ABSTRACT
If the current is alternating, the induction field effects near the conductor will exist as with steady current, and their strength will vary as 1/d2. However, there will be an additional effect. The alternating fields will be sweeping away from the conductor with the speed of light, in the form of energy-carrying electromagnetic fields. At a point remote from the radiating conductor, another conductor can be erected parallel to the electric flux lines, and it will have an alternating current developed in it at the frequency of the field alternations. Power can be abstracted from the passing ‘wave’, because the current developed in the second conductor can be passed through a load. This is the principle established by Maxwell in 1864. The radiated fields are thus a load on the energy source that is
used to develop the fields. This load can be represented by an imaginary resistor having radiation resistance or antenna impedance. Figure 6.1.2 shows, at a point remote from a radiating system, the field flux lines. They will be varying in phase, and are related to each other in quadrature in a plane at right angles to the direction of propagation. The whole field system, advancing along the direction of propagation, is termed a wavefront.
