e 'loss' resistance and is normally kept as low as possible because any power dissipated in this resistance is lost as heat, thereby reducing the efficiency and effectiveness of the antenna. The other resistive element is known as the 'radiation resistance'. This can be thought of as a virtual resistance and it arises from the fact that power is 'dissipated' when it is radiated. There are also reactive elements to the feed impedance. These arise from the fact that the antenna elements possess inductance and capacitance. At resonance where most antennas are used, the inductive and capacitive reactances cancel one another out leaving only the resistance. However, either side of resonance the feed impedance rapidly becomes either inductive if it is operated below the resonant frequency or capacitive if it is operated above resonance. Antenna systems 79 The value of the antenna impedance is very important if the system is to operate efficiently. In order to obtain the maximum power transfer, the antenna, the transmitter or receiver and the feeder should all have the same value of resistance. This value of resistance can be calculated using Ohm's law and by knowing the values of current and voltage at the feed point of the antenna. Gain and directivity One important characteristic of an antenna is the way in which it is more sensitive to signals in one direction than another. This is called the directivity of the antenna. To explain some of the features about directivity it is easier to visualize the operation of the antenna when it is transmitting. The antenna will then be found to perform in the same manner when it is receiving. Power delivered to the antenna is radiated in a variety of directions. The antenna design may be altered SO that it radiates more in one direction than another. As the same amount of power is radiated, it means that more power is radiated in one direction than before. In other words, it appears to have a certain amount of gain over the original design. The radiation of an antenna varies in a