The webinar is the second in a two-part series on phase stable cable assemblies and electrical length changes within cable assemblies in aerospace engineering and space technology development applications, among others. In this session, Dave Slack digs deeper into the impact of temperature on phase behavior. He explains the polytetrafluoroethylene (PTFE, also known Teflon™) “knee” that exists in some cables and how it impacts their phase performance. Slack also provides guidance on minimizing those effects with special materials, such as cables made from silicon dioxide (SiO2) and TF4.
Watch the video or read the session notes below.
A typical phase versus temperature signature of a flight-grade cable, circa 1995, would have been a 76% velocity cable made with a PTFE core. These cables were very rugged and resistant to damage during installation and maintenance.
Typically, there would be a relatively flat phase/temperature slope below room temperature. However, at room temperature, there is an abrupt jump that changes the phase temperature profile. This is due to a peculiarity of Teflon—a nearly perfect dielectric material for cable, RF and microwave applications because it has a constant dissipation factor as well as a constant loss tangent across a wide range of frequencies and temperatures. However, Teflon undergoes a mechanical or materials phase transition in which it changes density by about a percent and a half between 18-22 degrees Celsius, or 64-72 degrees Fahrenheit.
That change in density also causes a change in dielectric constant, which, as discussed in the Phase Stable Assemblies 101 webinar, creates a velocity change. This in turn produces an abrupt change in electrical length—a very common phenomenon known as the Teflon knee, or the PTFE knee.
A cable assembly gets electrically longer as it gets colder, and shorter as it gets warmer – contrary to what one might expect. Electrical length is proportional to physical length. Metals expand as they get warmer and contract as they get colder but as that happens, the dielectric constant expands and contracts as well and its density changes, altering the velocity. The dielectric effects of the plastic offset and dominate the metal effects.
Phase-matched cables are in pairs or groupings. As temperature changes to cold or warm extremes, they don’t exactly track together; the phase match degrades just slightly. That small amount of degradation is known as its phase tracking characteristic.
In a practical situation, a cable assembly might be phase matched to the initial assembly with a tolerance plus or minus a degree. The slope is the same, but it is offset by that initial match. The uncertainties and errors that accumulate with changes in temperature are the initial phase match plus the phase tracking; the two add to each other.
In another scenario, there may be two cable type families – a full-density and low-density PTFE. The vertical scaling is very large, in an 8,000 part-per-million window. Full-density solid Teflon actually takes up every bit of that scale. It has a relatively extreme slope below and above the knee, and a very pronounced step function during the phase transition temperature.
In the following example, we compare this against the low-density PTFE, with 10 cable assemblies of each family superimposed on top of each other, all phase matched at room temperature, tested and plotted across temperature. As it changes to extreme heat or cold, the cables are no longer perfectly phase matched; they differ from each other.
We tested two cable assemblies to look at phase versus frequency. One cable is a PhaseTrack® cable, and the other is a standard PTFE cable. When a freeze spray cools the cables down, the PhaseTrack cable stays consistent, whereas the PTFE changes because of the phase temperature knee. That causes antenna beam forming characteristics to defocus.
Because of these complexities, there is value in dealing with a supplier that offers a range of different technologies, and can provide optimized solutions for each unique application.
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