Editing Radio propagation (section)

==Direct modes (line-of-sight)==
{{Main|Line-of-sight propagation}}
[[Line-of-sight propagation|Line-of-sight]] refers to radio waves which travel directly in a line from the transmitting antenna to the receiving antenna, often also called direct-wave. It does not necessarily require a cleared sight path; at lower frequencies radio waves can pass through buildings, foliage and other obstructions. This is the most common propagation mode at [[VHF]] and above, and the only possible mode at [[microwave]] frequencies and above. On the surface of the Earth, line of sight propagation is limited by the [[horizon|visual horizon]] to about {{convert|40|mi|km}}. This is the method used by [[cell phone]]s,{{efn|Cellular networks function even without a single clear line-of-sight by relaying signals along multiple line-of-sight paths through cell towers.}} [[cordless phone]]s, [[walkie-talkie]]s, [[wireless network]]s, point-to-point [[microwave radio relay]] links, [[FM broadcasting|FM]] and [[television broadcasting]] and [[radar]]. [[Satellite communication]] uses longer line-of-sight paths; for example home [[satellite dish]]es receive signals from communication satellites {{convert|22,000|mi|km}} above the Earth, and [[satellite ground station|ground stations]] can communicate with [[spacecraft]] billions of miles from Earth.

[[Ground plane]] [[Reflection (physics)|reflection]] effects are an important factor in VHF line-of-sight propagation. The interference between the direct beam line-of-sight and the ground reflected beam often leads to an effective inverse-fourth-power {{nowrap|({{frac|1|distance<sup>4</sup>}})}} law for ground-plane limited radiation.{{citation needed|date=December 2020|reason=Need reference to inverse-fourth-power law + ground plane. Drawings may clarify.}}