Electromagnetic waves can be diffracted when they meet a sharp obstacle.
As radio waves are a form of electromagnetic wave, it means that they can also be diffracted.
As radio waves undergo diffraction it means that a signal from a transmitter may be received from a transmitter even though it may be "shaded" by a large object between them.
To understand how this happens it is necessary to look at Huygen's Principle. This states that each point on a spherical wave front can be considered as a source of a secondary wave front.
Even though there will be a shadow zone immediately behind the obstacle, the signal will diffract around the obstacle and start to fill the void. It is found that diffraction is more pronounced when the obstacle becomes sharper and more like a "knife edge".
For a radio signal the definition of a knife edge depends upon the frequency, and hence the wavelength of the signal.
For low frequency signals a mountain ridge may provide a sufficiently sharp edge. A more rounded hill will not produce such a marked effect. It is also found that low frequency signals diffract more markedly than higher frequency ones. It is for this reason that signals on the long wave band are able to provide coverage even in hilly or mountainous terrain where signals at VHF and higher would not.
The effect may also be important for very high frequency signals where items of furniture in the home may have a sufficiently sharp edge to enable diffraction to be seen. This may give slightly better coverage to items like mobile phones or for Wi-Fi systems.
