Radio System Characteristics

When a radio frequency current flows into a transmitting antenna ( aerial ) , power is radiated in a number of directions in what is called an electro-magnetic wave. The radiated energy (Fig. 3) will reach the receiving station by one or more of five different modes :-

       1) Surface wave                 4) Via a satellite
       2) Sky wave                      5) Scatter
       3) Space wave

Fig. 3 Radio propagation methods The surface wave is supported at its lower edge by the surface of the earth and is able to follow the curvature of the earth as it travels. The surface wave is used for world-wide communications in the low-frequency bands and for broadcasting in the medium frequency (MF) band.

The sky wave is directed upwards from the earth into the ionosphere ( 100 km or more above ground level ) whence,if certain conditions are satisfied, it will be returned to earth for reception at the required locality. The sky wave is used for high frequency radio communications systems, including long-distance radio-telephony and sound broadcasting.

The space wave generally has two components , one of which travels in a very nearly straight line between the transmitting and receiving locations, and the other travels by means of a single reflection from the earth. The space wave is used for sound and tv broadcasting, for multi channel telephony systems, and for various mobile systems, operating in the VHF , UHF ,SHF and higher bands.

The fourth method is a technique that utilizes the ability of a communications satellite orbiting the earth to receive a signal, amplify, and then transmit it at a different frequency back towards the earth. Communication satellites are used to carry multi-channel telephony systems, television signals, and data, utilizing UHF and SHF bands.

The fifth method listed, scatter (Fig. 4) , could be said to be the UHF / SHF equivalent of using sky wave transmission for long-distance HF radio links. The radio energy is directed towards part of the troposphere which forward-scatters the signal towards the receiver. ( The scattering region of the troposphere is about 10 km above ground level ) . Scatter systems operate in the UHF and SHF bands to provide multi-channel telephony links.

Fig. 4 Scatter propagation

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