Limitations on the sharing of radio frequencies in the HF spectrum are dependent on the propagation characteristics of ionospheric radiowaves. Frequency sharing at HF has been found to be extremely difficult because radiowaves propagating via the ionosphere do not attenuate quickly. Any attempt at frequency sharing must consider the facts that at HF, radiowaves normally propagate in all directions; radiowaves are refracted and reflected by the ionosphere and the ground and will continue until dissipated.