sweet & sour notes

I didn't grow up listening to shortwave radio but I recently started doing it and thought I'd share some tips on technical facts that can help in understanding how to receive shortwave broadcasts, what I've heard on the shortwave band, and some of the tools I've used.

My favorite shortwave listening location

What is shortwave, physically?

You don't need to have a deep understanding of physics to enjoy shortwave listening, but a few basic facts can help in navigating the hobby. Electromagnetic radiation (more commonly known as light) can be measured as an electrical field and a magnetic field radiating outward from some source and oscillating together in perpendicular sine-waves. Radio waves, assuming a definition that includes the microwave spectrum, are simply any electromagnetic radiation with a longer wavelength or a lower frequency than infrared light.

Wavelength is exactly what it sounds like and is measured in meters (abbreviated m). Frequency describes how quickly the fields oscillate in hertz, or cycles per second (abbreviated Hz or denoted $1 \over s$). Shortwave frequencies are high enough that they are typically denoted in kilohertz (abbreviated kHz; 1kHz = 1,000Hz), or more rarely in megahertz (abbreviated MHz; 1MHz = 1,000,000Hz). Mathematically, the wavelength $\lambda$ and frequency $f$ of an electromagnetic wave are inversely proportional and related by a factor of the speed of light. The speed of light in a vacuum is a known constant $c$ with a value of exactly 299,792,458 meters per second, and the speed of light through air is close enough that we tend to use $c$ when describing the relationship between wavelength and frequency for radio waves we transmit and receive on Earth. (If you're wondering how the speed of light happens to be an exact whole number in meters per second, it's because once the scientific community precisely measured the speed of light, they adjusted the international definition of the meter to fit that measurement exactly.) So the speed of light is the product of an electromagnetic wave's wavelength and frequency, $c = \lambda f$. Rearranging this formula, you can divide the speed of light to derive an electromagnetic wave's wavelength from its frequency or vice versa: $\lambda = {c \over f}$ or $f = {c \over \lambda}$.

Different parts of the electromagnetic spectrum propagate through the atmosphere and other matter in different ways. The exact boundaries of the shortwave band are debatable but a typical shortwave receiver might tune to frequencies between 1.7MHz and 30MHz, corresponding to wavelengths of about 175m to 10m. Electromagnetic radiation at these wavelengths can reflect strongly off the outer layers of Earth's atmosphere (the ionosphere), which has the practical benefit of enabling powerful ground-based radio transmitters to produce signals that can be received well beyond the horizon and even on the other side of the world. This form of signal propagation is called skywave.

If you have a typical contemporary “AM/FM” radio receiver, the shortwave band is between that radio's AM band (historically called medium wave) and the FM broadcast band (which has higher frequencies and shorter wavelengths than shortwave). Various kinds of signals are commonly transmitted in various parts of the shortwave band. There are simple Morse code transmissions in the interrupted continuous wave (CW) format which dates back to the early days of radio as a wireless alternative to the linear telegraph. There are signals in a number of digital formats used by everyone from military forces to amateur (“ham”) radio operators, which can encode all sorts of data including text and images. There is even a relatively new digital audio format intended for consumption by the general public, called Digital Radio Mondiale (DRM). But most broadcast shortwave transmissions are analog audio signals encoded by amplitude modulation (AM), found on frequencies within fourteen internationally agreed-upon slices of the shortwave spectrum called the shortwave broadcast bands. These bands are named for their approximate wavelengths: 120m, 90m, 75m, 60m, 49m, 41m, 31m, 25m, 22m, 19m, 16m, 15m, 13m, and 11m. The ham operators have their own assigned shortwave bands interspersed with these, and those bands have band plans laying out what kinds of signals are conventionally transmitted within specific frequency ranges and what operator license level is required to use them. And there are other assigned frequencies within the shortwave band, such as those reserved for marine and aviation use, the citizens band (CB) at wavelengths just below (or frequencies just above) the 11m broadcast band, or even the narrow bands assigned to the non-communicative radio signals produced by industrial, scientific, and medical (ISM) equipment.

