October 4, 2023

The World of Shortwave Signals

The shortwave spectrum from 3 to 30 MHz holds radio signals from all over the world. Here is a compact overview of the most commonly used signal types.

Introduction to Shortwave Communication

Shortwave or high-frequency (HF) describes the band of electromagnetic frequencies from approximately 3 to 30 MHz. Radio wave propagation in this frequency range has some interesting properties: Shortwave radio waves can be reflected by the ionosphere back to earth. Therefore beyond line-of-sight communication over a long distance is easily possible. This is unlike communications at VHF / UHF or higher frequencies, where typically line-of-sight is required to build up a link, which often limits the range on ground to roughly 100 km.

Shortwave radios can communicate over many 1000 km. The actual coverage is influenced by the state of the ionosphere and changes over daytime, season and solar activity. When propagation conditions are good, often only little equipment is sufficient for long distance communication, e.g. a compact transceiver with some 100 W output power and an antenna.

ionospheric propagation of shortwave signals
Propagation of shortwave signals through reflection at the ionosphere (source: NOAA)

In summary, the main advantages of shortwave or high-frequency (HF) communications are:

  • Long distance coverage
  • Simple equipment sufficient
  • Independent from large-scale infrastructure such as satellites, sea cabels, etc.
  • Hard to block by third parties

Due to these advantages many operators are using shortwave signals for communication including aviation, maritime, military, amateur radio, intelligence services and embassies (diplomatic).

Because of the potentially world-wide coverage, a single radio can receive many different signals from operators all over the world. Various modulation formats are present, of which some are easy to identify (e.g. Morse). However, the large number of different signals present in the spectrum can make signal identification a hard task.

spectral view of the HF band showing many different shortwave signals
300 kHz wide part of the HF band showing various different signal types

The well-known signal database sigidwiki.com lists over 220 different shortwave signal modulations. A similar number of modes is listed in Roland Prösch’s “Technical Handbook for Radio Monitoring”. However, many of the listed signals appear only very rarely in the spectrum or are outdated and no longer used.

From my experience roughly 100 shortwave signal types are used on a more or less regular basis in the shortwave HF band. Below an overview of these most commonly used signal types are given. It presumably covers > 95 % of the signal types one may find in the spectrum.

Overview of Shortwave Signals

General Signal Types

ModeModulationBandwidthWaterfall Image
Morse Codeon-off-keyingdepends on keying speed
USB Voice single-sideband (upper)2500 Hz
LSB Voicesingle-sideband (upper)2500 Hz
AM Broadcastingamplitude modulationoften 10, 15, 20 kHz
DRM BroadcastingOFDM5, 10 or 20 kHz

Amateur Radio Modes

Ham radio amateurs heavily use the shortwave spectrum for extensive communication in the decicated amateur bands. The most frequent modes are SSB voice, Morse code and digimodes like RTTY, PSK or FT4 & 8. In addition, a wide variety of other digital modes are frequently used.

ModeModulationBandwidthWaterfall Image
Ardopadaptive mode with single and multi-tone FSK and PSK200-2000 Hz
Contestia 8/2508-MFSK, 31 Bd250 Hz
Contestia 16/25016-MFSK, 16 Bd250 Hz
Contestia 32/100032-MFSK, 31 Bd1000 Hz
Dominoex 1118-MFSK, 11 Bd260 Hz
FreeDVdifferent OFDM modes1000 / 1600 Hz
FT-44-GFSK, 23 Bd, 4.5 s burst duration80 Hz
FT-88-MFSK, 6 Bd, 12.6 s burst duration50 Hz
Hell (Feld)on-off-keying75 Hz
JT-99-MFSK, 1.7 Bd16 Hz
JT-65 (A)65-MFSK, 2.7 Bd180 Hz
MFSK-1616-MFSK, 16 Bd300 Hz
Olivia 8/2508-MFSK, 31 Bd250 Hz
Olivia 8/5008-MFSK, 62 Bd500 Hz
Olivia 16/50016-MFSK, 31 Bd500 Hz
Olivia 32/100032-MFSK, 31 Bd1000 Hz
Packet 3002-FSK, 300 Bd / 200 Hz shift300 Hz
Pactor 12-FSK, 100 or 200 Bd / 200 Hz shift300 Hz
Pactor 22 tone PSK450 Hz
Pactor 3adaptive: 2, 6, 14, 16 or 18 tone PSK400 – 2200 Hz
Pactor 4adaptive: chirp, BPSK to QAM2400 Hz
PSK-31BPSK, 31 Bd80 Hz
PSK-63BPSK, 63 Bd160 Hz
PSK-125BPSK, 125 Bd320 Hz
Robust PacketOFDM with PSK500 Hz
ROS 8144-MFSK, 8 Bd2300 Hz
ROS 16144-MFSK, 8 Bd2300 Hz
RTTY 45 / 1702-FSK, 45 Bd, 170 Hz shift200 Hz
RTTY 75 / 1702-FSK, 75 Bd, 170 Hz shift200 Hz
Sitor-A2-FSK, 100 Bd, 170 Hz shift250 Hz
SSTVvarious analog types such as Martin, Scottie or Robot2400 Hz
Thor 1118-MFSK, 11 Bd260 Hz
Throb 49-MFSK, 4 Bd150 Hz
Vara HFadaptive: single and multi-tone MFSK, BPSK, QPSK, 8-PSK, 16-QAM, 32-QAM500 / 2300 Hz
Windrmdifferent OFDM modes2300 / 2500 Hz

Aviation and Maritime

Aviation and martime operators use shortwave signals to communicate weather, emergency and operational information over long distances where line-of-sight communication is not possible.

