 These two reasons mean that meteor scatter operation is generally confined to frequencies above 30 MHz. Operating above these frequencies has the further advantage that both galactic and artificial noise are less - a vital factor when considering the low signal levels involved in this mode of communication. Generally most meteor scatter operation takes place between 40 and 50 MHz although there is some between 30 and 40 MHz. The top limit is governed more by the fact that television transmissions previously occupied frequencies above 50 MHz, and still do in some countries. 50 Newnes Guide to Radio and Communications Technology Frequencies above 3 GHz At frequencies above about 3 GHz, the distances that can be achieved are not normally much in excess of the line of sight. This means that if greater distances are to be achieved, antennas must be placed higher above the earth's surface to increase the distance of the horizon. Other effects are also noticed. Signals are absorbed more by atmos- pheric conditions. Rain causes signals at these frequencies to undergo attenuation. The level of attenuation is dependent upon the frequency in use and the rate at which the rain is occurring. Gases also cause signal attenuation. There are peaks in the level of absorption due to water vapour at frequencies of 20 GHz and others at much higher frequencies around 200 and 350 GHz. Similarly oxygen gives rise to peaks in attenuation around 60 GHz and another just over 100 GHz. 3 Modulation Radio signals are used to carry information. This information may be sound as in broadcasting or mobile telecommunications, or it may be in another form as in a weather fax, or digital data used for sending text messages, for example. In fact there is an almost infinite variety of uses for radio signals, but in all cases they are used to carry information in one form or another. To be able to transmit any form of information a radio signal or carrier is first generated. The information in the form of audio, data, or other form of signal is used t