 always greater than one, and as a result the speed of the signal is always less than the speed of light. For many coaxial types of feeder the velocity factor is around 0.66 90 Newnes Guide to Radio and Communications Technology (i.e. 0.66 times the speed of light) although in some cases it can be as low as 0.5. For open wire type feeders the velocity factor may be much greater at around 0.98. Apart from reducing the velocity of the signal, the wavelength also changes and is reduced by the same factor. The frequency remains exactly the same. This results from the fact that the lower speed means that it travels a shorter distance in the same time. In many cases this does not cause a problem, but for applications where the feeder is cut to a specific number of wavelengths, this can be crucial. Types of feeder There are a number of different types of feeder that can be used dependent upon the requirements of the application. Some types are most suitable for more specialist or exacting requirements whereas other types like the very familiar coaxial feeder provide good performance in a wide variety of areas. Coaxial feeder This type of feeder is the most commonly used, being found in most homes for television or VHF FM down leads. Apart from this it is widely used for very many other applications where a radio frequency feeder is required. Coaxial feeder consists of two concentric conductors, spaced apart from one another by an insulating dielectric as shown in Figure 4.11. The inner conductor may be single or multi-stranded, but the outer one normally consists of a braid as shown in the diagram. The outside of the cable is covered by a protective sheath to prevent moisture ingress, as well as providing some mechanical protection. Outer Central protective conductor sheath Outer Dielectric braid spacing Figure 4.11 Cross-section through a piece of coaxial cable Antenna systems 91 Coax has a number of advantages over some other types of feeder that may be used. These include the fact that it is not affected by nearby ob