Or how radio waves travel through the air…
As radio waves travel through the air away from the transmitter, the path they can take falls into five different types with each type having it’s own particular characteristics and effects on the radio signal. Each type is a method of propagation.
The five types of propagation are:
- Ground waves
- Space waves
- Tropospheric waves
- Sky waves
- Free space waves
Lets look at each type in detail.
Sometimes called surface waves travel along the earth’s surface fairly close to the ground. The lower frequencies typically travelling for a distance of 100-150Km. At frequencies of around 30Mhz this distance can drop to as little as only 10Km.
Frequencies below approx 2Mhz make good use of ground wave propagation. This is where you find the “old” MW and LW broadcast stations.
Space waves do exactly what the name suggests.
They travel through space directly from the transmitter to the receiver by line of sight. The higher frequencies of UHF and microwaves use space waves.
This primarily affects VHF frequencies where the radio waves are refracted or bounced back to earth. Bouncing off the troposphere region of the atmosphere allows longer distances of communication than normal.
The weather also has a great effect on this method of propagation.
These are signals at frequencies below approx 30MHz which travel up to the ionosphere and escape out into space. Depending on the current atmospheric conditions, some of these waves may be reflected back to ground. These refracted waves then become known as ionospheric waves.
The ionosphere can change state quite quickly and is influenced by solar activity, sun spots, solar flares etc.
Solar activity can allow radio signals to be “bounced” between the earth’s surface and the ionosphere a number of times thus allowing huge distances of communication.
Free space waves
This type of radio wave is the most common.
At VHF frequencies and higher, the radio wave will just simply escape into space. They pass through the layers of the atmosphere with little resistance or reflection. It is these waves which are used for satellite TV and for communicating with the space station etc.