me of the power is dissipated as heat. To reduce this the conductors can be made thicker, but this increases the size of the whole cable, and increases the cost. Losses are also introduced by the dielectric. A dielectric material is used between conductors in the feeder to act as an insulator and spacer to keep conductors a given distance apart. Poor quality dielectric can dissipate some power. It is for this reason that many dielectrics are semi- air spaced, consisting of plastic with air holes of one variety or another. Moisture entering the feeder dielectric can increase the loss dramatically. As a result it is necessary to ensure that any feeders which are used externally need to be well sealed where required. Finally some power can be lost by radiation. In many types of feeder the amounts of power lost by radiation are relatively low. However, the reverse effect can be serious in some instances. If the feeder runs through an area where noise levels are high it may pick up significant amounts of interference which could adversely affect reception, despite having an antenna in a location where interference local levels are low. The loss introduced by a certain type of feeder is proportional to its length. As a result figures for this type of specification are given for a given length of feeder, often 10 metres. They are also expressed in decibels. The frequency of operation also has a major influence, the loss rising with frequency. As a result a typical specification for a feeder might be that it has a loss of 1.0 dB at 100 MHz and 3.8 dB at 1000 MHz for a 10 metre length. For the best operation of the antenna it is necessary to ensure that the loss is minimized in keeping with the cost of installing the feeder. Velocity factor When a signal travels in free space it travels at the speed of light. It would also travel at the same speed in a feeder if it did not contain an insulating dielectric. The speed of the signal is reduced by a factor of 1/VE where E is the dielectric constant. The dielectric constant is