Today we will cover the low end of the radio spectrum, namely the part below the longwave band on an ordinary radio receiver. This is the frequency range from 0 to 150kHz (corresponding to wavelengths of 2km and up).
Long waves tend to follow the curvature of the earth, hence long distance communication is possible in this part of the spectrum via ground wave propagation. As it does not depend on layers in the ionosphere, which come and go, propagation is very stable 24 hours a day and 365.25 days per year. At the lower end of the spectrum (below 30kHz) worldwide communication is possible, but it requires huge transmitter power and huge antennas and it provides little bandwidth to convey information.
As sea water is an electrical conductor, electromagnetic waves do not penetrate into sea water very well. But the lower the frequency, the deeper the waves can reach. At least some radio waves can reach submerged submarines.
The table below shows the major subdivision of the radio spectrum into bands. The names ELF, SLF and ULF are not universally used for the bands shown.
|ELF||Extremely Low Frequency||3-30Hz||10000-100000km||SLF||Super Low Frequency||30-300Hz||1000-10000km||ULF||Ultra Low Frequency||300Hz-3kHz||100-1000km||VLF||Very Low Frequency||3-30kHz||10-100km||LF||Low Frequency||30-300kHz||1-10km||MF||Medium Frequency||300kHz-3MHz||100m-1km||HF||High Frequency||3-30MHz||10-100m||VHF||Very High Frequency||30-300MHz||1-10m||UHF||Ultra High Frequency||300MHz-3GHz||10cm-1m||SHF||Super High Frequency||3-30GHz||1-10cm||EHF||Extremely High Frequency||30-300GHz||1-10mm|
So today we will cover all of the ELF, SLF, ULF and VLF bands, plus a healthy portion of the LF band. In terms of number of bands covered, this is quite a lot, but in terms of total bandwidth it is not. This entire portion of the radio spectrum has less bandwidth than a single FM broadcast transmission.
It is possible to send a constant current through a conductor and then a constant magnetic field arises. Between two points in space with a constant voltage difference, there is a constant electric field. A frequency of 0Hz corresponds to constant voltage and constant current situation (DC). Constant electric and magnetic fields exist, but they are not waves. In principle the frequency of an electric wave can be made arbitrarily low and hence the wavelength can be made arbitrarily long. But what are the lowest frequencies ever used?
In the 1980s the USA built a system to transmit messages to deeply submerged submarines (more than 100m below the surface). This system was called Seafarer, later Sanguine and operated at a frequency of 76Hz (I mean Hz, not kHz). It required insanely large antenna installations and insanely large transmitters. Apparently this system was discontinued in 2004. The Russians have or had a similar system at 82Hz. Even lower frequencies were proposed or even used. The available bandwidth is extremely low, less than 1 bit per minute. But this was enough to tell a particular submarine to get closer to the surface and listen to a slightly higher bandwidth signal.
The SLF, ULF and part of the VLF band correspond to audio frequencies. If these waves were sound instead of electromagnetic waves, you could hear them. There are very few man made transmissions below 10kHz, but there are naturally occurring radio waves, mainly caused by distant thunderstorms. If you build a suitable antenna (e.g. a large magnetic loop) and use a sensitive audio amplifier and a headphone or speaker, you can hear those signals without the need to demodulate them.
At about 10kHz the radio spectrum begins for real. The ITU starts frequency allocation at 9kHz. The main customers of the remaining part of the VLF spectrum (10-30kHz) are submarines again. Submarines have to be much closer to the surface (20m or less) to receive those signals, but much more bandwidth is available, enough for text messages. Transmitters are still power hungry and antennas are still large, but less so than for frequencies below 100Hz.
Let's go back to the very beginning of radio. Before 1903 there existed no vacuum tubes and during the first few years, they were not powerful enough for (large) transmitters. In the early days there were two types of transmitters.
Until the early 20s, long distance communication meant VLF frequencies and powerful machine transmitters. Radio Kootwijk in The Netherlands started with a VLF machine transmitter to exchange telegrams with its former colony (which we now call Indonesia). Sweden had the Grimeton VLF transmitter transmitter (callsign SAQ) to transmit messages to the USA. In those days, only morse telegraphy was used. The Swedish SAQ transmitter is the only remaining working machine transmitter in the world. Occasionally it still sends messages to the world at 17.2kHz (you could almost hear it without demodulation).
The first broadcast pioneers also used machine transmitters to transmit voice and music. Reginald Fessenden did it already in 1906. He inserted a large carbon microphone in the antenna line of a machine transmitter running at 50kHz. Today no voice communication (or broadcasting) can be heard below 150kHz; it is all digital data (and very few morse telegraphy transmissions).
Time stations were among the first radio stations aimed at the general public. They transmitted the time in morse code. Back to modern times. The band between 50 and 80kHz contains several standard time stations. Our part of the world is covered by DCF77, a station located in Frankfurt (am Main), Germany. It transmits at 77.5kHz, one pulse per second, where each pulse has a duration of 0.1 or 0.2 seconds. The end-of-the minute pulse is omitted. The durations of the remaining pulses encode the time and date. Using a very simple homemade receiver and a microcontroller it is easy to build a clock to decode those signals. Ready-made radio synchronized clocks are definitely cheaper.
Since a few years, radio amateurs in Europe have an amateur band in this part of the spectrum (135.7-137.8kHz). The entire band is not big enough for a single SSB voice signal, which would not be allowed anyway. Only very narrow band transmissions (such as morse telegraphy) are allowed. No ready made equipment is available for this band. Getting any of your generated signal radiated by your antenna is a real challenge in this band.
Most uses of this band are for sending information to seagoing vessels. Information is transmitted in digital form in a variety of modes.
Before GPS was invented, several forms of radionavigation existed, in particular for marine use. The most modern non-satellite system is LORAN-C, which operates at 100kHz.
The station DDH47 is the marine weather service station in Germany. It operates at 147.3kHz in old school RTTY (50 baud, 85Hz shift), so it is easy to decode with suitable RTTY decoding software. This is very close to the longwave broadcast band (whose lower edge is officially 148.5kHz). My Sony ICF-SW7600GR can just tune to this signal in LSB mode when tuned to 150kHz.
Most general coverage shortwave receivers only start receiving well above about 100kHz, even though most of them can tune lower, even down to 0Hz. A decent antenna certainly helps. My Lowe HF-225 can certainly pick up some signals in the lower part of the LF band (it starts at 30kHz), it can detect the DCF-77 signal. A few modern receivers (such as the AR7030) really perform well down to 10kHz and below. The Web-SDR at the University of Twente covers the LF band between 65 and 165kHz.
A few old professional receivers can receive (portions of) this spectrum very well. You have to visit the local army surplus store or hamfest to get one. Finally this band offers ample opportunities to experiment with home built receivers. Sound cards of modern PCs can sample at 96kHz or even 192kHz. Therefore they can directly digitize the entire spectrum up to 48kHz (or 96kHz). The signals can then be analyzed with software.
See you next time when we discuss the longwave broadcast band.
I added a band diagram. The Web-SDR at the University of Twente now covers the entire spectrum from 0 to 29.16MHz. It performs fairly well in the VLF and LF regions.