Ionosphere
The ionosphere (from the word “ions”) is the upper part of the Earth's atmosphere located above 50 km, highly ionized due to cosmic ray radiation originating from the Sun. The ionosphere extends above the Earth's surface from 60 to 600 km.
The concentration of electrons and ions in a unit volume of the ionosphere is distributed unevenly with altitude, therefore the ionosphere is divided into layers D, E, and F, which do not have clearly defined boundaries. Their position and density constantly change during the day, by season, and throughout the eleven-year solar cycle.
In relation to radio waves, the ionosphere behaves like a semiconducting medium from which radio waves can be reflected. The F layer is characterized by the ability to reflect radio waves, which makes it possible to transmit shortwave radio signals over long distances. The ionic composition of the F layer is determined by solar activity, but this does not affect the stability of reflecting electromagnetic waves with a frequency below 10 MHz.
Theoretical calculations show that waves longer than 10 meters are usually reflected from the ionosphere. Thus, for radio waves longer than 10 meters, the ionosphere is opaque, and waves of this range generally cannot leave the Earth's boundaries, except for very long waves propagating along the Earth's magnetic field lines. For radio waves shorter than 10 meters and for radio waves in the optical range, the ionosphere is a completely transparent medium.
At altitudes of 100–110 km, extensive layers or clouds with increased ionization, covering areas of up to hundreds of square kilometers, sometimes occur. This is the so-called Es layer or sporadic E. This layer is characterized by a high concentration of electrons (ne~105 cm?). Its thickness and duration are insignificant; however, in the equatorial region it is present almost constantly, in middle latitudes in summer it is observed daily with rare breaks, and in high latitudes it appears irregularly at night.
Electron clouds of the Es layer can move under the influence of atmospheric tides at speeds up to 250 km/h.
The E and Es layers have a significant effect on the propagation of medium and short radio waves reflected from this region of the ionosphere. Under certain conditions, even radio waves with frequencies of 50 MHz are reflected.
Ionospheric Radio Waves
Ionospheric waves are radio waves that propagate over long distances and encircle the Earth as a result of single or multiple reflections from the ionosphere (in the wavelength range longer than 10 m), as well as waves scattered by ionospheric irregularities and reflected from ionized meteor trails (in the meter wavelength range).
Medium waves (100–1000 m) are usually used for broadcasting and can propagate both as ground waves and as ionospheric waves. The range of ground wave propagation is limited to 500–700 km. Over longer distances, waves of this range propagate as ionospheric waves. At night, medium waves propagate by reflection from the E layer of the ionosphere, whose electron density is sufficient for this. During the day, the D layer is present in the path of wave propagation, which strongly absorbs medium waves. Therefore, with standard transmitter power, the electric field strength is insufficient for reception, and during daylight hours, medium waves propagate almost exclusively as ground waves over relatively short distances of about 1000 km.
Short waves propagate over long distances by reflecting from the ionosphere and the Earth's surface. This method of propagation is called skip propagation and is characterized by skip distance, number of hops, take-off and arrival angles, maximum usable frequency (MUF), and lowest usable frequency (LUF).
The distance along the Earth's surface over which a signal propagates when reflected from the ionosphere is called the skip distance. There is also an area known as the “dead” zone. Signals propagating toward the ionosphere cannot be reflected until they reach it, traveling distances much greater than those over which the surface wave completely disappears. This results in a zone where signals cannot be received. This zone is called the "dead" zone. It is especially pronounced for high-frequency signals, for which the surface wave attenuates very quickly, and the skip distance can be a thousand kilometers or more.
VHF radio waves behave more like light rays. They are practically not reflected from the ionosphere, bend very slightly around the Earth's surface, and propagate within the limits of line of sight. Therefore, the operating range of very high frequency waves is small.
Troposphere, Tropospheric radio waves,
Static interference,
Atmospheric interference
Satellite frequency table