The Sounds of HF
High Frequency, otherwise known as HF, is a spectrum, that roughly is defined to be around 2Mhz to 30Mhz and is used for broadcast, military, amateur, aviation and marine purposes. It is a relatively small amount of spectrum that posesses a very special quality - with the right solar weather conditions, signals within this band can propagate around the Earth by being reflected off the ionosphere. This means that there's no need to use satellites, and stations can be fairly cheap. As propagation conditions change throughout the day and night - the propagation quality and things you hear on HF change. It's like being able to hear transmissions from around the world, all from the comfort of your own home. Sure this doesn't sound that impressive now that we have the Internet and undersea cables and satellite linking every country, but HF is old-school, and it has a very interesting, albiet noisy and fadey quality about it. It's analog. It's real. And now, people are even sending Digital radio over it - aka Digital Radio Mondiale (DRM). It's interesting to me because it's just an extension of radio - just that HF radios can have ranges much much greater than your average VHF or UHF gear.
In order to listen to HF - it is essential to have a fairly large antenna, and a HF communications receiver, although a Shortwave radio is probably a good start. With one of those, you could spend some time chasing broadcast stations, claiming QSL cards (as I had initially done), and or you could focus more on the "unpublished" stuff - things like data and utility transmissions. There are many strange and weird sounding data transmissions around the band, if one is patient enough to listen periodically, scan the airwaves and wait. With some data transmissions, provided the coding can be identified, you can decode the data back into something you can read on your screen - or you can decode it into a picture. Some examples of that will be forthcoming in the future. Here is a small collection of some of the strange sounds you might hear on the air, but with that being said, in Australia, you don't often hear many Numbers stations or jamming signals. This is by no means a complete collection of sounds - some other things I just don't have recordings of yet - that might be for the future.
Contestia
A data mode commonly used by amateurs, related to Olivia, that encodes text as a sequence of tones. Is very robust against noise, but could be slow however depending on the tone-bandwidth-rate combination in use. Tuning accuracy is quite important.
Amateur Radio QSO using Contestia
HF Packet
A data mode commonly used by amateurs that employs FSK to modulate data in AX.25 frames at 300bps. Is most commonly used for APRS.
HF Packet Transmissions
Morse Code
Yes, good old morse code is still in use. It's by far the best mode to use in case of fade and low power operation (QRP) as it can be easily copied through the noise by a well trained operator without any sophisticated equipment whatsoever. Here is a recording of an Amateur calling CQ, another Amateur transmitting very noisy morse (could be a spark gap transmitter). Note that it's not only Amateurs who use Morse - some Marine operators use morse still as the Marine segment of the HF band is fairly small.
Amateur Radio CQ Call using Morse
Amateur Radio QSO with noisy Morse (spark gap maybe)
Amateur Radio QSOs using Morse - two are audible in this recording at the same time
Amateur Radio Beacon VK2RSY Sydney using Morse
Amateur Radio SSB Voice
AM usage in Amateur bands is fairly low due to the lack of spectrum available. SSB is a much more efficient mode which uses half as much spectrum and is better on power usage and so receives favour in these bands and also in marine bands. From time to time you can hear local transmissions and those further away as well.
Part of an Amateur Radio transmission from Tasmania
Part of an Amateur Radio transmission from an Unknown place in Unknown language
Amateur Radio FM Voice
Although SSB voice is most common, in the higher frequency bands such as 6m, we can see FM being used. FM is very clear and quiet, although this is only true for fairly high signal strengths. FM occupies quite a bit more bandwidth and is suitable only on the bands where spectrum isn't crowded.
Part of an Amateur Radio transmission from an Unknown place in Unknown language
Marine SSB Voice
There are marine transmissions on HF - the Bureau of Meterology operates a regular Voice Weather service for mariners.
Part of a BOM Voice Weather Forecast
Marine HF Fax
HF can also be used to transfer images such as Weather Charts and textual information by encoding the white and black dots into audio tones which are then transmitted using Frequency Shift Keying (FSK). There will be a section forthcoming where I can show you some of the images that I have received through radio FAX.
FAX Tuning Tone, Start Signal and Phasing Pips
FAX Scan Lines being transmitted
Analog SSTV
Also commonly used by Amateur Radio operators, this allows them to transmit colour images via their radios by encoding them into tones as well, however the timing and specifics are different to that used by FAX.
Part of an Amateur Analog SSTV Transmission
Automatic Terminal Information Service, Non Directional Beacons and VOLMET
HF is also used for Aviation where there are Morse Code only beacons used for navigation, known as NDB's or Non Directional Beacons. More advanced beacons have a voice transmission, known as an ATIS which permits them to broadcast Terminal Information such as runways in use, visibility, atmospheric pressure, temperature, time and station name. There is also another associated service which is called VOLMET, which is French for Flying Weather. This provides en-route weather for long distance flights.
Richmond Terminal Information Service - notice a lack of Voice initially as it is being updated at the time.
A sample of the Australian VOLMET service
High Frequency Data Link
HFDL is a system which allows aircraft to downlink and uplink data to terrestrial groundstations without the use of costly satellites. This system works over HF and provides 300bps to 1200bps data rates with a TDMA slot style system.
