What type of wave is a radio signal in empty space?

A radio signal in empty space is an electromagnetic wave.

Electromagnetic waves consist of changing electric and magnetic fields that sustain one another as they travel. Unlike sound waves, they do not require air, water, or another material medium, so radio signals can cross the vacuum between Earth and spacecraft.

Radio waves occupy the lower-frequency, longer-wavelength region of the electromagnetic spectrum. The same broad family also includes microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

In a vacuum, all electromagnetic radiation travels at the same speed. Frequency and wavelength differ between types, with their product equal to the speed of light. Radio communication encodes information by varying properties such as amplitude, frequency, or phase.