Waveguide RF & Microwave Coupler Basics

RF/Microwave couplers are crucial passive components used in a variety of systems, from high-power transmissions and electronic warfare (EW) to test and measurement instrument calibration. Given the range of use cases for RF couplers, there is a diverse scope of coupler types and configurations. One of the most common types of RF couplers used for millimeter-wave (mm-Wave), high-power, and high-performance applications is the waveguide coupler. This article strives to provide foundational guidance on the basics of RF couplers, such as coupler types and key performance parameters, with a focus on waveguide couplers. This article also provides some details on practical waveguide coupler considerations and applications.

Waveguide Coupler Basic Concepts

The main purpose of a coupler is to extract a controlled amount of the forward and/or reverse wavefront energy from a signal path while minimally impacting the primary signal path. The extracted energy from a coupler can be used in a feedback system to ensure accurate power levels, for sensing, test & measurement equipment, or other applications requiring precise power monitoring without siphoning off too much incident power. Unlike a power combiner/divider, the energy extracted to the coupled port(s) in a coupler is generally far less than half the signal energy, and a coupled port may be configured to primarily accept the forward or reverse directional energy.

There are several types of couplers that can be realized with waveguide technology. There is also a diverse range of port configurations for waveguide couplers, where coupled ports or even an inline port may be a coaxial connectorized port. However, the most common configuration for waveguide couplers is either a complete waveguide coupler with 3 or 4 waveguide ports, or one where only the coupled ports are coaxial connectorized ports with the primary ports.

Depending on the configuration of the waveguide coupler, the component may have 3 or 4 ports, as waveguide couplers have two primary (inline) ports with 1 or 2 coupled ports. The design of the coupled ports dictates if the ports extract energy from the forward or reverse waves passing through the primary ports. A typical directional coupler is a 3-port device that extracts either the forward or reverse wave to the coupled port. A bi-directional coupler is similar to a directional coupler but provides two coupled ports for both forward and reverse wave extraction. Lastly, there is also a dual-directional coupler, Where a bi-directional coupler uses a common coupled element (i.e. a single slotted waveguide section) to extract energy to the two coupled ports, a dual-directional coupler uses two separate coupled elements configured in such a way that both the forward and reverse waves are extracted independently. Hence, a dual-directional coupler can be realized with better isolation than a typical bi-directional coupler and the coupling factors can be different for the coupled ports.

Difference between directional, bi-directional and dual-directional couplers

There are two other common types of waveguide couplers, the 90/180-degree hybrids, or impedance transforming couplers. A hybrid coupler is a special case of a power divider/combiner and coupler where the signal energy is split evenly between two ports, but the coupled port phase is offset by 90 or 180 degrees to the primary port. With careful design of the terminating impedances of a coupler, it is also possible to produce an impedance transformation at the coupled ports. This can be done in one or more stages to increase the impedance transforming capability without sacrificing operating bandwidth.

Key Performance Parameters

Every design and configuration decision of a waveguide coupler comes with trade-offs and considerations. Balancing the key performance parameters for a given application with manufacturing practicality requires careful consideration of the machining, assembly, materials, and processing of a waveguide coupler.

Waveguide Coupler Key Performance Parameters

• Coupling factor/value
• Frequency range/bandwidth
• Directivity/Isolation
• Coupling flatness/Amplitude flatness
• Return loss
• Insertion loss
• VSWR
• Power handling

Graphical plots for key performance parameters

Of the key performance parameters, coupling factor/value and operating frequency range (bandwidth) are often the first considerations. The coupling factor is generally selected based on the power levels of the primary path and how much energy is required at the coupled ports for processing or feedback circuits. The bandwidth is chosen to cover the range of frequencies relevant to the system. The performance requirements of the system typically determine acceptable ranges for directivity/isolation, coupling flatness, return loss, insertion loss, and VSWR. Higher-performance systems, such as test and measurement instrumentation or radar transmitters, may require much higher levels of coupler performance than a telecommunication transmitter monitoring system, for instance. Power handling capability is a limiting factor based on the frequency range of operation and construction quality of a waveguide coupler.

