The high-frequency radio signal is the answer.
Radiofrequency (RF) radios are in the wavelength band of 100 to 10 meters and extend from 3 MHz to 30 MHz. Much of the HF bandwidth is allocated to mobile and fixed voice communications services that require transmission bandwidth of less than 12 kHz. International (Shortwave) Broadcast
Radio Frequency (HF) is an ITU term for the range of high-frequency electromagnetic waves (radio waves) from 3 to 30 MHz (MHz). It is also known as the decameter band or decameter wave because it has wavelengths in the range of 1 to 10 decameter (tens to hundreds of meters).
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