Shortwave Listening

Years ago, I was on a long drive home late at night with a friend, and we had the funny idea of checking out what kind of radio stations were still available on AM. I can't quite remember what lead to that experiment — maybe our phones were running out of battery so we didn't want to stream music anymore, or maybe cell reception was too patchy in the area, and then maybe the FM broadcasts that were available at that time and place were not to our liking, or maybe we were just bored, I don't know. The important thing is, we realized we could pick up more AM broadcasts than we had anticipated, that some of them seemed to come from quite afar, and that the same frequencies could be picked up over hundreds of kilometers. The experiment stopped when we finally got back to our city in the middle of the night.

The next morning, I tried to pick up the same broadcasts again, and... nothing. They were not there.

I suspected these radio broadcasts not being there anymore had something to do with it now being day instead of night. Some research on the Internet confirmed my suspicions: transmissions on the frequencies used by what we commonly refer to as AM radio (more on that later) can propagate farther away when the sun is not visible.

It was at that moment that I remembered a science teacher back in highschool saying something about a kind of radio that could pick up things from overseas during nighttime. I looked into it to find out what I was thinking of exactly: it was shortwave radio. So, out of scientific curiosity, I ordered a cheap shortwave radio receiver from Amazon. To my excitement, I was indeed able to tune in to international broadcasts using that radio. And just like that, I had found a new hobby.

Many years later, I decided to add this section to my website in order to share some of the knowledge I had gathered since then on shortwave listening and on radio in general.

Table of Contents

A Brief Explanation of Shortwave Listening

Shortwave radio refers to a type of terrestrial (as-in not satellite-based) radio transmissions that can travel all around the world under the right conditions. For instance, by using a radio receiver that supports shortwave, you can be in North America and listen to a radio broadcast being transmitted from Europe. Now, that might not sound very impressive in the Internet era, but keep in mind that shortwave radio is an ancient technology that does NOT use the Internet. Being able to pick up these broadcasts from overseas means some radio waves have managed to cross an entire ocean, without the use of satellites or submarine cables. Knowing that these waves that came from so far just happen to be present around us is impressive to me.

Radio waves are not usually capable of crossing oceans, as the earth being round means there is no line-of-sight between continents (in other words, the earth itself is blocking the way). However, shortwave broadcasts overcome this problem by bouncing off a part of the atmosphere called the ionosphere, than bouncing off the earth, than off the ionosphere again, and so on. In radio terminology, this is called "skipping". The results vary between day and night because the presence of the sun modifies the composition of the ionosphere. Other atmospheric conditions and solar weather can impact shortwave propagation as well.

Shortwave listening (SWL) is the hobby (which really exists and is also enjoyed by other people, I swear!) of picking up such transmissions out of technological and scientific curiosity. It is part of a broader radio hobby called DXing, where DX refers to "distance", as in receiving and identifying radio signals from far away, or in some cases establishing two-way radio communications over long distances.

Shortwave transmissions cannot be picked up on your typical AM/FM radio like the one in your car. You need a radio receiver that specifically supports shortwave in order to listen to shortwave radio.

A Much Longer Explanation of Shortwave Listening

Basic Radio Notions

Radio Spectrum

First of all, what is a radio wave? It is a type of electromagnetic radiation. OK, but what does that mean? Well, I do not have the physics background to explain exactly what electromagnetic radiation is, but what I do know is that light is also a type of electromagnetic radiation. Actually, we can think of radio waves as a kind of light that is invisible to the human eye. It being invisible can be explained by it having a different frequency than visible light. To understand the concept of frequency, we can think of another kind of waves: sea waves. If you were on the shore, staring at the sea, and were to count the number of waves passing a specific point every minute, you would be calculating the frequency of these waves. The same way, electromagnetic waves have a frequency, which is usually much higher than that of sea waves, and is therefore calculated per second instead of per minute.