ModePurposeModulationBandwidthWaterfall Image
GMDSS-DSCMaritime Comms2-FSK, 100 Bd / 170 Hz shift250 Hz
HFDLAircraft Position DataPSK bursts2400 Hz
ICAO SelcalAircraft selcall before voice linktwo two-tone codes1500 Hz
RTTY 50 / 450Weather data2-FSK, 50 Bd / 450 Hz shift500 Hz
WefaxWeather maps and dataanalog HF fax2400 Hz
Sitor-B / NavtexMaritime Information2-FSK, 100 Bd / 170 Hz shift350 Hz

Governmental, Diplomatic & Military

Signals from governmental, diplomatic and military users are often proprietary and not well documented in public. Therefore the real users, purposes and even signal names may remain obscure and information is typically not confirmed. The shortwave spectrum is heavily used by NATO, Russia and Japan, but also by many other countries. Especially Russia uses shortwave frequencies extensively for diplomatic and military purposes and transmits – from a technical point of view – some of the most interesting modulation formats.

ModeOriginModulationBandwidthWaterfall Image
CIS-12Russia12 tone OFDM3000 Hz
CIS-36 (Serdolik)Russia36-MFSK, 40 Bd1400 Hz
CIS-36-50Russia2-FSK, 50 Bd, shift 200 or 250 Hz300 Hz
CIS-45Russia45 tone OFDM3000 Hz
CIS-60Russia60 tone OFDM3000 Hz
CIS-112Russia112 tone OFDM3000 Hz
CIS-75/250Russia2-FSK, 75 Bd, 250 Hz shift400 Hz
CIS-200/1000Russia2-FSK, 200 Bd, 1000 Hz shift1000 Hz
CIS-3000Russia8-PSK, 3000 Bd3400 Hz
CIS-OFDM 121Russia121 tone OFDM3000 Hz
CIS-1200/800RussiaFSK, 1200 Bd, 800 shift1500 Hz
MazielkaRussia6-MFSK250 Hz
XPBRussia16-MFSK, 66 Bd2800 Hz
XPA2Russia14-MFSK, 7.5 Bd250 Hz
Chinese 4+4China8 QPSK carriers2500 Hz
Chinese Data Link 30China30 carrier BPSK2500 Hz
DPRK-FSKNorth Korea2-FSK, 600 or 1200 Bd / 600 shift1600 Hz
Israel Navy HybridIsraelMS-118-110A serial with characteristic preamble2400 Hz
Japan 8 ChannelJapan8 tone 2-FSK2400 Hz
Japan 32 ChannelJapan30 BPSK carrier + sync/pilot2400 Hz
Japanese Slot MachineJapanQPSK, 1500 Bd2000 Hz
Mahrs-ALEGermany8-PSK, 2400 Bd2600 Hz
MHF-50South Africa33-MFSK with 2-FSK preambles2200 Hz
Pol Intel 4 FSKPoland4-FSK, 100 Bd1500 Hz
ALE-2G (MS-188-141A)various8-MFSK, 125 Bd2400 Hz
ALE-3Gvarious8-PSK, 2400 Bd2800 Hz
FSK 50/850NATO2-FSK, 50 Bd / 850 shift1000 Hz
GM-2100variousvery similar to Link 11 Slew2500 Hz
Link 11 Slewvarious8-PSK, 2400 Bd2500 Hz
Link 11 Clewvarious16 tone QPSK2500 Hz
Link 22NATOQPSK / 8-PSK, 2400 Bd2800 Hz
MS-118-110A serialNATO8-PSK, 2400 Bd2500 Hz
Stanag 4197NATO39 tone OFDM QPSK2300 Hz
Stanag 4285NATO8-PSK, 2400 Bd2800 Hz
Stanag 4481 FSKNATO2-FSK, 75 Bd / 850 Hz shift1000 Hz
Stanag 4539NATOQPSK up to 64-QAM, 2400 Bd2800 Hz
Codanvarious16 tone QPSK1800 hz
Panther-HvariousFrequency Hopper, QPSK, 2400 Bd2500 Hz
Thales SkyhoppervariousFrequency Hopper, 8-MFSK & QPSK 2000 Hz
OTHR / RadarsRussia, China, Natofast repetitive sweepstypically 3- 50 kHz

Time Signals

Time signal stations convey information on the current time.

StationOriginWaterfall Image
BPMChina, Chinese Academy of Sciences
CHUCanada, Institute for National Measurement Standards
UTCUSA, National Institute of Standards and Technology, Stations: WWV & WWVH

Channel Markers

In the lower parts of the shortwave spectrum various stations can be found, that transmit repeating odd sounds. These shortwave signals do not convey any information, but their purpose is to keep the frequency occupied. Many of these signals have been given figurative names describing the sounds they make. Probably the most well-known signal is the “The Buzzer”, which has received some wider public attention. The channel markers are mostly attributed to Russian operators.

ModeFrequencyWaterfall Image
The Alarm4770 kHz
The Airhorn4930 kHz
The Buzzer4625 kHz
The Goose3243 (night) or 4310 kHz (day)
The Pip3756 (night) or 5448 kHz (day)
The Squeaky Wheeldifferent
T-Marker4182 kHz

Further Reading

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