The Relevance of Phase Stability over Temperature in Microwave Cables

Aug 6, 2019

Phase stability in microwave cables has become increasingly important in applications where temperature plays an essential role. The challenge is well known in the RF cable industry: PTFE, despite having excellent dielectric properties especially at high frequencies, making it an excellent insulator, also shows a non-linear change in phase when subjected to the 15–25 °C temperature range. This behavior, also known as PTFE knee, has caused several issues across major aerospace, defense and test & measurement equipment manufacturers.

In effect, there is a general need in the market to avoid the development of complicated systems for phase distribution monitoring, what would improve the simplification of diagnostic systems. This can be achieved through coaxial cables whose phase remains consistent over temperature.

Why is Phase Stability important? 

The following examples answer this question.

Example 1: Subsystems in aerospace and defense base their reliability on achieving the highest possible throughput. Some years ago, the use of low frequency copper cables for such applications was widespread. However, these result in losses that are unacceptable for today’s throughput expectations. In space applications, this efficiency optimization is translated into sending digital signals at higher speed rates (Mbit/s). The signals are transmitted via differential pairs.

In other words, every channel transmits the signal through two cables instead of one, and the receiving system responds to the electrical difference between the two signals. This process is necessary for noise elimination. On that account, phase matching is key, since both signals have to be completely equivalent for the electrical difference to be accurate. If the phase of one cable changes with respect to the other because of temperature variations, the signal will not be correctly detected and the system might be thus inefficient.

Example 2: According to U.S. Bureau of Labor Statistics, over the past 20 years electricity experienced an average inflation rate of 2.68% per year. In an attempt to reduce costs, system ventilation has therefore been economized. As a consequence temperatures are fluctuating in these systems in a much bigger cadence than were 20 years ago. As temperature in any part of the cable or at any given moment is not predictable, phase stability has been affected by it in a way that was difficult to counteract, until now.

Cable phase stability is critical for more specific applications, such as phased array antennas, where it is precisely the phase what steers the direction of the antenna’s signal; the newest body scanners which uses similar phase array technology, SAR Satellites where some of the challenges explained above are present; or network analyzers, where the phase invariance over temperature is vital for providing accurate measurements.

In order to offer an answer to above mentioned real life challenges, HUBER+SUHNER recently launched its CT cable family.

These products provide the industry leading phase vs. temperature performance, as well as a unique range of cable constructions to fulfill any demands. Depending on the application, there is a broad variety of product lines available:

  • Sucoform CT – Hand formable, form stable cable with tin soaked braid outer conductor
  • Semi-rigid CT – Form stable cable with tubular, aluminum outer conductor
  • Multiflex CT – Flexible alternative to Semi-rigid microwave cables
  • Minibend CT & Mini 141 CT – Flexible cable assemblies Minibend style with bend-to-the-end connector termination

Features include:

  • Excellent phase and insertion loss stability over temperature
  • Superior phase stability versus bending
  • Outstanding return loss and shielding effectiveness performance
  • Easy installation
  • Small bending radii

In the following video the difference in phase stability between a standard PTFE Multiflex and its CT equivalent – Multiflex CT can be appreciated, when both cables are cooled down.

The benefits of using these cables include the optimization of system accuracy over temperature changes, achieving system performance over multiple cycles and in conclusion, meeting the specified values not only at one single temperature, increasing the reliability of the final product. In the video below, learn, how HUBER+SUHNER CT product family for phase critical applications creates a stable and reliable interconnect solution to satisfy a huge range of customer applications where phase stability is key.

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