In mediumwave

Mediumwave

Medium wave is the part of the medium frequency radio band used mainly for AM radio broadcasting. For Europe the MW band ranges from 526.5 kHz to 1606.5 kHz...

 broadcasting

Broadcasting

Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...

, a mush zone is a region where the ground wave and sky wave

Skywave

Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...

 from a transmitter are received at approximately equal signal strength, resulting in interference between the two, which will typically cause fading and distorted audio.

The effect can reduce the coverage area of a transmission at night, even in the absence of interfering signals from any other source.

It can be mitigated to some extent by the use of a tall mast radiator

Mast radiator

A mast radiator is a radio mast or tower in which the whole structure itself functions as an antenna. This design is commonly used for transmitting antennas operating at low frequencies, in the VLF, LF and MF ranges, in particular those used for AM broadcasting. The metal mast is electrically...

of up to about 0.6 wavelengths height, which increases the ground wave field strength while reducing high-angle sky wave radiation.