Radio Antennas in the Wild
There are easily thousands of different types of antennas serving different purposes. Some are simple lengths of wire, others exotic assemblages of high math and science. Most of the antennas around us are pretty straightforward and have a set of identifiable characteristics.
With a few patterns in mind and some rules of thumb, you can usually make a pretty good guess at what most antennas are being used for, or at least the parts of the EM spectrum on which they’re operating.
Making electromagnetic waves
Antennas are driven from radios by feedlines consisting of two conductors (often bundled in a coax cable with a center conductor and a shield) which carry the two sides of the alternating current signal the radio produces. The simplest antenna, a dipole, takes the two conductors from the feedline and spreads them out away from each other. The antenna might appear to be a break in the circuit since the two arms of the antennas are separated instead of connected. What’s actually happening is that the two wires are acting like a big air-gap capacitor. Instead of the current being contained within the dielectric gap of a capacitor, the current is flowing in space as an electric field arc between the two arms of the dipole.
As the oscillating AC signal moves electric charges back and forth across the antenna arms, the accelerating charges create a magnetic field around the antenna, just like an electromagnet. Without trying to go too deep into Maxwell’s equations, rest assured that changing magnetic fields create electric fields, and changing electric fields create magnetic fields. The electrical energy applied to the antenna is thus converted into electromagnetic waves that self-sustain (magnetic and electric fields each relentlessly creating the other) and radiate away from the antenna. Radio!
This only works efficiently if the antenna has the appropriate length for the frequency of signal it’s trying to radiate. An antenna that is perfectly matched to a frequency is called resonant. When you change the free length of a guitar string by pressing on frets, you change the note (frequency) the string resonates at. When you change the length of an antenna, you change the frequency that it resonates at (though resonance has a number of additional technical meanings when used in electronics and radio).
Frequency and wavelength
Frequency is the number of times the wave completes a cycle in a given amount of time. The common unit of measure is the Hertz (Hz), or cycles per second. Since waves travel through space at the speed of light, the distance between peaks, called the wavelength (λ), is directly related to the frequency. This is generally measured in meters and derived by dividing the speed of light by the frequency. A signal with a frequency of 14.074MHz has a wavelength of 21.03m.
Ranges of frequencies are usually referred to as bands, such as the US 40m amateur radio band (7.000MHz–7.300MHz) or the US AM radio band (525kHz–1705kHz).
Antenna size and shape
Antenna lengths are generally whole-number multiples or fractions of a target wavelength (½λ, ¼λ, ⅛λ, 1λ, 2λ, 4λ, etc.), so they often provides clues about the intended frequency of operation. Long wavelengths/low frequencies tend to use long antennas and short wavelengths/high frequencies tend to use short antennas. Engineers have created a wealth of tricks to allow antennas to operate on multiple bands, but there are almost always limits. Shortwave antennas designed for wavelengths of tens of meters aren’t very useful for PCS cell wavelengths in the tens of centimeters, and vice versa.
If your initial image of an antenna is the wire sticking out of your car, don’t worry! That single wire (a monopole instead of a dipole) is doing the same thing as one arm of the dipole.
As a substitute for the other arm, monopole antennas connect that side of the feedline to a reference source, or ground plane, that’s usually perpendicular to the monopole (shown below the radiation pattern in the diagram). The ground plane could be your car, a set of wires, the earth, or in the case of cell phones, our bodies.
Shaping electromagnetic radiation
Monopoles and dipoles send their signals out in a donut-shaped 360° ring around the length of the wire. This is referred to as an omni-directional signal. However, electromagnetic (EM) waves interact with matter in different ways. In some cases it’s absorbed, in some reflected, and others refracted. Conductive items that respond to EM waves may, in turn, re-radiate them out again. If the secondary radiation interacts with the initial waves, the two signals may reinforce or cancel each other out depending on their phase relationship. This is referred to as constructive and destructive interference.
When carefully managed, this effect can be used to shape and direct the output of an antenna. Many antenna designs have a driven element called an emitter (frequently a dipole) and one or more additional elements (reflectors and directors) that shape the resulting combined field. While no antenna can add power to the initial signal, by using additional elements a power gain can be created in one or more specific directions by reducing power in other directions. The result is often called a uni-directional signal.
These are described as beam or directional antennas and are either fixed at a specific target to improve transmission and reception or rotated to change targets.
EM waves’ ability to reflect can be leveraged to great effect when they are aimed at a parabolic dish. Satellite dishes and space telescopes both use the same principle of concentrating a tight capture area into a small point for reception and focusing the output of a small point into a directional beam for transmission. The dish is a passive reflector element and the driven element is positioned at the focal point of its parabola.
