See https://sites.google.com/site/sub9khz/ for up-to-date information on all aspects of sub-9kHz amateur radio.
Maintained until sub-9kHz website created earlier this year. See above site for most recent information.
A very busy 8-9kHz amateur band in March 2011. This screen shot from DK7FC's grabber shows just how busy this band is now becoming and how much progress has been made in the last 12 months. Visible in the grab are DJ8WX, OE3GHB, DK7FC (local testing) and DF6NM.
Click on the image to see all the traces more clearly.
One of my interests is ELF/VLF with my final year university thesis (now nearly 40 years ago!) being on whistlers and related atmospheric phenomena below 9kHz. This interest started in the mid 1960s when experimenting with communications through the ground using frequencies below 9kHz (see my article in RadCom April 1975). This form of communications is sometimes known as Earth Mode or Earth Current. Currents are injected into the earth or rock via 2 electrodes and detected some distance away as a potential difference between 2 further electrodes driven into the earth or rock. Filters are usually needed to reject mains hum as this can be the limiting range factor in urban areas. In open areas well away from cables and pipes, signals attenuate very rapidly with distance (-18dB every time distance is doubled) so most people are unable to cover more than 0.5-1km with low power amplifiers and electrode spacings of 10-20m. With 500W - 1kW audio amplifiers ranges up to 10km have been achieved at around 6-9kHz. Greater earth mode DX is possible using modern weak signal techniques. Articles on this form of communications have appeared in radio magazines many times over the years. My own interest was first awakened by a couple in Practical Wireless in 1964 and 1965. In recent years earth current or earth mode communications has proved effective for cave communications and rescue work with depths of some 800m being covered into cave systems. Earth mode was popular in the USA during WW2 (see QSTs from 1942) when conventional ham radio was not allowed.
My own best DX with VLF earth mode using 4W QRSS3 at 838Hz and using a 20m earth electrode TX "loop in the ground" is currently 5.6km receiving the signal on an 80cm loop, preamp and PC running Spectran. See Earth Mode and Induction at VLF for regular updates.
With modern detection techniques, amateurs can radiate and detect signals over a reasonable distance in the 8-9kHz band. Some have speculated that ranges up to a few hundred kilometres might be possible, something considered impossible even very recently, and recent tests have confirmed remarkable distances are indeed possible, but clearly producing any serious amount of radiated power requires a well engineered transmitter and antenna system.
VLF WORK IN GERMANY
DK7FC did further tests on 8.97kHz on 15.3.10 using a 100m long kite mounted vertical Marconi antenna with a huge loading coil and 250W RF input (ERP 1.7mW). He was received in Germany (180kms) and in the north of the UK (857kms) although no identifiable callsign was received in the first test. A week later his next test transmission was copied by several stations across western Europe with the best DX reception being from 902kms away. Reception was only possible with very narrowband software based filtering using very slow QRSS but at least we now know that amateur signals on the 8.97kHz (33kms!) "Dreamers Band" can travel a very long way indeed. For more details of Stefan's equipment see http://www.qrz.com/db/DK7FC. For some plots of signals received in various DX locations see http://abelian.org/vlf/ss100321/ .
This is a screenshot of DK7FC's signal as seen at DF8ZR/Bernd's QTH in QRSS30 mode (16kms) during the first test on March 15th.
On the right shows Stefan's enormous base loading coil and the picture below the kite used to keep the vertical wire aloft.
UPDATE: On Aug 1st 2010 DK7FC experimented with a 200m kite antenna on 6.47kHz. Although QRN was high, several stations managed to copy his signals:
DF8ZR (QRSS-60 mode, 16km)
DK7FC (grabber in DFCW-240 mode, 40km)
DD7PC (congrats, first time! 53km)
DF6NM (insecure, fragments, 180km)
DL4YHF(impressive S/N!, 264km)
Paul Nicolson (859km) Best DX reception of the lowest signal, transmitted by an amateur so far (far field) - see image below and more details at http://abelian.org/vlf/ss100801/ and http://abelian.org/vlf/ss100801/todmorden4.png
HISTORY AND MILITARY USE
Engineers had long known that telegraph signals could travel a few hundred yards through the ground, but little use had been made of this form of wireless communication. In 1914 the enterprising engineer Gustave-Auguste Ferrié, who headed the French Radiotélégraphie Militaire before and during World War I, recognized two things: the newly available electron tube could significantly extend the range of this technique; and it might then be of enormous value in the fighting on the Western Front. Thus was born ground telegraphy or Earth-currents signaling.
Ferrié made improvements in the signal generator and in the receiver -- notably by the use of a triode amplifier -- and achieved a usual range of several kilometers. The transmitter was essentially a buzzer (an electromechanical device that interrupts the circuit at a very high rate) powered by a battery. The receiver was an amplifier, employing a triode electron tube. Earth connections were usually made by driving steel pins into the ground; often a short length of insulated wire was laid along the ground and anchored at each end by a spike.
These devices began to be used in large numbers in 1916, and by the end of the war the French had produced almost 10,000 of them for use by the Allies. The Germans also deployed a system of ground telegraphy; it was mainly the work of a young mathematician, Richard Courant, who became famous after the war for his work on quantum mechanics. The famous physicist Arnold Sommerfeld also contributed to the German development of ground telegraphy. In the United States, Lee de Forest patented a system of signaling by Earth currents.
