Reception of shortwave radio signals is always accompanied by variations in the received signal level, and these variations are random and time-dependent. This phenomenon is called radio signal fading. In practice, both fast and slow signal fading can be observed. The depth of fading can reach several tens of decibels.
The main cause of fast fading is multipath propagation of radio waves. In such cases, fading is caused by two rays arriving at the receiving point—one via a single reflection and the other via double reflections from the ionosphere.
Because the rays travel different path lengths, their phases upon arrival are not the same. Continuous changes in electron density in the ionosphere lead to variations in the path length of each ray, and consequently to changes in the phase difference between them. When rays from the same signal arrive at the receiver with equal strength but with a phase difference of 180°, they completely cancel each other out; with smaller phase differences, partial cancellation occurs. Such small changes in path length can occur continuously, so variations in the electric field strength in the shortwave band are frequent and deep. Observation intervals may range from 3–7 minutes at the lower frequencies of the shortwave band to as little as 0.5 seconds at frequencies closer to 30 MHz.
Additionally, signal fading can be caused by scattering of radio waves by ionospheric irregularities and by interference of the scattered waves.
Apart from interference fading, there is also polarization fading in the shortwave band. The cause of polarization fading is the rotation of the wave’s polarization plane relative to the receiving antenna. This occurs when a wave propagates along the Earth's magnetic field lines and when electron density in the ionosphere changes. For example, if both transmitting and receiving antennas are horizontal dipoles, a horizontally polarized wave, after passing through the ionosphere, will experience polarization rotation. This leads to oscillations in the electromotive force induced in the antenna, resulting in additional attenuation of up to 10 dB.
In practice, all the above causes of fading usually act together and follow the statistical distribution described by Rayleigh’s law.
In addition to fast fading, slow fading is observed, occurring with a period of 40–60 minutes in the lower-frequency part of the shortwave band. The cause of such fading is variation in ionospheric absorption of radio waves. The envelope of the signal amplitude during slow fading follows a log-normal distribution, with signal reduction of up to 8–12 dB.
To combat fading on shortwaves, the method of reception using spatially separated antennas is used. This works because increases and decreases in electric field strength do not occur simultaneously, even over a relatively small area. In practice, shortwave communications often use two fixed antennas spaced several wavelengths apart, with signals combined after detection. Another effective method is polarization diversity—simultaneous reception on vertical and horizontal antennas with subsequent combining of signals after detection.
Another approach to reducing fading is the use of directional antennas with a high gain (directivity factor) to prevent reception of signals from unwanted directions.
These methods are effective only for mitigating fast fading; slow variations in the signal cannot be eliminated, as they are associated with changes in ionospheric absorption of radio waves.
Related articles: Antenna, Shortwaves, Electromagnetic Wave Polarization.