l level area this may not be a problem. It is difficult to assess the exact level of attenuation because it depends on a variety of factors including the nature of the materials used in the house, the frequencies involved and their proximity to the antenna. For example, it is found that the level of attenuation increases when the roof is wet. There may be a 6 dB increase in attenuation at frequencies of the order of 500 MHz after 116 Newnes Guide to Radio and Communications Technology rain, and this may be maintained for many hours after the precipitation has finished if the roof tiles are absorbent. Another point to watch when mounting an antenna internally is that it is kept away from internal wiring and metalwork that might screen the antenna or affect its operation by detuning it. Often wiring or metal objects such as water tanks can have a significant effect. Another popular place to mount an antenna is on a chimney stack. This can be an ideal location as it is relatively high. However, check that any antenna mounted on the chimney does not overhang a neighbour's property. It is also necessary to check that the chimney will be able to withstand the antenna, particularly when it is windy. This is particularly important for large antennas. In some instances masts or towers may be required. Many varieties of masts and towers are available. Some of these are free standing while others need to have guys attached. Some can be wall mounted, using the side of the house or other building for support. Towers are ideal in many respects but they are more costly than masts using guys. However, a mast with guys will take up more space. In addition to this care and expertise are required when installing them. A further requirement for a tower or mast is that it should have lightning protection installed. A very good earth connection is required along with wide copper straps from the mast to the earth connection. In this way current flowing to the equipment will be reduced as far as possible and damage to the equipment and 