Relative permittivity of PTFE
| Value | 2.1 — |
| Status | Measured: ± 0.05 — (0.024 relative) |
| Source | Manufacturer data (typical value, 1 MHz–10 GHz) |
| Categories | ElectromagneticMaterial Properties |
| part per trillion | 2,100,000,000,000 ppt |
| part per billion | 2,100,000,000 ppb |
| part per million | 2,100,000 ppm |
| per cent mille | 210,000 pcm |
| basis point | 21,000 bp |
| per mille | 2,100 per mille |
| percent | 210 % |
| ratio | 2.1 — |
Learning zone
Roy Plunkett was working on refrigerants at DuPont in 1938 when a cylinder of tetrafluoroethylene stopped delivering gas but still weighed full. He sawed it open and found a waxy white polymer that nothing would stick to, dissolve or attack. PTFE's electrical virtues turned out to be as remarkable as its chemical ones: a relative permittivity of 2.1 that barely moves with frequency or temperature, and a loss tangent around 0.0002.
That combination made it the dielectric of choice for RF and microwave work. Semi-rigid coaxial cable, PTFE-filled connectors, and low-loss printed-circuit laminates all rely on it, and its low permittivity means a larger conductor diameter for a given impedance, which cuts loss further. Expanded PTFE with trapped air drops the effective value toward 1.4, giving the fastest propagation velocity of any practical cable dielectric — around 80% of the speed of light.