Read this: Most types of capacitor include a dielectric spacer, which increases their capacitance. These dielectrics are most often insulators. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Variable capacitors with their plates open to the atmosphere were commonly used in radio tuning circuits. Later designs use polymer foil dielectric between the moving and stationary plates, with no significant air space between them.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What sort of capacitors have traditionally been used in radio tuning circuits?
Variable capacitors