out. It can only be reduced by limiting the bandwidth of the set, and if the bandwidth is reduced too much then the wanted signal may be impaired. Other types of noise may appear as bangs or pops, and this type of noise is heard particularly on the lower frequency bands. This external noise comes from two main areas. It may be picked up by the antenna. In turn this can come from a variety of sources. It may be man- made. The variety of electrical and electronic equipment in use today generates noise which can be picked up over the whole of the radio spectrum. Electric motors, vehicle ignition systems, televisions, computers and even fluorescent lights all produce energy which can be picked up. Naturally occurring noise can also be picked up. Again there are a variety of sources for this. Static is one major type of noise. This comes from discharges in electrical storms. In view of the colossal amounts of energy involved in a static discharge it is hardly surprising it generates large amounts of radio frequency energy which can be heard over large areas. Noise also comes from outer space. This cosmic or galactic noise comes from many places in outer space, from our own sun, to the distant galaxies. Surprisingly this noise can be present at significant levels, despite the enormous distances involved. 170 Newnes Guide to Radio and Communications Technology The level of different types of noise varies with frequency, and this means that the design of radios is dependent to a certain degree upon this. Some forms of man-made noise extend to relatively high fre- quencies. Car ignition noise can extend to frequencies in excess of 400 MHz and noise from fluorescent lights can also be picked at frequencies well above 1 GHz. Naturally occurring forms of noise fall with increasing frequency. Atmospheric noise predominates at low frequencies, but above about 10 MHz galactic noise is the main constituent. Even this falls with increasing frequency leaving that generated within the receiver as the main source of noise at frequen