The Electromagnetic Spectrum

Figure 2-1 is a common depiction of the electromagnetic spectrum. Many know the term “electromagnetic spectrum” or “electromagnetic radiation,” but few can characterize exactly what these words signify. The electromagnetic spectrum encompasses all different types of electromagnetic radiation, all the way from the lowest-frequency radio waves to the highest-frequency gamma waves.

Our entire existence depends upon electromagnetic radiation. Electromagnetic waves can travel through a vacuum, which allows the sun to heat our planet across vast expanses of empty space. Far-flung galaxies and stars emit electromagnetic waves which allow us to explore our universe. Our eyes detect electromagnetic radiation in the form of light that allows us to see. All of the world’s economies, militaries, and technology industries are dependent upon electromagnetic waves to transmit information.

As mentioned, radio waves are just a small part of the larger electromagnetic spectrum. The property that makes them behave differently than another portion, such as visible light, is wavelength/frequency.

Radio Bands and Their Uses

ELF, SLF, ULF, and VLF

As we will explore later, lower-frequency waves cannot transmit information at a particularly high rate. This makes these bands only suitable for sending short, textual messages. An interesting property of this band is its incredible ability to penetrate the ground and ocean, as the ground and ocean normally block higher-frequency radio waves.

A practical use of this property is communication with submarines. Submarines normally have to come close to the surface in order to communicate with boats, satellites, and the mainland, but massive transmitters such as the one pictured in Figure 2-2 allow militaries to communicate with submerged submarines. This is a one-way link, as it is impractical to transmit ELF waves from a submarine. We will discuss transmitter characteristics such as this in the chapter on antennas.

LF, MF, and HF

Radio waves with long wavelengths (such as in these bands) can exploit interesting effects of the Earth’s atmosphere to travel extremely long distances. One can reflect waves in these bands off various layers in the atmosphere and ionosphere, making intercontinental communications possible. Figure 2-3 illustrates this property. These waves also can bend or diffract along the curvature of the Earth, making it possible to communicate over the horizon. We will discuss diffraction much more in the propagation section of this chapter.

Radio waves in these bands are capable of carrying low-quality voice signals, making these bands very popular for tasks such as maritime communications, some air traffic communications, and low-data tasks such as time signals for consumer clocks. AM radio also falls within this range.

VHF, UHF, and SHF

This section of the electromagnetic spectrum is considered the “beachfront property” of radio communications. Radio waves of these frequencies exhibit a good ability to transmit lots of information at relatively long distances. In these bands, you’ll find most of the consumer radio technologies that you’re already familiar with, such as FM radio, 3G/4G LTE, satellite radio, Wi-Fi, Bluetooth, and many more technologies. Figure 2-4 shows frequency allocations for this section of the electromagnetic spectrum; you're likely familiar with some of the technologies listed.

Due to the high value of certain bands within this range, governments actively regulate and police use of spectrum within their territories. Rights to very small slices of the spectrum, especially those used in cellular communications, can sell for billions of dollars at auction. In the United States, the Federal Communications Commission (FCC) levies millions of dollars in fines per year on individuals and corporations that operate unlicensed transmitters.

EHF

All radio waves within this band are classified as microwaves. The microwave range technically starts at around the UHF band, but different sources define microwaves in different ways. Microwaves are simply high-frequency radio waves; there’s no real practical distinction between the two. Many systems, such as the space telescope in Figure 2-5, make use of microwave frequencies to carry out their missions.

A useful feature of EHF-band communications is high data transfer rates. Space agencies around the globe utilize frequencies in this range to communicate with many satellites and science missions, and commercial communications satellites utilize this range for inter-satellite and satellite-ground communications. Point-to-point microwave links on Earth allow for companies to send large amounts of data across long distances. Cellular communications are also expanding into frequency ranges in the EHF band. Many planned 5G systems rely on channels in the tens of gigahertz to deliver ultrafast cellular data links to mobile customers. Radar systems often operate using frequencies in the EHF range. These are used in everything from police speed detectors to missile guidance and tracking systems. Many frequencies within this band are readily absorbed by atmospheric gasses. Additionally, frequencies within this band are blocked by any obstacles in the path of the transmitter. As a result, a direct line of sight between the transmitter and receiver is often needed.

THF

THF radio waves occupy an “in-between space” of radio and light. The frequency of these waves is too high to easily generate and detect using digital and analog systems, but too low to be visible by humans or exhibit characteristics of light and even higher-frequency radiation. As such, research into radio waves has been primarily focused below THF. I am not aware of any practical communications systems that utilize THF waves to transmit information.

