Mush zone

In mediumwave broadcasting, a mush zone is a region where the ground wave and sky wave 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 of up to about 0.6 wavelengths height, which increases the ground wave field strength while reducing high-angle sky wave radiation.

References

Radio frequency propagation

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