Radio waves can travel from one place to another without the need for a physical antenna by propagating through the medium they are traveling in. This propagation occurs through a process called electromagnetic wave propagation.
When an electrical current oscillates in an antenna, it generates electromagnetic waves. These waves consist of oscillating electric and magnetic fields that propagate through space or a medium. Radio waves, which are a type of electromagnetic wave, can travel through various mediums, including air, water, and even certain solids.
In free space or air, radio waves can propagate through a process known as free space propagation. The waves spread out in all directions from the source, forming an expanding sphere of energy. This is often referred to as "line-of-sight" propagation, where the radio waves travel in a straight line from the transmitting antenna to the receiving antenna.
In addition to line-of-sight propagation, radio waves can also undergo other propagation mechanisms, such as reflection, diffraction, and scattering. These mechanisms allow radio waves to travel beyond the line of sight and reach receivers even if there are obstacles in the path.
Reflection occurs when radio waves encounter an object or surface and bounce off it. This bouncing can redirect the waves and allow them to reach receivers that are not directly in the line of sight of the transmitter.
Diffraction occurs when radio waves encounter an obstacle or pass through an opening that is comparable in size to the wavelength of the wave. The waves bend around the obstacle or spread out through the opening, allowing them to reach areas that are otherwise obstructed from direct line-of-sight transmission.
Scattering occurs when radio waves encounter small objects or irregularities in the propagation medium. The waves interact with these objects, causing them to scatter in various directions. This scattering can help the radio waves reach receivers that would otherwise be in shadowed regions.
These various propagation mechanisms enable radio waves to travel from one place to another without a physical antenna connecting the two locations. Instead, they rely on the inherent properties of electromagnetic waves and the characteristics of the propagation medium to carry the signals.