In the previous two chapters we discussed energy conversion devices which are made from insulators and which are related to capacitors. In Chapters 4 and 5 we discuss energy conversion devices involving conductors and related to inductors. Maxwell's equations say that time varying electric fields induce magnetic fields and time varying magnetic fields induce electric fields. If a permanent magnet moves near a coil of wire, the time varying magnetic field will induce a current in the coil of wire. This idea is the basis behind motors and electrical generators, which are some of the most common energy conversion devices. However, they are outside the scope of this text because they involve magnets and coils. Instead, we will study two other types of energy conversion devices based on this same principle. In this chapter we discuss antennas, and in the next chapter we will discuss Hall effect devices.
- 4.1: Prelude to Antennas
- Antennas are energy conversion devices that convert between electrical energy and electromagnetic energy. Antennas can act as both transmitters and receivers. Transmitters convert electrical energy of the flow of electrons to energy of electromagnetic waves. Receivers convert energy from electromagnetic waves to the electrical energy of electrons in a circuit. The same physical antenna can operate in both ways depending on how it is used.
Thumbnail: Animation of a half-wave dipole antenna transmitting radio waves, showing the electric field lines. (Public Domain; Chetvorno via Wikipedia)