About

RF propagation is the behavior of radio waves when they are transmitted. As a form of electromagnetic radiation radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization and scattering.

RF propagation is affected by the daily changes of water vapor in the troposphere, ionization in the upper atmosphere due to the Sun, the frequency of the emission, the path over ground of the transmission. If the path is over-the-horizon path aided by refraction in the ionosphere, it will be influenced by factors that include sporadic-E, spread-F, solar flares, geomagnetic storms, ionospheric layer tilts, and solar proton events.

Propagation Modes

Surface modes (groundwave) — LF (between 30 and 3,000 kHz) have the property of following the curvature of the earth via groundwave propagation. In this mode the radio wave propagates by interacting with the semi-conductive surface of the earth. The wave "clings" to the surface and thus follows the curvature of the earth. Vertical polarization is used to alleviate short circuiting the electric field through the conductivity of the ground. Since the ground is not a perfect electrical conductor, ground waves are attenuated rapidly as they follow the earth’s surface. Attenuation is proportional to the frequency making this mode mainly useful for LF and VLF frequencies.

Direct modes (line-of-sight) — Line-of-sight is the direct propagation of radio waves between antennas that are visible to each other. This is probably the most common of the radio propagation modes at VHF and higher frequencies.

Ionospheric modes (skywave) — Skywave propagation, also referred to as skip, is any of the modes that rely on refraction of radio waves in the ionosphere, which is made up of ionized layers in the upper atmosphere. F2-layer is the most important ionospheric layer for long-distance, multiple-hop HF propagation, though F1, E, and D-layers can also play significant roles.

The D-layer, when present during sunlight periods, causes significant amount of signal loss, as does the E-layer whose maximum usable frequency can rise to 4 MHz and above and thus block higher frequency signals from reaching the F2-layer. The layers, or more appropriately "regions", are directly affected by the sun on a daily cycle, a seasonal cycle and the 11-year sunspot cycle and determine the utility of these modes.

• Meteor scattering
• Auroral backscatter
• Sporadic-E propagation

Tropospheric modes

• Tropospheric scattering
• Tropospheric ducting
• Tropospheric delay
• Rain scattering
• Lightning scattering

Other effects

• Diffraction
• Absorption