Electromagnetism
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.
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 |
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
Gain
Radiation pattern
Bandwidth
Polarization
Gain
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.
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
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
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
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
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.
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.