Radio propagation

Line-of-sight transmission on the surface of the Earth is limited to the distance to the visual horizon, which depends on the height of transmitting and receiving antennas.[a] At lower frequencies in the MF, LF, and VLF bands, diffraction allows radio waves to bend over hills and other obstacles, and travel beyond the horizon, following the contour of the Earth.Doubling the distance of a receiver from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions.Early long-distance radio communication (wireless telegraphy) before the mid-1920s used low frequencies in the longwave bands and relied exclusively on ground-wave propagation.[9] Non-line-of-sight (NLOS) radio propagation occurs outside of the typical line-of-sight (LOS) between the transmitter and receiver, such as in ground reflections.Obstacles that commonly cause NLOS propagation include buildings, trees, hills, mountains, and, in some cases, high voltage electric power lines.The acronym NLOS has become more popular in the context of wireless local area networks (WLANs) and wireless metropolitan area networks such as WiMAX because the capability of such links to provide a reasonable level of NLOS coverage greatly improves their marketability and versatility in the typical urban environments where they are most frequently used.For radio amateurs the WSPR mode provides maps with real time propagation conditions between a network of transmitters and receivers.For FM broadcasting (and the few remaining low-band TV stations), weather is the primary cause for changes in VHF propagation, along with some diurnal changes when the sky is mostly without cloud cover.In late spring and early summer, a combination of other atmospheric factors can occasionally cause skips that duct high-power signals to places well over 1000 km (600 miles) away.In urban (and to some extent suburban) areas with a high population density, this is partly offset by the use of smaller cells, which use lower effective radiated power and beam tilt to reduce interference, and therefore increase frequency reuse and user capacity.While this is generally transparent to the user thanks to the way that cellular networks handle cell-to-cell handoffs, when cross-border signals are involved, unexpected charges for international roaming may occur despite not having left the country at all.Created with the goal of formalizing the way radio waves are propagated from one place to another, such models typically predict the path loss along a link or the effective coverage area of a transmitter.As the path loss encountered along any radio link serves as the dominant factor for characterization of propagation for the link, radio propagation models typically focus on realization of the path loss with the auxiliary task of predicting the area of coverage for a transmitter or modeling the distribution of signals over different regions.For any model, the collection of data has to be sufficiently large to provide enough likeliness (or enough scope) to all kind of situations that can happen in that specific scenario.