jects. This means that it can be run almost anywhere without any undue detrimental effects. Although it will pick up or radiate very small amounts of signal this is normally small enough to be ignored. Where superior screening is required, the screen can be made up from a double layer of braid. In some instances it may be made from solid copper, although this makes it very difficult to bend. The cable can be considered to carry currents in both the inner and outer conductors. As they are equal and opposite they cancel one another out and all the fields are confined to the cable. In fact the signal propagates along the inside of the coax as an electromagnetic wave. It is for this reason that its operation is not affected by the proximity of nearby objects. There are a number of different types of coax. The main distinguishing feature is the impedance. For most television and domestic hi-fi antennas 75 ohms has been adopted as standard. For commercial and industrial radio frequency applications as well as amateur radio and citizens band equipment 50 ohms is used as the standard. Many computer applications use other impedances although 50 ohms is sometimes used. The impedance is determined by the dimensions of the conductors and the dielectric constant of the material between them. It can be calculated from the formula: where D is the inside diameter of the outer conductor or braid d is the outer diameter of the inner conductor E is the dielectric constant of the material between the two conductors Apart from the loss of a feeder that has already been covered, the other aspect of a coaxial feeder to note is that it is what is called an unbalanced feeder. This means that one of the conductors is connected to earth. As one would expect the outer braid is always connected to earth as it acts as a screen. The fact that coax is unbalanced means that it must be used with a system which can tolerate unbalanced feeders. This is normally no problem with transmitters and receivers, but some antennas require a balanced feed. I