstrip couplers that eliminate the complexity of multilayer structures, suspended substrates, or extensive via usage.
Component Innovation: Development of a compact UWB in-phase power divider and a novel five-port power-divider configuration specifically tailored for integration into the hybrid topology.
Design Flexibility: Demonstration that the topology can be adapted to trade off between circuit size and fractional bandwidth by selecting different constituent building blocks (e.g., WPDs vs. five-port dividers).

Results
Both proposed couplers demonstrated wideband operation and excellent agreement between simulated and measured results:

Frequency Range: Both designs operate effectively over the 4–9 GHz range. Work II extended the measured fractional bandwidth to 87% (3.5–9.5 GHz), an improvement over Work I's 73%.
Return Loss: Input return losses at the $\Sigma$ and $\Delta$ ports were better than 12 dB across the operating bands.
Isolation: Isolation between the input ports ( $\Sigma$ and $\Delta$ ) exceeded 30 dB, while isolation between output ports remained better than 15 dB across the entire band.
Amplitude and Phase Balance: Amplitude imbalance was low (within $\pm 0.5$ dB to $\pm 1$ dB). Phase deviation from the ideal 0° (in-phase) and 180° (out-of-phase) responses was maintained within approximately $\pm 5^\circ$ to $\pm 10^\circ$ .
Compactness: Work II achieved a footprint reduction of approximately 34% compared to Work I, with dimensions of 37.6 mm $\times$ 37 mm, while maintaining a single-layer configuration.

Significance
The paper claims that the proposed topology offers a favorable combination of wide bandwidth, compact size, and fabrication simplicity. By avoiding multilayer structures and complex via interconnections, the designs are presented as attractive candidates for integration into modern microwave and RF front-end systems. The work validates that a systematic combination of specific power division and phase-shifting techniques can yield high-performance 180° hybrids without the drawbacks of size and complexity associated with traditional or previously reported advanced structures. The flexibility of the topology is highlighted as a practical foundation for developing future wideband couplers with customized performance requirements.

The Problem with the Old Tools

The New Solution: A Modular "Lego" Approach

Two New Designs

Why This Matters

The Bottom Line

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