A HFDL Signal which has been corrupted by some other transmission in the vicinity
Navigation
Before GPS, LF and HF was populated by signals which allowed some sort of rough positioning to take place. I'm not sure if the following transmission sample is a navigation sample, but that being said, I can't think of what else it could be.
A possible LF Navigation signal
Timing and Frequency Standard
HF has been used to distribute time and standard frequency for quite a long time. Some clocks are capable of automatically synching to these signals, and science experiments have been using these signals to keep time. Recently, however, GPS has taken over from these time standards stations - Australia used to operate one and it was since shut down early 2000's.
Timing Ticks from WWV, WWVH and BPM altogether
Announcements on the hour from WWV, WWVH and BPM altogether
Broadcast AM
The most common transmissions on HF by far are the Shortwave broadcast radio stations transmitting in AM mode. While inefficient, the broadcasters do put out lots of power and it allows cheaper radios to be used to receive them. This is important in developing and underprivledged countries. Both of these samples represent a very good reception condition. With many weaker stations or poor propagation conditions, you would find the result to be much more noisy. As frequencies are scarce, often multiple broadcasters will timetable to share given frequencies at different times on a rotating schedule to make the best use of the frequencies available to target their audience. Sometimes they will use several relay transmitters as well to make sure the signal is good. But sometimes this, along with good propagation conditions, can result in a garble of stations as evidenced in the third sample recording.
CRI English Header
CRI English Transmission
Shortwave Multipath and Multiple Transmitters
Marine Data - PACTOR, Global Link Network, RTTY
Mariners often cannot afford the steep costs of Satellite Internet Access, and so they instead choose to use their HF transcievers along with a PACTOR modem to access the internet via shore-based gateways. The data has a characteristic sound, depending on the type of transmission. Note that RTTY (aka Radio Tele-type) is often used by radio amateurs as well.
Pactor 3 Link Establishment
Pactor 3 Shore Station sending lots of data to Ship
Global Link Network Station Free Signal (I think ...)
Global Link Network Station Transmission
A GLN and PACTOR station working Side by Side
RTTY Transmission 1
RTTY Transmission 2
Ionospheric Research Sounders and Over the Horizon Radars
Considered as nusiance transmissions by some, these devices cause strange sounds. Most of the time, these research stations move around the HF band to do their work - and so they can step on people who are using those frequencies at that time. Their transmissions are usually brief, but repetitive and periodic. Ionosphere sounders are used to measure the state of the Ionosphere and provide important data which can be used to estimate HF propagation. Over the Horizon radars allow countries to patrol their borders in such a way that they're not limited by the curvature of the Earth and can detect invasion threats early. Below are different samples, all sounding different.
Ionospheric Sounder or OTHR Radar 1
Ionospheric Sounder or OTHR Radar 2
Ionospheric Sounder or OTHR Radar 3
Ionospheric Sounder or OTHR Radar 4
Ionospheric Sounder or OTHR Radar 5
Ionospheric Sounder or OTHR Radar 6
Ionospheric Sounder or OTHR Radar 7
Ionospheric Sounder or OTHR Radar 8
Ionospheric Sounder or OTHR Radar 9
Radio Pirates and Jammers
Since radio transmissions on HF can traverse the globe, some transmissions can enter politically sensitive territories, where some governments do not allow the consumption of foreign media. Other times, people there take it upon themselves to block such transmissions through the use of jammers which try to transmit over the top of someone else's transmission, thus making it difficult to understand the original transmission or making it annoying to listen to. Furthemore, as HF radio has great range, other unlicensed users feel that they can roam the bands and transmit wherever they please. These transmissions are deemed pirate - that is, they are not legal. Some fishing villages in South-East Asia have this problem to a great extent.
Sounds like a Vietnamese Fishery
Other Pirates
Warble Jammer
OFDM and MFSK Modes
As there are more demanding users of the HF spectrum that require higher data rates or more robust transmissions, there are many MFSK or OFDM type transmissions on the air. Almost none of them are decodable, and their modes cannot always be reliably identified. Most of these do not originate from civilians for civilians, but are rather used by utility users directly, or by military users around the world. There are some military specific systems, however, I have no recordings of them. OFDM is also utilized for digital radio mondiale which allows AACplus encoded audio to be transmitted through HF.
DRM OFDM
Packetized OFDM
OFDM Transmission with strong Frequency Selective Fading
OFDM Unknown 1
OFDM Unknown 2
OFDM Unknown 3
Wireless Keyboards, Mice, Remote Control Toys and Interference
27Mhz is often used by basic entry-level wireless Keyboards, Mice and Radio Control toys. As a result, their transmissions can be received over some fairly good distances at time. Also, as the data rate of many digital circuits also result in clock rates within the HF spectrum, their interference and potentially the data that is being transacted can be extracted by a radio.
Wireless Keyboard and Mouse
Unusual Interference
Other Unusual Interference
There is also ALE, XSL and other modes which I have no good examples of at the present moment and hope to include in the future.