What is the PTFE Knee?

For many years, Polytetraflouroethylene (PTFE), or Teflon™ has been widely used in coaxial lines and is considered the best dielectric material due to its low attenuation and excellent electrical and mechanical properties. As the market demanded enhanced performance at increasing frequencies, the manufacturing technologies of PTFE dielectrics started to evolve to achieve the lowest dielectric constant (εr) by striving for maximal air content and unsintered PTFE. The low density PTFE offered an even lower attenuation and increased velocity of propagation (vp). Another method used tape-wrapping technology, instead of the typical extrusion process, to offer a more phase stable material.

Even after the tremendous advancements in PTFE manufacturing technology, the one limiting factor of PTFE was not resolved. At approximately 18°C, PTFE undergoes a structural transition altering the dielectric constant of the material and resulting in a substantial change in the delay of the transmitted signal. This non-linear phenomenon, known as the Teflon™ Knee [See Figure 1], is a property of the molecular structure of the PTFE material and cannot be eliminated regardless of advancements in dielectric manufacturing technology. Coaxial cable manufacturers have made many efforts to minimize this effect while designers of phased array radar systems have been struggling to overcome the Teflon™ Knee due to the lack of options.

Figure 1 – Relative Phase Change (ppm) vs. Temperature (°C) for low density PTFE (76% vp)

In a typical microwave coaxial cable, the inner and outer conductors are separated by a polymer dielectric, and they all are encapsulated by a polymer jacket as shown in the Figure below. 

The speed of a signal traveling through a coaxial cable depends on the dielectric constant of the insulating material between the center conductor and the outer conductor. Thus, cable assemblies use poly-tetrafluoro-ethylene (PTFE, a.k.a. Teflon™) or expanded PTFE (e-PTFE) as the dielectric, to minimize signal delay. But PTFE undergoes dimensional changes with temperature that can be measured via a thermo-mechanical analysis (TMA).

If we see the typical TMA curve for unconstrained, solid PTFE, at approximately 19°C there is a dramatic change in the dimensions of PTFE due to a change in the crystal structure. The crystals rearrange themselves in a structure that is more energetically favorable. 

This drastic change can be seen in the phase versus temperature curve of a PTFE cable assembly (See image belwo) and is commonly known as the “Teflon™ Knee” or PTFE Knee.