Which of these will be most useful for insulation at UHF frequencies?

Mica is particularly effective for insulation at UHF frequencies due to its excellent electrical properties and stability across a range of temperatures. It has a high dielectric strength, meaning it can withstand high voltages without breaking down, making it ideal for applications that require insulation from electrical current. Additionally, mica exhibits low loss characteristics at high frequencies, which helps minimize signal degradation.

In contrast, while air can serve as an insulator, it is not as effective at higher frequencies due to its variable dielectric properties. Glass provides good insulation and is often used in high-frequency applications, but it is not as effective as mica in terms of maintaining signal integrity and handling high power. Plastic can be a useful insulator as well but varies widely in its properties depending on the type. Mica stands out as the better choice due to its unique combination of high dielectric strength, low loss factor, and thermal stability when working with UHF signals.