Explanation: The relative dielectric constant, also known as permittivity, is a measure of a material's ability to store electrical energy in an electric field compared to a vacuum. A vacuum serves as the baseline, having a relative dielectric constant of exactly 1. Air, being predominantly empty space, has electrical properties remarkably similar to a vacuum. For all practical radio frequency applications, the difference is negligible, and therefore, air's relative dielectric constant is considered to be 1. This makes air the common reference point when evaluating other insulating materials used in circuits or antennas. Values like 2 or 4 represent materials that can store significantly more electrical energy than air, while 0 would imply an inability to support an electric field.