ELECTROMAGNETIC SPECTRUM TUTORIALS AND BASIC INFORMATION

For electrical engineers the word electromagnetics typically conjures up thoughts of antennas, transmission lines, and radio waves, or maybe boring lectures and “ all-nighters ” studying for exams.

However, this electrical word also describes a broad range of phenomena in addition to electronics, ranging from X-rays to optics to thermal radiation. In physics courses, we are taught that all these phenomena concern electromagnetic waves. Even many nontechnical people are familiar with this concept and with the electromagnetic spectrum, which spans from electronics and radio frequencies through infrared, visible light, and then on to ultraviolet and X-rays.

We are told that these waves are all the same except for frequency. However, most engineers fi nd that even after taking many physics and engineering courses, it is still difficult to see much commonality across the electromagnetic spectrum other than the fact that all are waves and are governed by the same mathematics (Maxwell’s equations).

Why is visible light so different from radio waves? I certainly have never encountered electrical circuits or antennas for visible light. The idea seems absurd. Conversely, I have never seen FM radio or TV band lenses for sale.

 So why do light waves and radio waves behave so differently? Of course the short answer is that it all depends on frequency, but on its own this statement is of little utility. Here is an analogy. From basic chemistry, we all know that all matter is made of atoms, and that atoms contain a nucleus of protons and neutrons with orbiting electrons.

The characteristics of each element just depend on how many protons the atom has. Although this statement is illuminating, just knowing the number of protons in an atom doesn’t provide much more than a framework for learning about chemistry.

Continuing this analogy, the electromagnetic spectrum as shown in Figure 20.1 provides a basic framework for understanding electromagnetic waves, but there is a lot more to learn.

To truly understand electromagnetics, it is important to view different problems in different ways. For any given frequency of a wave, there is also a corresponding wavelength, time period, and quantum of energy. Their definitions are given below, with their corresponding relationships in free space. frequency, f , the number of oscillations per second wavelength, λ , the distance between peaks of:

λ  = C/f

time period, T , the time between peaks of a wave:

T = 1/f

photon energy, E , the minimum value of energy that can be transferred at this

frequency:

E =  h x  f

where c equals the speed of light and h is Planck’s constant.

Depending on the application, one of these four interrelated values is probably more useful than the others. When analyzing digital transmission lines, it helps to compare the signal rise time to the signal transit time down the transmission line.

For antennas, it is usually most intuitive to compare the wavelength of the signal to the antenna length. When examining the resonances and relaxation of dielectric materials it helps to compare the frequency of the waves to the resonant frequency of the material’s microscopic dipoles.

When dealing with infrared, optical, ultraviolet, and x-ray interactions with matter, it is often most useful to talk about the energy of each photon to relate it to the orbital energy of electrons in atoms.

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