What is the frequency range of signals that propagate best over the surface of the earth?

The frequency range of signals that propagate best over the surface of the Earth is typically within the Extremely Low Frequency (ELF) to Very Low Frequency (VLF) range. This is approximately 3 Hz to 30 kHz. These frequencies are used because they can travel long distances and can even penetrate water and earth to some extent.

Here's a step-by-step explanation:

1. Radio waves are a type of electromagnetic wave, and they can travel through different mediums including the air, vacuum of space, and even through solid objects to some extent.

2. The ability of a radio wave to travel over the surface of the Earth is influenced by its frequency. Lower frequencies can travel further than higher frequencies.

3. Extremely Low Frequency (ELF) waves, which range from 3 to 30 Hz, can travel great distances. They can even penetrate water and earth, which is why they're used for communication with submarines.

4. Very Low Frequency (VLF) waves, which range from 3 kHz to 30 kHz, can also travel long distances and are used for long-range communication, including with aircraft.

5. As the frequency increases, the ability of the radio waves to travel over the surface of the Earth decreases. This is why higher frequency bands, like those used for TV and cell phone signals, require a clear line of sight to the receiver or the use of repeaters to boost the signal over longer distances.