3.3 Classical Radiation

The fields associated with sine wave voltages and currents provide insight into the radiation process. We have already seen that any electrical activity involves both fields. In ac circuit theory, the energy associated with a magnetic field in an inductor is returned to the circuit twice per cycle. The same process takes place with a capacitor except that this energy is in an electric field. In an ideal component, very little of the field energy leaves the component space. In high frequency circuits the component space is not that well defined and the fields are also not well confined. In early rf circuitry components were often placed in metal cans to avoid cross coupling.

At low frequencies (below 100 kHz), the field energy associated with electrical activity moves into position without an apparent delay. The field moves out from the circuit at the speed of light. To begin our discussion of radiation, consider a sine wave magnetic field associated with an inductance. As the frequency is increased, a component of the magnetic field cannot return to the circuit in phase with the current generating the field. This occurs because the field travels out and back at the speed of light. This is the magnetic field component of the radiated field. A changing magnetic field is always associated with a changing electric field. This is one of Maxwell's equations. This electric field also moves out into position at the speed of light. As the frequency is increased, a component of the electric field cannot return to the circuit in phase with the generating voltage. These two fields, taken together, leave the circuit as radiation.

The wave character of a field near a radiator is given by the ratio of E and H field intensities. Near the radiator, the field is a composite of radiating and returning field energy. Near loops, the magnetic field intensity is high and the ratio between E and H field intensities is low. Near radiating dipoles, the electric field intensity is high and the ratio of E to H field intensity is also high. Since the ratio of E to H has units of ohms, it is usual to describe the field character of waves in terms of wave impedance. The wave impedance near loops is low. The wave impedance near isolated conductors (dipoles) is high. The radiating intensities for a loop and for a dipole are shown in Figure 3.1.

Figure 3.1 Radiated field intensities from a dipole and from a conducting loop. (a) The radiated field near a half dipole; (b) the radiated field near a current loop.