The unit used to express the frequency of an electromagnetic radiation is Hertz (Hz), which means "events per second". Other units based on Hertz are used to express different multiples of a Hertz:

  • 1 kilohertz (kHz) is 1000 Hz
  • 1 megahertz (MHz) is 1,000,000 Hz
  • 1 gigahertz (GHz) is 1,000,000,000 Hz
  • 1 terahertz (THz) is 1,000,000,000,000 Hz

Electromagnetic waves with different frequencies have different properties, such as their visibility and the way they propagate. All the possible frequencies of electromagnetic radiation together constitute the electromagnetic spectrum, which is divided into categories based on their frequency ranges:

  • Radio Waves (up to 300 GHz)
  • Infrared Waves (300 GHz - 400 THz)
  • Visible Light (400 THz - 750 THz)
  • And then all the dangerous stuff (Ultraviolet, X-Rays and Gamma Rays) with even higher frequencies.

(Depending of who you are talking to, the radio range can also overlap with the lower end of the infrared range, up to 3 THz, but this is not of concern for us.)

And then, frequencies in the radio range are further divided into multiple categories, called bands, that constitute the radio spectrum:

Frequency Band Frequency Range
Extremely Low Frequency (ELF) 3 Hz — 30 Hz
Super Low Frequency (SLF) 30 Hz — 300 Hz
Ultra Low Frequency (ULF) 300 Hz — 3 kHz
Very Low Frequency (VLF) 3 kHz — 30 kHz
Low Frequency (LF) 30 kHz — 300 kHz
Medium Frequency (MF) 300 kHz — 3 MHz
High Frequency (HF) 3 MHz — 30 MHz
Very High Frequency (VHF) 30 MHz — 300 MHz
Ultra High Frequency (UHF) 300 MHz — 3 GHz
Super High Frequency (SHF) 3 GHz — 30 GHz
Extremely High Frequency (EHF) 30 GHz — 300 GHz

The bands that appear in bold are the most commonly used in consumer tech.

As for the electromagnetic spectrum in general, transmissions in different radio bands behave differently, including in the way they propagate. Therefore, different bands have different uses. For instance, Wi-Fi uses frequencies both in the UHF and SHF bands, while regular FM radio broadcasts are in the VHF band.

Another way to express frequency bands is in terms of wavelengths. To make a parallel with sea waves again, imagine that you could freeze the ocean and then measure the length between the peaks of two waves using a measuring tape. That would be the wavelength. Wavelengths are expressed in meters. The higher the frequency, the shorter the wavelength. For example, wavelengths in the MF band range from approximately 100 meters (at 3 MHz) to 1 km (at 300 kHz), while SHF ranges from approximately 1 cm (at 30 GHz) to 10 cm (at 3 GHz).

The Shortwave (SW) band corresponds roughly to the High Frequency (HF) band (3 MHz to 30 MHz), with some definitions also including the top of the Medium Frequency (MF) band and the bottom of the Very High Frequency (VHF) band. Wavelengths in the HF band range approximately from 10 meters (at 30 MHz) to 100 meters (at 3 MHz). It can seem today that the name "shortwave" makes no sense when the wavelengths of VHF, UHF and SHF are much smaller, but keep in mind that the name comes from a time where these frequency bands were not much used. In fact, frequencies in the UHF band and above are also known as Microwave — rings a bell?

Transmission Modes

We have established that the radio spectrum is divided in multiple bands, or wavelengths, one of which is High Frequency (HF) and is roughly synonymous with shortwave (SW), with frequencies ranging from 3 MHz to 30 MHz. Now, when a radio transmitter produces radio waves in a given frequency, it needs a way to modify (or modulate) some property of these waves in a way that conveys information. The most obvious property that can be modulated is the frequency itself. By increasing and decreasing the frequency of a radio signal over time, it can be made to carry information. This method is called Frequency Modulation (FM). Another property that can be modulated is the amplitude of waves. If we were looking at sea waves again, the amplitude would be the height of the waves. Amplitude Modulation (AM) consists of modifying the amplitude of the signal over time in a way that represents information. In any case, the frequency that we need to actually tune in to pick up a specific radio transmission is called the carrier frequency.

Now, when it comes to technology in general, information is most commonly represented either in an analog way or a digital way. With analog methods, information is represented by varying a physical value in a way that is analogous to the variation of another physical value (the one to represent). Think for instance of a gradated thermometer: the higher the temperature, the longer the line that represents it. As for digital methods, they rely on the binary system (0s and 1s) to encode information. It is these 0s and 1s that are represented by variating a physical value (ex: higher voltage for 1s and lower voltage for 0s). The meaning of 0s and 1s in a digital system is entirely made-up by humans, while an analog representation involves a value that varies in a way that actually looks like what it is representing physically.

