 describe television picture and sound broad- granted and which, among other things, provides for the casting. There are many words used to describe forms of broadcasting to our homes of entertainment, news and radio system including satellite communications, radar, information of every kind in both the radio and television microwave links, mobile telephones, etc. spheres. The most recent development, resulting in the Despite the profusion of terms in use to describe the domestic satellite dish antenna, brings the quasi-optical transmission of intelligence by electromagnetic waves, nature of microwaves to the notice of the consumer. the nature of these waves is basically the same, the The use of semiconductor devices (transistors) has variable being the way in which the intelligence (signal) become commonplace and as a result the mass and is added. It is therefore convenient to refer to these volume of electronic products for a given function is electromagnetic waves as 'radio waves' and the frequen- much less than that of their earlier counterparts which cies of the waves as 'radio frequencies'. used electronic tubes. However, in the high power transmitter field electronic tubes are still the mainstay of transmitters. These use very high voltages, depending on The nature of radio waves power output. A voltage of 40 kV or more is not unusual for very high power equipments. High power systems Most readers will be familiar with the fact that an such as MF and HF broadcasting systems need consid- alternating current or voltage which is undistorted has erable provision for cooling the vacuum tubes used and in an amplitude which varies with time and reverses some cases the resulting heat is transferred to the station direction at each 180°, one cycle taking 360°. This heating system! pictorial representation of a current or voltage is Semiconductor devices are being used in transmitters referred to generally as a waveform and the description of more modest power and also in spaced array radar above is that of a sine w