 amount of power that is being reflected. This is particularly important where transmitters are used because the high current or voltage values may damage the feeder if they reach very high levels, or the transmitter itself may be damaged. The figure normally used for measuring the standing waves is called the standing wave ratio (SWR), and it is a measure of the maximum to minimum values on the line. In most instances the voltage standing wave ratio (VSWR) is used. The standing wave ratio is a ratio of the maximum to minimum values of standing waves in a feeder. The reflection coefficient (p) can be defined and this is the ratio of the reflected current or voltage vector to the forward voltage or current vector. It is therefore very easy to calculate the SWR. The minimum value of standing wave is (1 - p) and the maximum is (1 + p). The standing wave ratio then becomes: SWR = (1 From this it can be seen that a perfectly matched line will give a ratio of 1:1 while a completely mismatched line gives oo:1. Although it is perfectly possible to quote SWR values of less than unity, it is normal convention to express them as ratios greater than one. Even though the voltage and current vary along the length of the feeder, the amount of power remains the same if losses are ignored. This means that the standing wave ratio remains the same along the whole length of the feeder. Often the forward and reflected power may be measured. From this it is easy to calculate the reflection coefficient as given below: where Pref is the reflected power and Pfwd is the forward power. Antenna systems 89 Another important factor about a feeder is the loss that it introduces into the system. The ideal scenario would be for the same amount of power to appear at the far end as entered from the generator. In reality the radio frequency power leaving the far end of the cable is always less than that which entered. There are a number of reasons for this. The first is that the conductors in the feeder have a certain resistance, and as a result so