Propagation

Propagation is the way a wave travels through a medium. Electromagnetic waves have different propagation modes, with the type of mode generally depending upon wavelength and environmental characteristics. All the communication methods further discussed in this book propagate through the line-of-sight mode, although different propagation modes were mentioned in the section on LF, MF, and HF waves.

The term line of sight is somewhat misleading as it implies that a receiver must have a direct line of sight to the transmitter in order to get a signal. Obviously this is not the case for many wireless communications systems; one needn’t see a router in order to have a Wi-Fi connection! A direct line-of-sight path is one in which the transmitter and receiver are visible to each other, such as in Figure 2-6. Direct line-of-sight paths are utilized in microwave data transmission links, among other things.

The general line-of-sight propagation mode can be more thought of as allowing one to communicate with something that one could see in the absence of any obstacles, that is, something not blocked by the curvature of the Earth or large geographic features like mountains.

The three general wave properties that govern line-of-sight propagation are diffraction, reflection, and absorption. I posit that through a good understanding of these properties, you can gain an intuitive understanding of radio propagation.

Diffraction

Diffraction is a general wave property that occurs when a wave meets a sharp transition. Upon meeting such a transition, the wave will spread out, or diffract, around the edge. In the context of radio communications, this means that a receiver shaded by some obstacle, such as a hill, can still receive signals from a distant transmitter. An example of diffraction is shown in Figure 2-7.

Diffraction occurs differently for waves with different frequencies. In general, lower-frequency waves are better at diffracting around large obstacles. Radio waves with a frequency lower than about 400 MHz, such as FM radio, can spread over a large geographic area because they can diffract around hills and buildings. Radio waves with a frequency higher than this are less effective at diffracting around geographic obstacles so are limited to shorter-range communications.

Diffraction is still extremely important in higher-frequency communications systems. The 2.4 GHz and 5 GHz radiation emitted from a Wi-Fi router diffracts around walls in houses to give you adequate coverage in your home (along with reflections, which we will discuss soon).

Reflection

Reflection is another general wave property that allows a receiver to communicate with an occluded transmitter. If a wave propagating through one medium reaches a different medium, the second medium will reflect back a portion of the wave’s energy. Radio waves can reflect easily off of many large objects; Figure 2-8 illustrates this principle.

These reflected waves can still carry useful information to a receiver. In urban environments, cell phone signals (usually in the range of 700 MHz–2 GHz) bounce off buildings as one way of reaching their target. Many wireless signals can take multiple reflected paths to a receiver. This is called multipath propagation, which can be both good and bad for wireless communications systems.

In general, good conductors (such as metal) reflect most of an electromagnetic wave’s energy. Other materials like rock reflect some energy, and many insulators such as plastics reflect little energy. Areas covered in metal are well shielded from electromagnetic radiation, because the metal will reflect much of the incoming energy back.

Absorption

The energy not reflected by a medium will pass into the medium. Some materials allow electromagnetic radiation to pass through them without attenuation better than others. This is obviously another frequency-dependent phenomenon, as many materials do not allow visible light to pass at all but do pass lower-frequency radio waves. Radio waves can travel through most nonconductive materials, as illustrated in Figure 2-9.

There’s not much of a general trend for how much energy a material absorbs vs. how much it lets pass. The important things to understand here are general characteristics. Many signals pass somewhat well into aboveground structures, but are still slightly attenuated by them (try observing how your cell signal changes as you move outside). Other materials, such as thick concrete, block radio waves pretty well. In general, the ground will block most radio waves; this means that underground structures are well isolated against stray radio signals.

Noise

Noise is a concept that has an analog you already understand. In a noisy room, it can be difficult to hear something someone else says. This idea is no different for electromagnetic waves. Errant noise around the same frequency at which one is transmitting can vastly limit data transmission rates and force transmitters to use more power. In the example of speech, this represents talking slower and louder to make yourself easier to understand.

You’ve likely already “seen” electromagnetic noise. Televisions that receive wireless TV transmissions are prone to displaying static, which is nothing but the visual representation of electromagnetic noise. Figure 2-10 shows such static on a wireless television set.

Certain slices of the electromagnetic spectrum are extremely noisy, such as the unregulated 2.4 GHz band. Most of this noise is due to other devices transmitting in this frequency range. There is also a “noise floor” of background radiation that is always present.

