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Phase Stability

Performance of RF coaxial cable assemblies depends on a variety of factors and is often defined in different ways. Depending on the performance requirements, attributes like phase stability become very important. Phase stability is an important attribute of RF cables, yet it remains a concept not fully grasped.

Previously, we compared RF cables to a string quartet or a 4 piece band. Let’s revisit this metaphor to better understand phase stability.

What is phase stability? 

Phase stability is typically measured by two variable factors: temperature and flexion. Unlike phase matching, which compares cable performance against a standard, phase stability poses a different question: “How does the cable’s performance fluctuate with changing environmental conditions?”

Imagine our string quartet sets up outside on a winter day. Our musicians have bundled up for the cold and begin to play. As they play, they gradually shed layers as the sun emerges and they warm up. Now, when they finish the song they’ve been playing, how closely does their last note align with what it would have been had they remained cold throughout?

This scenario illustrates phase stability over temperature. Many applications which use RF coaxial cables experience significant temperature fluctuations, either due to the system heating or transitioning between hot and  cold environments. The change in temperature causes a change in the dielectric constant, a property of coaxial components that defines how fast RF energy travels through the cable assembly.

Materials like Teflon™ and generic alternatives, such as PTFE, have a well-known change, called a phase shift, as the temperature fluctuates. The so-called “Teflon™ knee” occurs when temperatures change from below to above room temperature. Cable manufacturers employ various techniques to minimize this phase shift, requiring engineers to make careful selections based on the application’s requirements.

Phase stability is expressed as a percentage of change.  In the case of phase stability over temperature, phase change is defined as “+/- x% over y degrees change.” 

Phase stability over flexion

Let’s imagine our quartet taking their performance to the streets, where they’re asked to move around while playing.How would that movement affect their synchronization? This scenario mirrors the concept of “stability over flexion,” a crucial consideration when discussing phase stability in coaxial cable assemblies. How does moving or repeated bending of the coaxial cable assembly affect its performance?

The most common application where stability over flexion is critical is test cables used with RF test equipment, often called network analyzers or VNAs. These cables endure frequent handling, bending, and twisting, comparable to our musicians navigating the streets. However test cables are expected to perform exactly the same regardless of how they are bent or where they are rested. Having cables that are phase stable over flexion is important to get consistent, reliable test results over many cycles.  

Applications where phase stability is critical

We’ve already noted the importance of phase stable cables in test cables, but it also extends to various industries and applications requiring precise timing or phase relationships between signals:

  • Aerospace and defense applications, including electronic warfare (EW) systems and missile defense systems, demand phase stable cables that can withstand extreme temperature variations, from scorching desert heat to icy altitudes.
  • Space applications, such as LEO satellites and Lunar landers, use RF coaxial assemblies for collision avoidance, mapping and other research missions.  In the harsh conditions of space, where temperatures fluctuate dramatically, phase stability over temperature becomes imperative.
  • Medical systems like MRIs use cables that must be phase stable over flexion as the elements of the system are moved in ways necessary to obtain the best images possible to aid in the identification, diagnosis, and treatment of medical conditions.  

Performance for the mission at hand

Phase stable cables, whether stable over temperature or stable over flexion, play a vital role in the performance of systems that often have a high cost of failure.   By understanding the requirements, environmental factors, and expected lifespan, engineers can specify and produce phase stable cables tailored to meet the demands of each unique mission.

Understanding phase stability is crucial for ensuring the reliability and performance consistency of coaxial cable assemblies across diverse applications and environments. By delving into this concept, we empower engineers to make informed decisions, ultimately enhancing the efficiency and effectiveness of critical systems across industries. If you’d like to learn more feel free to contact us.


  • Jessica Chandler

    Jessica Chandler is a Digital Marketer with over ten years of experience within the digital marketing landscape. She has been proudly working with Winchester Interconnect since 2023.

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