© Alex Wulff 2019
A. WulffBeginning Radio Communicationshttps://doi.org/10.1007/978-1-4842-5302-1_3

3. Antennas

Alex Wulff1 
(1)
Cambridge, MA, USA
 
Antennas are the interface between the world of electronics and the world of electromagnetic radiation. All electronic devices depend on current, the flow of charge, and voltage, electric potential, to perform their intended function. An antenna can transform an alternating current (AC) into a radio wave and vice versa. In this chapter, we’ll explore the physics that governs how antennas work and how you can use antennas to communicate. We'll cover everything from basic wire antennas to massive radio dishes such as those shown in Figure 3-1.
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Figure 3-1

Radio telescopes use parabolic antennas (“radio dishes”) to detect radio waves from spacecraft and celestial objects

Electromagnetism

Electromagnetism is the field of study that encompasses charge, current, magnetism, and electric and magnetic fields. Electromagnetic waves are governed by the principles of electromagnetism; an electromagnetic wave is nothing but alternating electric and magnetic fields. You're already familiar with one type of electromagnetism: lightning. Lightning bolts, such as those shown in Figure 3-2, can produce intense electromagnetic waves.
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Figure 3-2

Lightning is a burst of current that produces very strong electromagnetic waves

An alternating current within a conductor will produce an electromagnetic wave; antennas exploit this simple property to transmit information in the form of electromagnetic waves. Electric currents are very easy to produce and control, so antennas serve as an effective way to transmit and receive information without the use of a direct connection.

Electromagnetism provides the fundamental basis of all electromagnetic waves; but, thankfully for those learning about radio communications, very little knowledge of electromagnetism is required to understand how radio communication systems operate.

Current and Voltage

The “driving force” behind the electromagnetic waves emitted from an antenna is an electric current. Current is the flow of charge (electrons) per unit time, which is measured in amperes (A). The more charge flowing per second through a circuit, the higher the current. In general, the more current one can induce in an antenna, the stronger its emitted radiation, so almost all antennas are made from good conductors of current (metals).

In order to generate an electromagnetic wave from a conductor, one needs to apply an alternating current (AC). An alternating current is one in which the electrons move back and forth in a sinusoidal manner inside the conductor at a set frequency. Wall outlets supply alternating current to our devices at a frequency of 60 Hz, so there’s a lot of 60 Hz noise that makes its way into radio systems from power lines and electrical wiring.

Another property of electromagnetism is electric potential, or voltage. Voltage is measured in units of volts (V). An applied potential in a circuit will coincide with a current. If there is no potential difference between two places in a circuit, then no current will flow between these two points. Charge always wants to move to areas of lower potential, and it takes energy to raise these electrons back to a higher potential. In many circuits, chemical reactions inside of a battery supply this energy to keep electrons flowing.

Current and voltage are related in an equation known as Ohm’s law: V = IR, where V is voltage, I is current, and R is resistance. Therefore, for a given circuit, current and voltage are proportional.

Resistance is an intrinsic property of an object that depends on its geometry and material. The resistance of an object is a measure of how much it impedes the flow of electrons. Resistance is measured in ohms (Ω). Metal wires have very low resistances, on the order of 0.1 Ω per meter of wire, whereas insulators such as plastics have resistances many orders of magnitude greater than that.

For the purposes of this book, you can think of a voltage applied across a conductor as causing a current in that conductor, although in actuality the underlying mechanisms of electromagnetism tell a slightly different story. Wall outlets in the United States supply a potential of 120 V to electronic devices in our homes. Many devices have circuitry that lowers this voltage and converts the resulting current into direct current (DC), which is a linear flow of charge without any oscillation. Direct current flowing through a conductor does not produce electromagnetic radiation, but it is useful in sensitive digital equipment such as smartphones and computers which operate entirely using DC. In fact, one of the only uses of alternating current in such circuits is for radio systems.

Power

Power is a measure of energy per unit of time. In the context of antennas, the power dissipated by an antenna shows how much energy this antenna radiates per unit of time. Power is measured in watts (W) and is given by P = IV (power is current times voltage), or P = I2R (the current squared times the resistance of a material) for resistive loads. The amount of energy you utilize in your home is often measured in kilowatt hours (kWh). If a home consumes 1000 kWh in a month, one could expend the same amount of energy by consuming 1000 kW (1 MW) of power over the span of 1 hour or, equivalently, 1 kW for 1000 hours.