AM is a very simple and powerful way to transmit audio signals. In brief, it works like this: Sound is a compression wave that propagates through a medium like air or water. Unlike light, it cannot pass through a vacuum, because it is made up of the oscillating compression of its medium. Electric audio equipment converts these sound waves to and from waves of oscillating voltage; the peaks and valleys of this electrical wave represent the peace and valleys of compression measured by a microphone. An AM broadcaster transmits a powerful electromagnetic signal, the carrier signal, at once specific frequency, the dial frequency, but modulates the amplitude (“brightness” or intensity) of the signal according to the peaks and valleys of the sound wave that is being encoded. This fast modulation of the signal also produces additional, symmetrical electromagnetic signal in the frequencies immediately above and below the dial frequency, called the upper and lower sidebands. The frequency range occupied by the whole signal including sidebands is the signal bandwidth and is determined by the strength of modulation used; for shortwave AM broadcasts this is typically no greater than 9kHz.

AM has the advantages of carrying intelligible signals over relatively long distances and being the easiest form of audio transmission for a radio receiver to receive and decode. In fact, AM is so easy to receive and decode that soldiers in World War II would use headphones borrowed from communications personnel to listen to AM transmissions on makeshift “foxhole radios” which could be made with only a flashlight battery, some scavenged copper wire, a cardboard cylinder, a razor blade, and a safety pin. The primary disadvantages of AM are a susceptibility to certain kinds of signal distortion and the high electrical power requirements for transmitters.

Ham operators often transmit audio with more power-efficiency by transmitting only a single sideband (SSB) without a carrier signal; depending on the conventions for the transmission frequency, this can be either the upper sideband (USB) or lower sideband (LSB). SSB transmissions require a more sophisticated receiver capable of essentially recreating the carrier signal, and exhibit more extreme forms of audio distortion when the receiver is slightly detuned or if the wrong one of the two SSB modes is used. The sound of struggling to tune into an SSB transmission is much like the distorted voice transmissions heard in the underground rebel war room on Hoth in The Empire Strikes Back. But if you get the tuning just right it can sound pretty much like an AM transmission. Some time stations, like the Canadian government's CHU, opt for a compromise format like USB with the carrier wave; this requires more power to transmit than without the carrier wave, but less than full AM, and it is a little more distortion-prone than AM, but crucially it can be decoded by a normal AM receiver.

What is shortwave, anthropologically?

What makes shortwave socially distinct from other forms of radio broadcasting is that skywave propagation carries signals beyond regional, national, and continental boundaries, and so shortwave attracts broadcasters who are motivated to reach distant audiences. These include public broadcasters and missionaries who are trying to reach remote populations who have little access to other communications networks, propagandists both national and private, cults and political extremists who face stricter regulations in other media, organizations promoting cultural exchange and extending soft power, and spy networks.

Because skywave propagation is more effective in different bands at different times of day, and because a broad range of different transmitters are sharing the same frequencies all around the globe, shortwave broadcasts tend to be limited to a few hours in length, operating on fixed daily or weekly schedules. A good resource for navigating these schedules is the public database at short-wave.info.

For me the main draw of shortwave listening is the high likelihood of hearing something unfamiliar. Most of what I've received here has been in English or Spanish, but I can often find other languages on the dial, and I've received broadcasts over the air in languages that I hadn't even heard of before (Waorani, Aromanian), which is a lot more linguistics variety than I get on commercial medium wave and FM broadcasts locally. I've heard intense signal jamming and gone to the broadcast directories looking for answers about who was jamming those frequency bands; I could only speculate about why. I've heard cult missionaries sermonizing on obscure theologies, and the mouthpieces of foreign governments denouncing transnational political movements that have no physical presence where I live. Voice of Türkiye can tell me that the PKK is a terrorist organization again, but they've said it often enough that I hardly register the words anymore.

