Multipath is a phenomenon where waves from a radiating source travel to the receiver along two or more paths. If the waves remain coherent, two or more components can superimpose and cancel each other, which is known as multipath fading.
Frequency selectivity and temporal spreading
The phase relationships between components differ for signals at different frequencies. Therefore, interference effects vary with frequency; this property is called frequency selectivity. Correspondingly, because different paths have different delays, portions of a signal emitted at the same time arrive spread out in time at the receiver, producing temporal overlap. The following are several common propagation paths:
Direct path
The direct path is the radio wave that propagates directly from the transmitting antenna to the receiving antenna, i.e., the line-of-sight component, plus the additional distance caused by atmospheric refraction and diffraction around the Earth's curvature. This range can be extended by raising the height of the transmitter or receiver antenna, or both.
Ground-reflected wave
Ground-reflected waves are radio waves that reach the receiver after reflecting from the Earth's surface. Cancellation occurs when the ground-reflected component and the direct component arrive at the receiver simultaneously with a 180-degree phase difference.
Ground wave
Waves that propagate along the Earth's surface are ground waves; they result from the Earth's electrical properties and diffraction along the Earth's curvature. The ground-wave strength at the receiver depends on transmitter output power and signal frequency, the terrain and conductivity along the transmission path, and weather conditions.
The Earth's conductivity and permittivity affect the ground-wave component, i.e., the surface wave. When the transmitting and receiving antennas are at or near the ground, cancellation between the direct and ground-reflected waves can cause the receiver to receive predominantly the surface-wave field, which decreases with increasing height. Because the ground absorbs some energy, the field strength of the surface wave decays more rapidly; this attenuation depends on the relative conductivity of the propagation surface.
Skywave (ionospheric propagation)
Skywaves are waves that reach the receiver after reflection from the ionosphere. When radio waves encounter charged particles, the particles oscillate. The oscillating particles absorb electromagnetic energy from the radio wave and then reradiate it, causing changes in polarization and in the wave path.
Higher frequencies require greater ionization density to refract radio waves back to the Earth's surface. The ionosphere's F layer refracts higher frequencies because it has the highest degree of ionization. Variable ionization in the E layer produces anomalous behavior, refracting medium-, high-, and low-frequency radio waves. The D layer has the least ionization and mainly absorbs radio waves; minor refraction is possible but unpredictable.
Critical frequency and incidence angle
For any given time and ionospheric region, there is an upper frequency limit below which vertically transmitted radio waves are refracted back to Earth. This limit is the critical frequency. Vertically transmitted radio waves at frequencies above the critical frequency will pass through the ionosphere into space.
Radio waves with frequencies above the critical frequency will be refracted back to Earth if they propagate at an incidence angle smaller than the critical angle. At angles equal to or greater than the critical angle, waves at frequencies above the critical frequency will pass through the ionosphere. At frequencies above about 30 MHz, almost all energy penetrates the ionosphere. As frequency increases, the required incidence angle for refraction decreases.