Practical Waveguide Coupler Design & Use Case Considerations

RF couplers are available in a variety of packages and form factors, such as bare die, surface mount, planar circuit, transmission line-based coaxial connectorized, and waveguide. For low-loss, high-power, and high-frequency applications, the best option is often mmWave waveguide couplers. Waveguide interconnects, components and devices generally have the lowest loss and highest power handling of the various interconnect and electromagnetic energy conditioning technologies. The reason for this is that the primary mode of a given waveguide type is a very efficient form of electromagnetic energy containment and guidance, even compared to coaxial transmission lines. Moreover, waveguide structures are generally fabricated from solid and highly electrically conductive metallic structures with good thermal conductivity. For waveguide components like couplers, slots, resonators and passive structures, using the same conductive materials as the waveguide body is often feasible. These materials are intrinsically rugged and can readily be designed for very wide temperature operation and reliable operation in harsh environmental conditions.

At mmWave frequencies, many aspects of the fabrication and design of waveguide couplers are more critical than at lower frequencies. Since waveguide structures are dimensionally proportional to the operating wavelength, higher-frequency waveguide structures are smaller than lower-frequency waveguide structures. This also means that physical features, such as surface finish, dimensional tolerances, alignment, and parallelism must be ensured to a very high degree in order to manufacture mmWave waveguide components with high performance. Hence, it is generally good practice to plate mmWave structures with highly corrosion-resistant yet conductive coatings, such as gold and silver. An additional plating layer can help to prevent corrosion and ensure a more conformal and higher quality surface finish for the interior of the waveguides. In some cases, the outside of the waveguide coupler is painted with anti-corrosion paints or the waveguide structure is completely plated for aesthetics and to simplify processing.

Specifically for waveguide couplers, which are generally broad wall or narrow wall coupled, the alignment, spacing, and dimensions of the slots in the walls are vital criteria to ensure proper performance. The two or three waveguide sections for directional, bi-directional, or dual-directional couplers must also be carefully aligned and joined with tight tolerances to avoid unwanted reflections and increased losses to the primary path. Soldering/brazing is a common method of joining copper-based waveguide sections, and it may be necessary to use post-processing methods to remove any unwanted solder/brazing material from the waveguide coupler slots to ensure the desired level of performance.

Applications of waveguide couplers

mmWave Waveguide Coupler Applications

mmWave waveguide couplers are often used in waveguide test and measurement applications and other test applications where the performance of the coupler is a priority. Waveguide couplers are banded, so the selection of the appropriate waveguide size and coupler frequency of operation is important. Compared to coaxial transmission line-based couplers, waveguide couplers are often able to offer superior coupling value tolerance, VSWR, loss, and coupling/amplitude flatness. These performance areas are often a focus for precision test applications and with test equipment, such as network analyzers where directional couplers are used for the test ports for sampling the forward and reverse waves.

Another common application for mmWave waveguide couplers is sampling the output of high-power mmWave transmitters. Many of these transmitters, especially traveling wave tube amplifier (TWTA) based transmitters, benefit from sampling and analysis of the output of the transmitter to ensure proper operation. Sampling such transmitter outputs with a precision waveguide coupler can allow for feedback and adjustments of the bias and control inputs to limit dispersion, enhance linearity, correct power levels etc. In a similar way a precision waveguide coupler can be used for signal generator leveling in some lab test setups, such as receiver sensitivity/intermodulation testing.

Waveguide Bi-directional and dual-directional couplers are also often used as part of reflectometry setups, which can be extremely useful in identifying faults in a waveguide path during setup or troubleshooting. Reflectometry is also used in test and measurement applications.

Conclusion

Many space, defense, and even commercial applications are moving to mmWave frequencies beyond ka-band to avoid interference and congestion at lower frequencies. Given the high losses at mmWaves, waveguide interconnects, components, and devices are increasingly beneficial as elements in the latest test, communications, and sensing systems are being developed at these frequencies due to their low loss and high performance. MmWave waveguide couplers are an essential component in these systems where precision is key.

Click here to learn more about waveguide directional couplers from Marki Microwave.