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.

Antennas are all around us. Cell phones and laptops have antennas, and antennas are mounted on the roofs of most cars. Antennas relay information about the electrical grid to the local power utility, and antennas on satellites transmit weather maps to weather stations on earth. Antennas are even built into RFID tags on shirts in stores, and these tags are used to track inventory and prevent theft.

Electrical engineers study both electrical energy and electromagnetic energy, and the words used to describe these phenomena are similar. Is this really an energy conversion process? The answer is yes. Electrical energy involves the flow of electrons through a wire. We often think of electrons as particles. We often use the term electromagnetic wave to describe the flow of electromagnetic energy transmitted by an antenna. However, electrons have both wave-like and particle-like properties. Similarly, electromagnetic waves have both wave-like and particle-like properties. The wavelengths involved are orders of magnitude apart, so it is convenient to only discuss either the wave-like or the particle-like properties. There are fundamental differences between electricity and electromagnetic waves. Fermions are elementary particles with half integer spin quantum numbers and with quantum mechanical wave functions which are antisymmetric when two particles are interchanged [46, p. 391]. Bosons are elementary particles with integer spin quantum numbers and with wave functions which are symmetric when two particles are interchanged [46, p. 391]. Electrons are fermions while electromagnetic waves are bosons. So, antennas are energy conversion devices. A complete discussion of the differences between fermions and bosons requires the study of quantum mechanics and quantum field theory which are beyond the scope of this book.

Figure \(\PageIndex{1}\): Center-fed half-wave dipole and quarter-wave monopole antennas.

An antenna may be as simple as a single metal rod, it may be a copper trace on a printed circuit board, it may be a cone shaped horn, or it may be a complicated arrangement of multiple wires. Some antennas even resemble planar or volume fractals [47] [48]. Hundreds of types of antennas have been developed. Seventy five types are discussed in [49], and 91 types are discussed in [50].

The simplest antenna is just a piece of wire. It may be straight and taut, or it may be carelessly strung from a tree. For an antenna designed to operate at wavelength \(\lambda\), the length of the antenna is often approximately \(\frac{\lambda}{2}\). A straight antenna of length \(\frac{\lambda}{2}\) with signal supplied to the center is called a center-fed half-wave dipole or a \(\frac{\lambda}{2}\) dipole. Some antennas are placed above a conducting plate, or above a conductive surface, which acts as a reflector. A straight antenna of around length \(\frac{\lambda}{4}\) supplied by a signal at one end with a reflector beneath is called a quarter-wave monopole or a \(\frac{\lambda}{4}\) monopole. Figure \(\PageIndex{1}\) illustrates both dipole and monopole antennas. While a random wire will act as an antenna, an antenna with frequency response, impedance, radiation pattern, and electromagnetic polarization designed for the specific application will perform much more efficiently, and these factors are discussed below in Sec. 4.4.