Reflection also allows EM waves to be routed through shaped hollow pipes called waveguides. Though there are examples of waveguides for all kinds of waves, these tools are a common feature used in antennas used for extremely high-frequency waves, commonly called microwaves when above 1GHz (though other definitions start microwaves at 300MHz). Waveguides allow a low-loss way to direct EM waves while also serving as a high-pass filter, rejecting waves below a certain frequency.
The diagram illustrating the electrical wave arching from one end of a dipole to the other implies an orientation that antennas create for the waves they generate. This effect is called polarization and it is an important aspect of antenna design. Antennas have different polarizations if they are mounted vertically or horizontally. A receiving antenna operates best if it has the same orientation as the transmitted signal it’s picking up. Other polarization schemes are used as well. Helical antennas are an example of a circular polarization design that can receive both horizontal and vertically polarized waves. This is especially useful for receiving signals from satellites that are changing orientation (and therefore polarization) as they orbit or experiencing an effect called Faraday rotation as their signal traverses the ionosphere.
Most antennas operate by interacting with the electrical field of an EM wave. But the magnetic field is always there too, and that’s the aspect used to transmit and receive with most loop antennas. These antennas can be circular, hexagonal, square, or any closed geometry. There may be more than one loop serving the same purpose as the additional elements on a beam antenna. Small loop antennas can have especially sharp nulls where they can’t receive. This makes them especially useful for radio direction finding. By turning the loop until a signal disappears, you can determine the direction from which it is coming.
How electromagnetic waves travel
EM waves can travel, or propagate, in a variety of ways, though different frequencies have different abilities in this regard. These propagation types are: ground waves, direct waves, and sky waves. Extremely low-frequency (used for communication with submarines around the world) to medium-frequency signals (3Hz–3MHz, such as AM radio) hug the Earth as they travel and are called ground waves. High-frequency (HF) waves in the 3MHz–30MHz range (and medium-frequency waves) have the ability to refract off the layers of the ionosphere as sky waves, if they’re at a shallow enough angle (otherwise they head into space), often multiple times, and reach around the world.
Very high-frequency (VHF) and above waves (30MHz and up) rely on direct, or line-of-sight, transmission. These waves are obstructed by the curvature of the Earth or other obstacles and tend to have limited range compared to lower frequencies. Of course, that depends on where you point them. Aim them at a satellite and even a handheld radio can contact low Earth orbit!
The higher an antenna is mounted, the farther its line-of-sight radius is. Tall antenna towers provide broad coverage for antennas relying on direct propagation. Cellphone towers, commercial radio, amateur radio repeaters, microwave data transmission, emergency services, and other UHF/VHF/microwave antennas all tend to be mounted as high as feasible to increase their range.
Even HF antennas attempting to bounce signals off the ionosphere benefit from height. Some part of the signals they output bounce off the Earth below them. When they are less than half a wavelength above the ground, these reflected waves interfere with the transmitted wave and tend to make most of the signal go upward at an angle so steep that it either passes through the ionosphere or bounces straight back down. This means that an antenna operating on the 40-meter amateur band would ideally be at least 20 meters (65 feet) above the ground to achieve long distance propagation. The images below show the comparison of this band being broadcast at 5 meters high and 20 meters high. In some cases, if you want to communicate locally, aiming up and bouncing down is desirable. This is referred to as near-vertical-incidence skywave (NVIS) operation.
All these characteristics provide clues when spotting an antenna in the wild. The length of the elements relates to the frequency of operation. The height offers clues to the coverage area. Odd shapes like horns suggest waveguides which are most likely used for high-bandwidth microwave transmissions. Directional antennas are usually pointed at a target such as a receiver at a given compass bearing. If they’re pointed at the sky, that target is probably a satellite.
We spend our lives constantly bathed in the radio waves propagating around us. Much of our daily activity relies on radio-enabled cellphones, tablets, GPS, and Wi-Fi. Recognizing the amazing proliferation of antennas can be an exciting way to gain awareness of these tools for manipulation of the invisible forces underlying our modern world. And with that, let’s take a look at some of the more common antennas you’ll see in the wild …
Antenna spotting field guide
Antennas are all around us, but what does each kind actually do? Here are a variety of different types, and how they’re used. See how many you can find near you!
Dipole
Dipoles consist of two arms, each connected to one side of the radio circuit feedline. Can be constructed from stretched lengths of wire or rigid metal tubing. “Rabbit ear” TV antennas are a common example. Used for nearly every radio band. Size ranges from huge to tiny. Particular favorites of the amateur radio aka ham community.
Horizontal, Vertical, and Diagonal — Mounted horizontally, a dipole radiates broadside to the length of the wires, with nulls (directions in which it can’t receive) at either end. Mounted vertically, a dipole radiates in an omnidirectional 360° pattern (since the nulls point up and down). When one end is mounted higher than the other, it’s called a sloper and the radiation pattern changes with the angle of slope.