Communications and electrical engineers often measure the energy output of antennas in terms of watts. Table 3-1 gives the output or input energy of various systems.
Table 3-1

A list of various electronic systems and their associated output power

System

Typical Power

Wi-Fi router

100 mW

Cell phone antenna

3 W

Voyager spacecraft transmitter

~20 W

Geostationary communications satellite antenna

~100 W

FM radio station antenna

1 kW

AM radio station antenna

50 kW

Most powerful AM radio stations’ antennas

2 MW

Power output of a nuclear power plant

1000 MW

The Voyager spacecraft are a fascinating case. Using around 20 W, Voyager 1 is able to communicate with Earth over a distance of more than 13 billion miles. Estimates place the power received on Earth from this spacecraft at around 10^-16 W, which is only detectable by extremely large radio dishes and is barely above the power of ambient radio noise. NASA utilizes a network of global network of these dishes, such as those shown in Figure 3-3, to receive this puny signal.
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Figure 3-3

Radio dishes used to communicate with the Voyager spacecraft as a part of the Deep Space Network

Resonance

A given antenna can be utilized to transmit across many different frequencies, but antennas are always most effective around their resonant frequencies. For an antenna receiving radiation, incoming radiation of the resonant frequency helps magnify currents inside the antenna from previous radiation, creating a compounding effect. Other frequencies can be particularly ineffective at inducing current in an antenna because incoming radiation can actually create currents that oppose existing current. This same principle occurs in reverse for transmitting.

The resonant frequencies of an antenna are mostly determined by its length. Many antennas resonate at odd multiples of ¼-wavelength. This means that if a quarter of a given frequency’s wavelength is some odd multiple of the length of the antenna, the antenna will likely resonate at that frequency. Let’s take 144 MHz waves as an example. The wavelength is 300,000,000 m/s / 144,000,000 Hz ≈ 2 m, so a resonant receiving antenna would be approximately 50 cm (or odd multiples thereof).

Antenna Properties

Similar to radio propagation, there are a number of properties of antennas that govern their use. Through an understanding of these properties, you can gain an intuitive understanding of how an antenna will operate without knowing much about electromagnetism. The properties that we will explore are
  • Gain

  • Radiation pattern

  • Bandwidth

  • Polarization

Gain

The gain of an antenna refers to how much energy an antenna radiates in a particular direction. Antennas do not radiate energy equally in all directions, which is a useful property that one can exploit to make highly directional antennas. Gain in the general sense of the term is nothing but the ratio between the output and input power of a system. As such, the gain of a system must always refer to some baseline. In the case of antennas, this baseline is a theoretical antenna called an isotropic antenna (see Figure 3-4).
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Figure 3-4

A drawing of an isotropic antenna. The antenna is the dot at the center, and its radiation pattern is drawn

An isotropic antenna radiates any input energy equally in all directions. This is impossible to construct in practice, although there are types of antennas that have similar characteristics. The gain of an antenna in any given direction is the ratio between the radiated power in that direction and the expected radiated power from an isotropic antenna. This is known as isotropic gain.

Isotropic gain can be expressed as the raw ratio, but it is more often measured in units of decibels-isotropic, or dBi. The decibel scale is used extensively to characterize the gain of many systems in scientific and engineering contexts. The decibel scale is always logarithmic, meaning that a doubling of output to input ratio results in a linear increase in gain. The definition of isotropic gain is
$$ {G}_{dBi}=10{log}_{10}left({G}_{ratio}
ight). $$

A doubling of power with respect to an isotropic antenna yields a gain increase of approximately 3 dBi. You can easily show this through the gain formula: 10 · log10(1) = 0 (a ratio of 1 corresponds to zero gain), 10 · log10(2) = 3.01, 10 · log10(4) = 6.02, and so on.

An antenna will only have a fixed amount of input power to radiate; if the radiated power of an antenna is greater in a particular direction than an isotropic antenna with the same input power, then the radiated power must be less than that of an isotropic antenna in another direction. Antennas are passive elements, meaning that they do not contribute any energy to a system; antennas merely transform electromagnetic waves into current and vice versa. There will always be this trade-off between more radiated power in one direction and less in others for any type of antenna.

Antenna gain is symmetric for transmitting and receiving. If an antenna radiates more power in a particular direction, then this antenna will also be better at receiving electromagnetic waves in that particular direction.

Beam/Radiation Patterns

One can represent the gain of an antenna in all directions through a plot of its radiation pattern. The radiation pattern of an antenna is nothing but a 3D or 2D plot of an antenna’s isotropic gain as measured from an observer a fair distance away. An antenna can be constructed in a computer-aided design (CAD) program that will then simulate this antenna’s radiation pattern. The radiation pattern of antennas can be physically measured by moving a highly calibrated receive antenna around the transmitting antenna to capture its gain at specific points.