Just like with other technology, a radio signal is either analog or digital. Analog radio is mostly used to transmit sound. Interestingly, sound is also made of waves — not electromagnetic waves, but mechanical waves, most often transmitted through air vibrations. In the analog world, FM radio varies the frequency of the radio signal in a way that is consistent with the variations of the sound waves to represent, while AM radio does the same thing by varying the amplitude of the radio signal instead of its frequency.

AM has a variant called Single Sideband (SSB), which I have yet to fully understand. When using AM to transmit audio, three radio signals are actually produced (the "Why" is the part I don't fully understand): the carrier signal, then a second signal at a slightly lower frequency, called the Lower Sideband (LSB), and then a third signal at a slightly higher frequency, called the Upper Sideband (USB). The actual information to transmit is represented in the two sidebands, and both convey the same information. Therefore, it is possible to suppress the carrier signal and one of the sidebands and still transmit the same audio. The advantage is that this kind of transmission requires less power, and also uses less bandwidth (that is, it uses a smaller range of frequencies to transmit the same information since only one sideband is kept — therefore, less "room" is occupied on the radio spectrum). The best explanation I have found for this so far is here. Even without fully understanding, we can view SSB as another transmission mode that is a subset of AM, and itself has two versions: LSB (when only the lower sideband is kept) and USB (when only the upper sideband is kept).

(Sidenote here: one of the things I like about radio is that you don't need to understand all the theory behind it in order to enjoy it as a hobby, as you can also get a practical understanding of things just by experimenting.)

Another analog transmission mode that exists is Continuous Wave (CW). With CW, the signal being transmitted is always the same, but is being successively turned on and off, which allows to transmit Morse code.

There are therefore mainly 5 modes of transmission in analog radio:

  • AM
  • FM
  • LSB
  • SSB
  • CW

Theoretically, all transmission modes could be used all across the radio spectrum. In practice though, FM transmission is mostly used in the VHF and UHF bands, while AM is mostly used in the LF, MF and HF bands. In the common language, "FM radio" usually refers to VHF radio broadcasts happening in the frequency range of 88 to 108 MHz (if you listen to FM radio, you will notice that the channels are always comprised in that range), while "AM radio" is usually meant as the AM broadcasts available in the Mediumwave band, that is, roughly between 530 kHz and 1700 kHz (the exact limits of the band vary between areas of the world). The Mediumwave band is sometimes referred to as the "AM band", which is a mistake, as AM is also abundantly used on Shortwave. However, if you buy a radio that advertises itself as an AM radio (or, more commonly, an AM/FM radio), it will most likely not support Shortwave.

While FM is not usually used on Shortwave (although exceptions are always possible), all the other aforementioned analog modes (AM, LSB, USB and CW) are commonly used on it. Some digital modes are also used on Shortwave, mostly for transmitting text and images.

Propagation

Now is where it gets really interesting.

Again, the frequency of a radio signal determines the way it behaves. It can for example impact how far it can travel, how well it can penetrate buildings, how much it gets absorbed by the atmosphere or whether it can be reflected by some layer of it, etc. An occurrence of this in everyday life is with dual-band Wi-Fi: the 2.4 GHz band has better range and penetrates obstacles more easily, but is more susceptible to interference for this exact reason, while the 5 GHz band is better at staying constrained between your walls — and your neighbor's.

The division of the radio spectrum in bands (HF, VHF, UFH, etc) was designed so frequencies inside a same band behave in a similar way. Frequencies inside the HF or Shortwave band have the property of being reflected by the ionosphere. This is what allows them to travel great distances, by bouncing multiple times between the ionosphere and the Earth (this is called skipping). Without such property, it would be impossible for terrestrial waves to travel such long distances, as the curvature of the Earth means the Earth itself gets in the way eventually (pretty much only ELF waves, which are extremely hard to produce and are not suitable for audio transmissions, can penetrate the Earth — they are also the only radio waves that penetrate seawater well enough to allow communication with submarines past a certain depth).

Propagation of Shortwave is not constant though. It varies depending on factors such as the specific frequency being used, time of day, season, atmospheric conditions and solar activity. Just because you could pick up a certain frequency at a certain time does not mean you will always be able to hear it (there is also the fact that most Shortwave stations are not transmitting 24/7). This is part of what makes Shortwave interesting, as transmissions can seem to just "appear" on the air randomly.

