er may result in the whole antenna system being degraded. It is therefore very important that a suitable feeder is used. The operation of a feeder may not be as straightforward as might be expected at a first look. There are a number of characteristics that need to be taken into consideration. Feeder impedance One of the first aspects of a feeder is its impedance. Just as an antenna has a certain value of impedance, and a receiver or transmitter has an input or output impedance, a feeder has what is called its characteristic impedance. Again this is measured in ohms. The impedance is partic- ularly important because it has to be matched to the value of impedance for the transmitter or receiver and the antenna if the optimum performance is to be achieved. The impedance of the feeder is governed by a number of factors. The dimensions of the feeder have a large bearing on the impedance, as does the dielectric constant of the medium in and around the feeder. By controlling these factors it is possible to manufacture feeders of the required impedance. Standing waves When the whole antenna system is perfectly matched the maximum power transfer is obtained. When this does not happen, standing waves are set up in the feeder. When power is transferred from a source into the load, the maximum power transfer occurs when the load and source have the same impedance and are said to be matched. In the case of a feeder and an antenna, the feeder acts as the source and the antenna is the load. If there is a poor match only a proportion of the power will be transferred from the feeder into the antenna. The remaining power from the feeder cannot just disappear and is reflected back along the feeder. When this happens the voltages and currents in the feeder add and subtract at different points along the feeder. The result of this is that standing waves are set up. The way in which the effect occurs can be demonstrated with a length of rope. If one end is left free and the other is moved up and down as shown in Figure 4.8, the wave m