Inverted V and L — Dipoles are often mounted with the highest point in the center and the two arms angled down, usually at a 120° or 90° angle. Inverted-vee dipoles provide a more omnidirectional pattern than a horizontal. Dipoles can also be mounted with one arm horizontal and one vertical, in an L-dipole configuration. This is usually due to space constraints.
Folded — When the far ends of the dipole are connected with a parallel wire, the result looks like a flattened oval and is called a folded dipole.
Monopole
Extremely common anywhere that room for a dipole isn’t available. Frequently used in commercial radios to save space. If one of the arms of the dipole is replaced with a connection to a ground plane (wires, car, person, earth), the antenna becomes a monopole.
Long Wire — While they require a tuning circuit to work well, long lengths of random wire can be used as antennas.
End-Fed Half Wave — By using a transformer to connect to a wire that’s a half-wavelength long, the antenna will be resonant on the primary wavelength plus its harmonics, making it a multi-band antenna.
Car — Car radio antennas are usually around 75cm long, approximately one quarter of the center wavelength of the FM radio band (2.78m–3.41m).
Handheld — Walkie-talkies, handheld radios, and older cellphones typically have antennas sticking out of them. They may be telescoping whips or wire coiled in a plastic extrusion. Our bodies serve as their ground planes!
Directional
These allow for transmission/reception toward specific locations, while lowering functionality in other directions. Generally referred to as beam or directional antennas.
Yagi — While not all antennas with a central shaft and parallel extrusions are specifically Yagi antennas, the term often gets used generically for this and similar styles of directional antenna (including Uda and log-periodic types). Yagi-style antennas are loved by hams for the gain they offer. High-end broadcast television antennas are often Yagi antennas since they can be aimed at a stationary broadcasting station.
Hexbeam — The hexbeam directional antenna uses a W-shaped driven element in front of a U-shaped reflector element, all arranged as a hexagon. Hexbeams are a favorite of ham radio operators for 20m–6m operation. Longer wavelengths make the construction too large.
Parabolic (Dish) — A transmitting/receiving element placed at the focal point of a parabola collects and emits focused radio waves from the dish. Commercial and government installations often use parabolic antennas, as does satellite TV.
Waveguide — These are channels that direct and focus radio waves in the microwave range. Frequently used for high-speed digital signals between buildings or relay towers.
Helical — Antennas produce polarized radio waves. These can be horizontal, vertical, or circular depending on the orientation of the emitting elements. Orbiting satellites change position, and therefore polarization, so a helix does a good job of dealing with those changes. Helical antennas are used whenever it’s difficult to get the receiver oriented to the same polarity as the transmitter.
Cellphone Tower Sector Antennas — Cellphone tower antennas are designed to work in a very narrow angle, or sector. Sets of these provide 360° coverage without sacrificing signal strength. Sector antennas are the most common configuration for cell service since they provide excellent coverage.
Tiny and huge
Other types of antennas, from PCBs in personal devices to massive pieces of engineering used for commercial, military, and scientific purposes.
2.4GHz (Bluetooth, Wi-Fi, etc.) PCB Antennas — Some devices operate on wavelengths short enough to allow traces on a printed circuit board to serve as antennas. The zig-zag trace shown is a Bluetooth 2.4GHz (12.5cm) antenna along with the ground plane of the board. The ESP8266 board includes a built-in Wi-Fi antenna, extremely common in IoT and mobile devices. You probably have a few around you right now!
High-Band (Millimeter Wave) 5G Antennas — Very tiny, used by 5G cellular devices operating near 30GHz (10mm).
ALLISS — Commercial, government, and high-power shortwave stations with coverage that needs to be changed directionally may use giant, steerable ALLISS antennas (from the French towns of Allouis and Issoudun).
“Elephant Cage” (AN/FLR-9, AN/FRD-10) — During the Cold War, massive ring antenna systems were set up to monitor and locate radio transmissions worldwide. The last of these “elephant cage” antennas can still be seen in Canada and Alaska.
Loop
Unique in that many of them respond to the magnetic rather than the electrical portion of the electromagnetic waves. Have very sharp nulls, making them useful for direction finding. Very portable; common in receivers and low power (QRP) operations.
Circular — Loop antennas can be circular, rectangular, or any closed geometry. They are often easier to deploy than large wire antennas.
Quad — Loop antennas can also include multiple elements that modify their directional transmission and reception abilities.
Radio Direction Finding Loop — Radio direction finding (RDF) usually uses small receiving loops that are inefficient for transmitting, but have extremely sharp nulls to assist in finding the direction of a radio source.
All images by Tim Deagan unless otherwise noted. Featured graphic Adobe Stock/bitontawan02.
This article appeared in Make: Vol. 78. Subscribe today to get the latest content.
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