There is also a clear divide between day and night on Shortwave propagation, because the Sun changes the composition of the ionosphere. As a general rule of thumb, frequencies in the lower part of the Shortwave band (up to about 13 000 KHz — Shortwave frequencies are usually expressed in kHz rather than MHz) are easier to pick up at night, while daytime favours higher frequencies.

The possibility to pick up a certain transmission also depends on the geographical locations of the transmitter and the receiver, and on what area of the world is being targeted by the transmitter. Some international broadcasters actually change their targeted area depending on time of the day.

What Can You Listen To on Shortwave?

There is quite a few things that can be picked up on Shortwave:

  • International Broadcasts: that is the most obvious category. They are the "regular" radio programs targeted at the general public, made of talk and sometimes music. There are fewer international broadcasters than there used to be, but there is still enough on the air to be able to pick something up for sure. International broadcasters include a mix of public broadcasters (ex: the BBC, Radio Romania International, NHK World-Japan, Voice of America, etc), religious broadcasters (mostly from the United States), privately owned broadcasters that rent airtime to anyone (some famous ones are WRMI from Florida and WBCQ from Maine, which end up also broadcasting a lot of religious programs, but also transmit other things) and propaganda radios targeted at specific countries (the most famous probably being Radio Martí, an American government-funded radio targeting Cuba). International broadcasts are usually in AM mode.
  • Utility stations: these are signals that have a practical use. The most basic example of these is time signals: both the United States and Canada operate Shortwave stations that broadcast the current time in UTC. Other examples include weather information for aircrafts and mariners, air traffic control, radio beacons and the like. USB seems to be the most common mode for these transmissions, but AM, LSB and digital modes are also used. I find the propagation of these signals to be quite random and this is what makes them interesting to me.
  • Amateur radio: Amateur radio (also called ham radio) is another radio hobby where licensed radio operators communicate with each other on frequency ranges that are allowed for this usage. Some of these frequency ranges are on the HF band, and communications can therefore be picked up by Shortwave receivers. LSB, USB, CW, AM and some digital modes are used by Ham operators on the HF band.
  • Citizens band (CB): CB is a two-way radio service that operates on HF around 27 MHz. In some countries, it can be used without a license. Since CB is on the HF band, it can be picked up on a Shortwave radio. Transmission modes that can be used on CB vary by country, and sometimes even include FM. CB is supposed to be meant for short-distance communication, but transmissions sometimes travel much farther. Some CB operators actually make a hobby on reaching out to people far away, in ways that are not always legal.
  • Pirate radios: These are broadcasters that operate illegally. Some of them transmit on HF, and can therefore be picked up by Shortwave radios. For obvious reason, there are no official schedules and frequencies for these broadcasts, and they can be hard to pick up since they usually use low transmitting power.
  • Numbers stations and other mysteries: Numbers stations are mysterious broadcasts that appear around the HF band from time to time and transmit series of numbers, either spoken by a synthetic voice or using Morse code or even digital modes. They are believed to be used by government agencies to communicate information to spies in foreign countries. Aside from numbers stations, there are other mysterious shortwave signals out there, such as The Buzzer from Russia, that repeatedly transmits a buzz tone 24 hours a day.

Equipment

In order to listen to shortwave radio, the first thing you need is a radio receiver that supports it. "Regular" radios support only the so-called AM and FM bands (as seen earlier, what is commonly referred to the "AM" band is actually the Mediumwave band), so you need to look for a radio that advertises as also supporting Shortwave or SW. The next thing you need to look at is the actual frequency range supported by the radio, as not all Shortwave radios support the full HF band. Finally, you will also need to look at the modes supported by the radio: the cheapest ones tend to only support AM, which pretty much limits you to international broadcasts. For the full Shortwave experience, look for a wide band Shortwave radio that supports SSB. Ultimately, your choice will depend on what you actually want to be able to do with your radio.

You can also get a Software-defined radio (SDR), which is a radio that you connect to a computer and control using software. These have the advantage of allowing to not only listen to, but also "see" signals. They usually allow to do more than just Shortwave.