I haven't received much of these on my own equipment, but using radio receivers connected to the internet in various locations I've heard a few transmissions of a more inscrutable nature. There are the elusive “numbers” broadcasts, which have no published schedule or acknowledged origin. These often start and end with a short melody and maybe some codewords, but most of the transmission consists of a voice robotically reading out a series of digits, like: “four one nine zero three four zero zero” and so on. These are assumed to be spy communiques, encrypted by some trustworthy method like a one-time pad. There is the afformentioned jamming, which I have mainly heard on bands that are carrying broadcasts from Taiwan; the jamming transmissions are believed to originate from mainland China. Jamming can be any signal that is transmitted with the intent of interfering with the reception of another transmission, and the literature on jamming testifies to a number of creative techniques that were deployed during the Cold War, such as transmitting randomly sampled recordings of speech. But the kind of jamming I've mostly heard is basically just a wall of electromagnetic noise across a fairly wide band of frequencies, manifesting as a sort of jackhammer sound on my receiver. And then there are the real anomalies, transmissions of unknown purpose that defy categorization like the station known as UVB-76 that broadcasts mainly the sound of an intermittent buzzer on 4625kHz from a site near Moscow.

The sounds, culture, and mystique of shortwave radio has even inspired music. My personal favorites in this microgenre are Kraftwerk's album Radio-Activity and Boards of Canada's album Geogaddi.

Listening without your own radio receiver

Maybe you're curious about what you can hear on the shortwave band, but not ready to buy a receiver. Maybe you want to know what broadcasts you might be able to receive clearly in your region before you spend any money. Fortunately, a number of enthusiasts around the world who have software-defined radio (SDR) receivers have connected them to public web servers.

rx-tx.info is a great place to start connecting to SDR receivers located all over the world; there you can view them either in a list or on an interactive map, and filter by the frequency bands they receive. There are many that receive the shortwave (HF) bands, but there are also receivers that can tune to other frequencies, from longwave up to microwave. It's good to keep in mind as you peruse these SDR sites that they are community resources largely hosted by individual radio enthusiasts. They generally have small user quotas, and they may go offline from time to time without dedicated 24-hour support staff. Your turn tuning the receiver may be terminated after a while if other people are waiting, or you may have to wait in a queue. But usually I can find a shortwave receiver with no queue at any given time.

The web interface for many of these receivers will provide a “waterfall” view by default; this shows you at a glance a wide band of frequencies, with a color gradient representing their amplitude; the horizontal line spanning these frequencies scrolls downard at a constant rate, forming a waterfall-like moving graph that clearly shows what frequencies are carrying the strongest signals and even provides visual clues about the encoding of those signals. There will be some controls for tuning the exact frequency of the receiver, which is usually indicated by a single marker at the top of the graph. In some cases the top of the graph may also have a number of community-supplied labels flagging frequencies where particular kinds of transmissions tend to occur; for example “AMers” to mark where amateur radio operators often transmit AM signals or “STANAG” where modems transmit digital data according to the STANAG 5066 standard. There will also likely be controls for changing the bandwidth of the tuning, and for switching between different kinds of decoding to apply (AM, FM, CW, LSB, USB, and typically a few digital modes such as DRM).

Strongly-received AM signals tend to appear as bright streaks of color with a straight line running down the middle (the carrier wave) and symmetrical splotches on either side (the sidebands). Weaker AM signals may appear without visible sidebands or with only faint indications of them, but with careful tuning you may be able to hear a trace of audio from them.

Single sideband transmissions will look like half of an AM transmission, without the bright line of the carrier wave. The color-splotches are clustered more toward one side of the signal; this is the side where the carrier wave would be if the signal were broadcast in full AM. If they cluster more toward the lower-frequency end of the signal, it's an upper sideband (USB) transmission, and if they are clustered more toward the higher end it's lower sideband (LSB).

