berkeman said:
Oh, it may be a language/translation issue. I call that "Multipath" interference. To me "Fading" means attenuation.
This is a massive topic and can't really be dealt with briefly but:
Some history: fading is a phenomenon which was observed long before it was understood. LF and MF signal propagation was found to work way beyond what was expected but long distance signals faded (varied)
periodically but the absolute level couldn't be predicted accurately That wasn't explained until the modes propagation were understood. We know that's due to diffraction / interference / multiple paths and the presence of the Earth (curvature and ground resistance), Ionosphere and Troposphere. Multipath effects can be variable (shifting reflection levels) and cause fading or they can be stable and cause steady increases or decrease in received signal levels.
Height Gain is due to reflections from the ground which produce an image of the transmit antenna which causes a vertical interference pattern. For appropriate heights, the beam pattern gets sharper and sharper at the centre, which increases maximum received signal level (=gain) and also sidelobes. With horizontal polarisation, the reflected signal is in anti phase which cancels the horizontal part of the beam and tilts it upwards. Great for HF which is not used for local reception and the angle can be used to select an appropriate ionospheric level for a 'good' skywave path.
For MF transmission, the ground wave propagates over the curve of the Earth, which tilts the wave forward, rather than 'letting it go'. The wave in higher angles will hit the lowest 'D' layer which is relatively dense and is absorbed in the day, giving clean reception right out to where the signal level fails. When the Sun goes down, ions quickly recombine and the D layer disappears. The sky wave carries on up and bounces from the E or F layer (depending on time of day, sunspots etc.) and hits the ground again, producing terrible skywave interference (and also interference from other distant MF transmitters.
[Edit: Antifading MF aerials are made very tall with top loading. That 'pulls' the current maximum up the mast and tends to depress the main beam, making the ionospheric reflection further away and reducing fading in the near service area.]
Height gain at VHF and above is more a matter of getting the propagation path over the height of buildings and trees and extending the horizon.