diameter of the conductors E is the dielectric constant of the material between the two conductors Open wire or twin feeder is not nearly as widely used as coax, although it provides an ideal solution for a number of applications, especially those in the short wave part of the spectrum. It has a velocity factor of about 0.98 when the open wire version is used, and can offer very low levels of attenuation if it is kept away from other objects. The main drawback is that it is affected by nearby objects and as a result it cannot be taken through buildings in the same way as coax. This limits its use considerably, and it is rarely seen in domestic applications except for use with temporary VHF FM antennas. Waveguide A third major type of feeder is called waveguide. This is only used for microwave frequencies and it consists of a hollow pipe along which the signals propagate. Waveguides can be circular, but it is more common to see rectangular types as shown in Figure 4.13. They are different to the more conventional forms of feeder in that there are not two conductors. Instead the signal is introduced into the waveguide and this carries it as far as required, the walls preventing the signal escaping. A signal can be entered into the waveguide in a number of ways. The most straightforward is known as a launcher. This is basically a small probe that penetrates a small distance into the centre of the waveguide itself as shown. Often this probe may be the centre conductor of the coaxial cable connected to the waveguide. The probe is orientated SO that it is parallel to the lines of the electric field that is to be set up in the waveguide. An alternative method is to have a loop that is connected to the wall of the waveguide. This encompasses the magnetic field lines and sets up the electromagnetic wave in this way. However, for most Hollow centre Conductive Figure 4.13 A rectangular waveguide 94 Newnes Guide to Radio and Communications Technology Coax and Outer of coax feeder connected to waveguide Coax inner conductor p