The speed of radio waves in a vacuum medium is the same as the speed of light in a vacuum, which is approximately 299,792,458 meters per second (or about 186,282 miles per second). This speed is often denoted by the symbol 'c' and is considered a fundamental constant in physics.
Radio waves are a form of electromagnetic radiation, just like visible light, X-rays, and gamma rays. They all travel at the same speed in a vacuum because they are all part of the electromagnetic spectrum and are composed of oscillating electric and magnetic fields.
It's important to note that the speed of electromagnetic waves can vary when they propagate through different materials. For example, when radio waves pass through a medium other than a vacuum, such as air, water, or a solid material, their speed can be slightly slower due to interactions with the atoms and molecules in the medium. This decrease in speed is described by the refractive index of the material. However, in a vacuum, radio waves (as well as all other electromagnetic waves) travel at the maximum possible speed, which is the speed of light.