We will now discuss basic types of antennas, their merits, and their radiation patterns. I have modelled all these antennas in one of the aforementioned CAD programs and included images of the calculated radiation patterns. The gain of most of these images is measured in dBi; when represented in ratio form, many of the nuanced features of the radiation patterns disappear, as logarithmic scales tend to accentuate smaller features and make more powerful features less prominent.

Dipole

A dipole antenna is one of the most widely used antenna types. It is oftentimes used on its own to transmit and receive radio waves, or it can be a part of a larger antenna structure containing multiple elements. As shown in the radiation pattern, dipoles radiate most of their energy along their length. They exhibit a null, or area of low radiation, around the top of the antenna. See Figures 3-5 and 3-6 for the appearance of a typical dipole antenna and a simulation of its radiation pattern.
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Figure 3-5

A model of a dipole, which is just a wire with the feed (red dot) in the middle

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Figure 3-6

Dipole antenna’s radiation pattern in dBi

Dipoles are usually comprised of two colinear quarter-wavelength elements that are connected to the source. Since the total length of the antenna is half a wavelength, this particular variety is called a half-wave dipole. A very common use of dipole antennas is in cellular communications. Most cell towers look similar to the one in Figure 3-7.
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Figure 3-7

A standard cellular communications tower with sector antennas

The rectangular-looking objects on the cell tower are called sector antennas , which are nothing but a few dipoles stacked vertically. A metallic surface behind the dipoles reflects their energy outward to cover roughly 120 degrees in azimuth around the tower.

Monopole/Whip

Monopole antennas are similar to dipole antennas in their radiation pattern and their operation. The difference is that monopole antennas generally have their feed point at the bottom of the segment of wire, and the total antenna length is generally one quarter-wavelength. As shown in Figures 3-8 and 3-9, there are many similarities between the monopole and dipole antenna types.
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Figure 3-8

A monopole antenna model; note the red feed point at the bottom as opposed to the middle with the dipole

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Figure 3-9

The radiation pattern of a monopole antenna; it is very similar to that of a dipole

Monopole antennas only have one length of wire that radiates energy, hence why they’re called “monopole” antennas as opposed to dipole antennas which have two lengths of wire that radiate. One can commonly see monopole antennas in the form of mast radiators, or radio towers as they are colloquially known. Figure 3-10 shows a collection of mast radiators.
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Figure 3-10

A variety of mast radiators

Radio towers are often employed to transmit radio waves with long wavelengths across long distances. Some towers utilize the entire length of the tower as an antenna, while other towers merely support a radiating element at the top of the tower. Many of the towers in Figure 3-10 support a large monopole antenna at the top of the tower.

Yagi

Yagi antennas employ a variety of driven and parasitic elements to create a highly directional beam. The longest and farthest back element is a reflector, which gives the antenna forward directionality. Next is the driven element, which is a dipole antenna connected to the source. Subsequent elements are not connected to the source, just like the reflector; their job is to absorb and reradiate the energy emitted from the dipole to give the antenna further directionality (this is why they’re known as parasitic elements). Figures 3-11, 3-12, and 3-13 show the appearance and radiation pattern of a Yagi antenna.
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Figure 3-11

An isometric view of a model of a Yagi antenna. The radiating element is the second from the right. Most elements are slightly different lengths

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Figure 3-12

A gain-scaled model of a Yagi’s radiation pattern. Take no note of the fact that the radiation emanates from the element in the middle; this point was merely chosen to be the origin

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Figure 3-13

A dBi-scaled model of a Yagi’s radiation pattern

One can commonly see Yagi antennas on the roofs of buildings. Back when most television was still transmitted over the air, many homes used directional Yagis to achieve good reception. One item to note in the radiation patterns pictured above is the difference between the raw gain-scaled image and the image that is scaled in dBi. Logarithmic scales accentuate features of lower magnitude, so it’s possible to see minute details of the radiation pattern that disappear on the gain-scaled image. In actuality, these features are hundreds of times lower magnitude than the main beam pictured in the gain-scaled image.

Log-Periodic

Log-periodic antennas look very similar to Yagi antennas but operate in a different manner. Log-periodic antennas have multiple driven elements which allow a given antenna to operate across a wide range of frequencies. We will explore this concept further in the section on bandwidth. Log-periodic antennas are also directional.

