Size Reduction in Rat-Race Coupler Designs

Key Takeaways

• Rat-race couplers are used in phase shifters, power dividers, and power combiners in microwave and RF circuits. 

• The rat-race coupler is a four-port device that is capable of dividing power equally between output ports. 

• To achieve size reduction in hybrid type rat-race couplers, T-type equivalent circuits of three lumped elements are employed in the transmission line sections between ports 1 and 4 and the central part of the bottom half of the ring.

Couplers in RF and microwave circuits are used for phase shifting, power combining, and power dividing

In RF and microwave circuits, couplers are devices used for phase shifting, dividing, and combining power. They are most often found in power amplifiers, Butler matrices, and phase array power feeding networks. The most widely used power combining and dividing devices in microwave and RF circuits are the Wilkinson power divider, branch line, and rat-race coupler. 

The rat-race coupler is a four-port device that is capable of dividing power equally between output ports. Depending on the application and integrating requirements, the rat-race coupler designs used in microwave and RF wireless communication systems vary. In this article, we will discuss conventional rat-race couplers as well as some other novel designs. 

Conventional Rat-Race Coupler Design

The rat-race coupler divides power equally between port 2 and port 3. Ports 1 and 4 are isolated from each other and function as input ports. Rat-race couplers are usually manufactured in the shape of a ring or circle.  Each port is placed at one-quarter wavelength (λ) distance away from the other around the top half of the ring. The bottom half of the ring is three-quarter wavelengths (λ) in length, as shown in the figure below. Rat-race couplers operate at a specific frequency and are associated with a characteristic impedance which is √2 times the port impedance. They can function as a power combiner as well as a power divider. It is possible to obtain both in-phase output signals and out-of-phase output signals from rat-race couplers. When the output signals are 180 ° out-of-phase, the coupler is called a 180 ° hybrid. 

Modes of Operation  

The modes of operation of rat-race couplers are:

Mode 1: Input is supplied to port 1 and the output signal is obtained at ports 2 and 3. The output signals are equal in amplitude and in-phase. Port 4 is isolated in this mode of rat-race coupler.

Mode 2: Input is fed to port 4 and the outputs obtained at ports 2 and 3 are equal in amplitude and 180 ° out of phase. Port 1 is isolated in this mode.

Mode 3: In this mode, the rat-race coupler acts as a power combiner. The in-phase input signals given to port 2 and port 3 give the sum of the inputs at port 1 and the difference of the inputs at port 4. Considering this mode, port 1 is also called the sigma (Σ) port and port 4 is called the delta (Δ) port

Rat-race couplers have been applied on transmission media such as microstrips, striplines, coplanar waveguides, and waveguides. Conventional rat-race couplers occupy a large area on the circuit layout. Reducing the size of rat-race coupler designs is a topic of interest among RF circuit designers and layout engineers. New rat-race coupler designs are quantified by parameters such as insertion loss, return loss, bandwidth, isolation, and output imbalance. 

Hybrid Type Rat-Race Coupler Design 

One technique to reduce the size of rat-race couplers is the use of coupled line-based rat-race couplers, which utilize a T-type equivalent circuit. There are a few of these hybrid type rat-race couplers, including those that use three lumped elements and those that use lumped elements, open stubs, and short stubs.

Hybrid Type Rat-Race Couplers Using Three Lumped Elements

In this type of rat-race coupler, three lumped elements are used. For size reduction, T-type equivalent circuits of three lumped elements are employed in the transmission line sections between ports 1 and 4 and the central part of the bottom half of the ring. The circuit and the equations to calculate the values of the lumped element inductor and capacitor are given in the figure above. The size reduction achieved in this hybrid type rat-race coupler is 78.5% of the conventional rat-race coupler.

Hybrid Type Rat-Race Couplers Using Lumped Elements, Open Stubs, and Short Stubs

In this hybrid type rat-race coupler design, size reduction is achieved by using a T-type equivalent circuit consisting of transmission lines, capacitors, open stubs (jBo), and a short stub (jBs). The size reduction achieved in this hybrid type rat-race coupler is 77.9% of the conventional rat-race coupler.

Advanced rat-race coupler designs are utilized in modern microwave and RF wireless communication systems for the addition and subtraction of RF power.  Using a quality set of design and analysis tools can support you in the authentication of rat-race coupler designs with a reduced footprint. 

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