CW transmissions will tend to manifest as a dashed line or as a pair of lines, one solid and one dashed. The SDR interface will typically reproduce these as a series of beeps when properly tuned, but many of them can also automatically decode the transmission to text if the signal is recognizable Morse code.

Some digital signals will appear as a basically a solid stripe of colorful noise with clearly defined upper and lower frequency bounds. Others may take more exotic forms.

You may also see various kinds of interference. Natural phenomena like solar flares can produce fuzzy pseudo-signals on the graph. Jamming may appear as crude diagonal stripes across a wide frequency band. Even over-the-horizon (OTH) radar systems used for things like missile defense can produce blips across the radio spectrum. Before the collapse of the Soviet Union, secret Ukraine-based OTH radar systems produced such powerful interference in the shortwave bands that the phenomenon was known colloquially as the “Russian woodpecker.” But this kind of interference is much less prevalent today.

How to use a portable shortwave receiver

There are a few features to look for in a portable shortwave receiver. Analog receivers may offer smoother continuous tuning than their digital counterparts, but digital receivers will allow you to instantaneously dial in a precise frequency. It's also worth noting whether a digital receiver has band-scanning capabilities and memory for storing frequencies, because the shortwave bands are very large compared to commercial AM/FM bands and it can take a long time to find signals by manually adjusting the frequency of a digital receiver. Some receivers will also be capable of decoding single sideband transmissions, which may be useful if you want to listen to the shortwave ham bands.

Once you have your own portable shortwave receiver you can start to experiment to find the ideal conditions for receiving signals in your local area. There are several factors that contribute to clarity signal reception.

Location is a major factor. The ideal location for radio reception has clear views out to the horizon and is far from sources of interference. I have very poor reception for shortwave or even commercial AM/FM bands in the room closest to my house's connection to the electrical grid, and it's worth keeping in mind that electronics in general can be a source of interference. But I find that I get particularly good reception on a hilltop farm near my home that is open to the public as a historical landmark. You may not have regular access to a location like that, but some places may be degrees closer to that ideal than others, and those degrees can make a noticeable difference. It's worth trying a portable radio in a few places to experience that difference for yourself.

Weather is another factor. Of course, you should never operate a radio attached to an antenna that is vulnerable to lightning strike in an electrical storm. But you'll find that reception is much poorer during an electrical storm even if you are safely operating your portable receiver indoors. However, cloud cover per se does not doom shortwave reception, especially in very good locations. It is worth scanning the bands in overcast weather as long as there's no threat of a storm.

Time of day is not to be underestimated as a factor, not only because of when transmissions are scheduled but also because of how sunlight affects atmospheric propagation. I tend to find that my best shortwave listening experiences overall happen at night, but there are different bands that are more active and easier to receive in the daytime. It's worth listening day or night, but over time you'll learn what you receive best before or after sunset.

A good way to boost reception strength and maybe compensate somewhat for deficiencies in the other factors is to extend the length of your antenna. There are DIY methods to do this, but one convenient method is to purchase a portable wire antenna. This typically comes spooled up in a reel so you can easily carry it when not in use, with some kind of clip on the reel so you can hang one end of the antenna from window-blinds or a tree branch. The other end of the wire may have a clip that's meant to attach to the end of your receiver's built-in antenna, or it may have a 3.5mm TS jack to connect to a portable receiver that has a corresponding port for an external antenna. Check whether your receiver has such a port before you buy an antenna with one of these connectors.

You can never be totally sure what you'll hear out there. If you catch a broadcast that's unfamiliar to you, try checking short-wave.info to find out where it's coming from. For me, it's still kind of a transcendent experience to turn on my radio in Connecticut and hear a broadcast carried on beams of light from as far away as Algeria or China. I hope you can experience this magic too. Good luck, and happy listening!

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Where not otherwise noted, the content of this blog is written by Dominique Cyprès and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.