You also need an antenna. Most small Shortwave radios come with a telescopic antenna, and you will probably be capable of picking up signals just using this antenna. You will however get more signals if you hook up an external antenna to your device. This can be as simple as a long wire that you clip on your telescopic antenna, or that you plug into your radio's external antenna port if it has one. Be aware though that some small radios do not perform well with too long antennas as they tend to get overloaded. If your radio has an external antenna port, it is a good sign that it should work well with a longer antenna. If you go the route of using a wire as an antenna, you should ideally install it outside, as high as possible, for example by attaching the other end to a tree (make sure to do this safely and to not put your antenna anywhere near a power line).

Personally, there are two radios that I use at this time for Shortwave listening:

  • SDR
    • Here I actually use a setup comprising of multiple devices:
      • The SDR itself, a Nooelec NESDR SMArt v5, based on RTL-SDR.
      • This SDR is actually not great for HF applications, because frequencies under 25 MHz are only supported through a method called "direct sampling" which is less efficient and causes aliasing, that is, signals appear on frequencies other than their actual ones. So I also use an upconverter, which is a device that "shifts" incoming frequencies up so the SDR sees them as frequencies that it actually supports. The upconverter I use is the Ham It Up Nano.
      • Another problem with that SDR is that when used in a city where there are nearby FM stations, it gets overloaded by these strong signals, and therefore, they appear all over the spectrum. A way to remedy to that is to use a FM bandstop filter, which blocks the offending frequencies. I use the Flamingo+ FM.
      • Finally, I also use a LaNA HF, which is a signal amplifier.
      • The parts of my setup are connected in this order: Antenna -> Flamingo+ FM -> LaNa HF -> Ham It Up Nano -> SDR -> Laptop.
      • I use the SDR++ software on Linux.
  • Physical Radio
    • I use the Retekess TR110, a portable radio receiver that does way more than just Shortwave. I bought this for camping, in order to avoid carrying my laptop with all my SDR paraphernalia.

Identifying a Signal

Websites exist that are useful to identify signals based on frequency, time of the day, language, and the nature of what is being heard. The following websites provide databases of most international broadcasts and some utility station broadcasts:

Keep in mind that these websites usually express times in UTC rather than any local timezone.

A website called HF Underground has a forum where people share reports of radio transmissions being picked up, and is particularly useful to identify pirate broadcasts. It also has a wiki that can help identify all kinds of transmissions, including utility stations.

This web page contains a list of CB channels with their corresponding frequencies.

SigIDWiki.com is helpful to identify "weird" signals (not only in the HF band) and even contains screenshots to show what these signals look like on an SDR.

Sending reception reports

Part of the Shortwave listening hobby, and the broader DXing hobby, is to send reception reports to broadcasters of which we have picked up the signal. Broadcasters will sometimes respond by sending a QSL card. A QSL card is a postcard that confirms reception of a particular broadcast, with the date, frequency and location of the receiver.

Good information to include in a reception report is the date and time of reception in UTC, the frequency, details about what was airing on this frequency at the time, the signal quality, and the equipment used. Some stations accept reception reports by email, while others will only reply with a QSL card to reports sent by postal mail and including a self-addressed stamped envelope.

Other Radio Hobbies

Shortwave Listening is not the only radio hobby out there. Others include:

  • Amateur Radio (ham radio): as explained previously, amateur radio is a hobby that consists of using radio technology to communicate with other amateur radio operators, who can be anywhere in the world. This requires a license in most countries.
  • CB: Some people make a hobby of operating CB radios. The requirement for a license and the rules of operation vary by country.
  • TV and FM DX: The hobby of picking up TV and FM signals from far away, which is made possible when some unusual atmospheric conditions are met.
  • Scanning: This is the hobby of using a radio scanner to listen to short-range communications that happen on the VHF and UHF bands, for example, public safety communications. The legality of this varies by country. In Canada, my understanding (and I am not a lawyer, so treat this with caution) is that listening to unencrypted radio communications is legal, but sharing what you have heard with other people is not. Transmitting on the frequencies used by public safety and the like is of course prohibited.
  • Radio Astronomy: I do not know much about this one, but apparently, even a cheap SDR can be used as a radio telescope.
  • Satellite reception: The hobby of picking up and decoding signals from satellites, such as images from weather satellites.

Useful Resources