Parabolic

Most antennas commonly known as “radio dishes” are actually parabolic antennas. Parabolic antennas are highly directional and help the operator collect a lot of energy, making long-distance communications feasible. These are perhaps the easiest type of antenna to identify and are seen in numerous places. Figures 3-14, 3-15, and 3-16 demonstrate the appearance and radiation pattern of the parabolic antenna.
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Figure 3-14

A simple parabolic antenna comprised of a dipole and a parabolic reflector

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Figure 3-15

A gain-scaled image of this parabolic antenna’s radiation pattern

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Figure 3-16

A dBi-scaled image of this parabolic antenna’s radiation pattern

Parabolic antennas are commonly comprised of a metallic parabolic dish and some feed element offset in the center of this dish at a very precise place called the focal point. Parabolic antennas function by reflecting incoming waves off the dish once, with all the reflected waves combining at one point, called the focal point. This concept is easy to understand after looking at how such a dish functions in Figure 3-17.
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Figure 3-17

Parabolic antenna operation

Many cell towers with sector antennas, such as the one in Figure 3-18, also have drum-looking structures mounted in a different location.
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Figure 3-18

A cellular communications tower with parabolic antennas covered by a radome

These objects are actually parabolic antennas covered by a radome. Radomes are radio-transparent devices (radio waves pass through them unhindered) that protect the parabolic antennas from the weather. These parabolic antennas provide high-capacity data links to and from the tower that allow it to communicate with other towers and stations on the ground. This minimizes the amount of wiring and infrastructure necessary to deploy new towers. Such links are highly directional and oftentimes require a direct line of sight between other dishes.

Another common application of parabolic antennas is in radio astronomy and satellite communications. Since radio antennas are highly directional and have the capability of collecting lots of energy from a distant receiver, they are well suited to this use case. The only downside of such antennas is that they must be pointed directly at their target to be effective. A difference of only a few degrees can cause stations on Earth to lose communications with a spacecraft; such a spacecraft is shown in Figure 3-19.
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Figure 3-19

Spacecraft such as Voyager (pictured here) employ high-gain parabolic antennas to communicate with distant targets. Voyager’s parabolic antenna is the large dish at the center of the spacecraft

Bandwidth

Due to the principles of resonance that we discussed above, many antennas can only transmit and receive across narrow bands of frequency. A way to express this quality of a given antenna is bandwidth. Bandwidth is a measure of how good an antenna is at radiating energy across a varied frequency range. Some antennas have a very narrow bandwidth, while others can transmit across large frequency ranges.

A narrow bandwidth can be both good and bad depending upon the application. If one wants to transmit or receive information at many different frequencies, a narrow bandwidth is undesirable. Alternatively, a narrow bandwidth can be a useful property to filter out noise from adjacent parts of the spectrum for receiving. This idea of fundamental trade-offs is a recurring theme throughout the realm of radio communications; many properties of communications equipment and the electromagnetic spectrum can be exploited for one application while introducing complexities for another application.

Polarization

Most antennas emit polarized radiation , or electromagnetic radiation that is directionally oriented. In the context of antennas, the concept of polarization requires that transmitting and receiving antennas must be oriented in the same direction in order to function properly. Dipole and monopole antennas are polarized, which means that antennas derived from monopoles and dipoles, such as some parabolic antennas and Yagis, are also polarized. A 90-degree difference between the transmitting antenna and the receiving antenna can attenuate the received signal by as much as 20 dB, or around a factor of 100.

Summary

The physics that governs antennas, and radio waves in general, lies primarily in the domain of electromagnetism. Radio waves are nothing but alternating electric and magnetic fields. The length of an antenna is a function of the wavelength of radio wave it’s designed to emit. As shown by the radiation patterns in this chapter, one can arrange basic antenna types in creative ways to serve a variety of tasks. The monopole and dipole antennas form the building blocks of the more complex antennas, such as the directional Yagi and parabolic antennas.

Hopefully, antennas are now not just some strange device you see on the side of homes or on top of large poles; antennas are the backbone of many communications networks, so understanding various antenna properties is crucial to gaining a complete picture of radio communications. There is still so much to learn about antennas. This chapter glossed over many of the details of how antennas actually radiate, the role phase plays in these systems, and many other points; so, if your interest has been piqued, I highly recommend doing more of your own research.

Much like with radio wave propagation, it’s not necessarily important to have a concrete mathematical basis of how antennas operate. Rather, make sure you understand the basic principles and antenna types discussed in this chapter, and you’ll be well on your way to becoming proficient in radio communications. In the next chapter, you’ll get to take your first dive into radio communications by downloading information from National Oceanic and Atmospheric Administration (NOAA) weather satellites via radio waves.

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