What is the best frequency to use for communication?

Sky waves are radio waves reflected by the ionosphere. Therefore, the propagation of these waves is not limited by the curvature of earth. So, these can be used for beyond the horizon communication. Thus, sky waves are used for communication over long (inter-continental) distances.

There are two types of frequencies: VHF and UHF. VHF is the most suitable for businesses that must communicate across long distances . UHFprovides high yet short wavelengths, best for communicating within smaller spaces and metallic buildings.

The 7000 kHz (7 MHz or 40 metre) band is useful for daytime communication up to a distance of 800 km via E and F layers. Long distance world-wide communication takes place in this band through F2 layer.