The Doppler Effect

The Doppler effect is a general wave phenomenon that occurs with a moving transmitter or receiver (Figure 2-11). If a transmitter is moving at sufficient speed, electromagnetic waves will “bunch up” in front of the transmitter, making the waves appear at a higher frequency. Similarly, the waves will “spread out” behind the transmitter, making the waves appear at a lower frequency. Figure 2-11 provides a visual demonstration of this concept.

Some radar systems use this effect to measure the velocity of objects; a radar gun can bounce radio waves of a set frequency off of cars and measure the returned frequency, allowing it to calculate the speed of the car.

We will directly observe this effect in some later examples on satellite communications. Satellites are fast-moving objects, and as we observe transmissions from some satellites, the transmit frequency will appear to shift as the satellite moves closer to us and then recedes.

An interesting corollary of the Doppler effect is the shifting of infrared, visible, and ultraviolet light from distant galaxies and other celestial objects. Many celestial objects move at a significant fraction of the speed of light relative to observers on Earth. As a result, light from these objects can appear shifted due to their motion. If an object is rapidly receding, its emissions will appear more reddish (or “redshifted”), and if an object is rapidly approaching, its emissions will appear more blueish (or “blueshifted”). Scientists can use this information to estimate the speed and range of distant celestial objects. A famous image from the Hubble Space Telescope, shown in Figure 2-12, captures this phenomenon.

An Aside on “Radiation”

Radio waves are classified as nonionizing radiation, which means that they lack the energy necessary to separate electrons from atoms. Ionizing electromagnetic radiation, such as ultraviolet light or x-rays, is energetic enough to cause adverse biological effects like DNA damage. On the other hand, the only known effect of radio waves on the human body is surface heating of tissue, which is completely harmless for all but the most powerful of transmitters. This is the principle that microwave ovens exploit to heat food.

The radio waves we interact with on a day-to-day basis pose little threat to human life. Scientists are still gathering information on the effects of long-term exposure to electromagnetic radiation such as that emitted by a cell phone, but for now there’s no evidence to indicate that such exposure causes cancer or other adverse health effects. You may, however, encounter electromagnetic warning labels such as the one in Figure 2-13.

Such labels generally indicate the presence of a high-powered transmitter. Engineers do take precautions to limit exposure to electromagnetic radiation around these devices. Tissue heating can be dangerous, and the FCC sets limits on how much exposure to radio frequency (RF) radiation an individual should allow.

General Trends

In case you haven’t already noticed, I described many of the preceding principles in a very broad manner. This is a recurring trend that you will see all throughout the domain of radio communications. A good communications engineer or electrical engineer has a comprehensive intuitive understanding of the concepts he or she uses. Having a general idea of what will happen in any given scenario is important, and this can be developed through practice and experimentation. There is a concrete mathematical basis for almost every phenomenon in radio communications, but very little of this is needed to understand the magic of radio.

So you can combine the preceding principles to generate an understanding of how radio waves will behave with no knowledge of the math and formulas behind them. In general, lower-frequency radiation propagates farther but can carry less information. This simple statement represents one of the fundamental trade-offs in radio communications systems. We will explore this throughout the rest of the text.

Summary

Radio waves are a fascinating phenomenon with broad impacts on our daily lives. As part of the electromagnetic spectrum, radio waves exhibit many of the same behaviors of light. Many of the differences in the behavior of radio waves as compared to light are a consequence of their relatively long wavelength (and thus low frequency). The difference in wavelength between different radio waves also leads to different properties among radio waves, which we classify through “bands.”

In general, lower-frequency bands such as LF, MF, and HF can utilize special properties of radio waves and the Earth’s atmosphere to travel long distances. These bands are not suitable for high data transfer rates, however. Bands of somewhat higher frequency, such as VHF and UHF, can still travel long distances but support high data transfer rates. This makes them very valuable for applications such as cellular data. The highest-frequency radio waves in the microwave region allow for extremely high data transfer rates, but oftentimes a direct line of sight between the transmitter and receiver is required.

Radio waves of the frequencies that we’ll discuss in this book propagate through the line-of-sight propagation mode. Diffraction around sharp edges and reflections off obstacles allow transmitters and receivers without a direct line of sight to still communicate, although a direct line of sight will always yield the strongest signal.

In the next chapter, we will discuss antennas: the physics that govern their use, different types of antennas in use today, and how one can utilize an antenna for radio communications.