Film coils with polypropylene as insulator

The conductor of the coil is a film made of (Angelique) copper, silver, or (Angelique) silver-gold. Unlike enameled copper wire, for example, the film is not coated with enamel for insulation; instead, a polypropylene film wound together with the film conductor serves as the insulator.

Electrical strengths

• Compared to wire enamels, polypropylene has a low dielectric constant and very low dielectric losses → very low parasitic capacitance
• The polypropylene film is thicker than a typical wire enamel coating → further reduced parasitic capacitance.
• A foil coil tightly wound with PP film forms an extremely solid block. The softer PP film also provides strong damping against mechanical vibrations.

Advantages of Polypropylene Compared to Wire Insulation Enamel

Each coil also functions as a series of many individual capacitors, since the individual turns are separated by a dielectric (the insulation). At first glance, one might think that these many individual capacitors are short-circuited by the conductor itself. However, the conductor is not an ideal, resistance-free object; rather, it has a physical volume, a DC resistance, and an AC impedance. Depending on the geometry of the conductor and the coil, as well as the frequency of the alternating current, the current may flow not only through the actual conductor — which also acts as an inductor with an impedance that increases with frequency — but also increasingly through the parasitic capacitance formed by the closely spaced turns.

This effect degrades the coil’s high-frequency response. The thicker the insulation and the lower the relative permittivity, the less pronounced this undesirable effect is.

Compared to conventional wire enamels, polypropylene has a low permittivity and extremely low dielectric losses. As a result, unlike wire enamels, it does not dampen the signal energy through internal molecular friction and releases it quickly and with precise phase alignment.

The Decisive Advantage: the Quietest Wrap

Film coils are structurally much more stable than round-wire windings—and, in practice, the smoothest in terms of mechanical operation. Precisely because the foil is flat, dimensionally stable, and tightly wound, the winding is particularly resistant to mechanical movement under load. In this regard, foil coils are noticeably smoother — even compared to very high-quality round-wire coils with baked varnish.