Users of ground telegraphy discovered that their receivers frequently could pick up telegraph and telephone signals from lines buried nearby. They were thus used to tap enemy lines and also to receive one's own telegraph or telephone signals when a line had been severed. These receivers came to play a large role in eavesdropping. Its portability and its freedom from electrical lines made ground telegraphy an important means of communication during the Great War. It was a technique, however, that scarcely outlived the war. Even before war's end it began to be displaced by another wireless communication technique. This, of course, was radio, the technology to which Ferrié devoted most of his efforts.
The military used "earth mode" in the 1960s for secure communications between buried nuclear installations. More recently they have used extensive ELF arrays to communicate with submarines but this is actually using radiated signals rather than conduction currents.
Radio amateurs have tried "earth mode" at 73 and 136kHz with some success, as have cavers. With modern weak signal detection methods, e.g. waterfall displays on PCs, signals far too weak to hear can be "seen" as traces on the screen.
Another mode useful for Earth Mode is PSK31 which is a very narrow band data mode. Most PSK31 stations use PC sound cards to do the digital signal processing making this one of the easiest digital modes to use. The data rate allows up to 50wpm in bandwidths of about 31Hz (hence the name).
Foremost in sub-9kHz through the ground communications was John, G0AKN, who sadly is a silent key. A paper on his work is available as an attachment on this page (see bottom of page).
9kHz RECEIVING EQUIPMENTJim M0BMU has produced a neat loop/preamp design suitable for listening on the 9kHz band. His design has sufficiently low noise to provide a state-of-the-art receiver for this band when used with the many excellent software based VLF receivers such as Spectran and Spectrum Lab. This (rev B) circuit was posted on the LF Reflector on March 23rd 2010.
Jim M0BMU has since modified his circuit so it functions as a complete 8.97kHz direct conversion receiver. He posted a description and a schematic on the LF-reflector (see below).
Other possible receiver front ends and antennas include variations of the voltage probe antennas using high impedance input stages and short whip antennas. Examples include the BBB-4 from S.McGreevy, possibly with some changes to increase the response around 9kHz. See http://www.auroralchorus.com/bbb4rx3.htm
MODERN DX RECORDSThe table below gives a summary, as best I know them, of the distances amateurs have so far covered below 9kHz. If you have more data, please let me have this so I can update the chart. In the coming months with several German and UK stations experimenting, the DX record is likely to be smashed. It is doing the impossible that has always driven radio amateurs. At VLF everything is "up for grabs" as far as records go.
19th century DX
William Preece spanned 5kms in 1892 with induction communications across the Bristol Channel (see J.J.Fahie's book below) albeit with huge induction coils at each end. This pre-dates Marconi's true radio tests by several years.
Laurence K6VLF is also active and says VLF radio enthusiasts are not limited to listening, some are sending out signals using home built equipment. His samples here are from an experiment to find out if VLF can be used for amateur communications. It was done in the Sierra Nevada Mountains in California. The transmitter sent out an intermittent CW (continuous wave) signal at one mile from the receiver, then again at two miles. The frequency used was 1.8 Kilohertz, well within the VLF range.
More about this experiment can found here: VLF Transmitting Experiment #2
Earth-mode (through the ground conduction) and induction communication (mutually coupled coils) have an inverse cubed attenuation with distance. Think about this: I reached 0.3km with earth mode for an audible CW signal with around 4W RF and a simple 5m base receiver and 10m base transmitter. To double this range, all other things being equal, means increasing power by 18dB to 250W. To double range again means raising the power to kWs. Conversely, improving the detectable sensitivity threshold by 18dB (increasing the electrode spacings at each end, using WSPR or QRSS, etc) reduces in power needed for a given range dramatically: my 4W signal could be detected at 0.6kms or a just a 63mW signal could be detected by earth mode at 0.3km. In reality, especially in urban areas, water pipes and cabling may help "propagation" and achievable ranges may be greater. DX is relative, especially with conduction and induction at VLF.
very beautiful recording of chorus was made recently by Paul Nicholson: it sounds like birds in a forest at dawn, but the sounds are all generated by auroral electromagnetic activity.
SOME READING MATERIAL (from the late G0AKN's article)
Bishop, Leon 'The Wireless Operators Pocketbook of Information and Diagrams' ,
Bubier Publishing Co. 1911.
Fahie, J.J 'A History of Wireless Telegraphy 1838 - 1899'. William Blackwood and Sons. 1901. pp 1-5, 130 - 176.
Rabson (& Gibson and Gill) 'Cave Radio and Electronics Group' Journal 33, Sept. 1998
Hardy, James K. 'Electronics Communications Technology'. Prentice / Hall International Inc. 1986 p. 331
IEEE Transactions on Communication Vol. Com-22, No.4, April 1974
'Early Development of the Project Sanguine Radiating System' B.E.Keiser
Meulstee, Louis 'Earth Current Signalling. The History of the Power Buzzer'. Journal of
the Royal Signals, Spring 1988.
Stanley, Rupert 'Textbook on Wireless Telegraphy', Vol. 2. Wireless Press. 1919.
Kendall, G.P 'Earth Wireless, Some Notes on the Power Buzzer'. Vol. IX, Wireless World. 1921. pp 409 - 411.
QST Magazine. 'Experimenter's Section' April to October inclusive. 1942.
Bradley, C.R. 'Communications Through The Ground' Practical Wireless May 1964.
'Improved Communications Through The Ground' Practical Wireless February 1965.
Lapthorn, R. 'Radio communications at frequencies below 10kHz'
Radio Communication April 1975
Pickworth, G. 'Earth Current Signalling' Electronics Today International
February,March, April, June 1990.
VLF Earth Loop Antennas Electronics Today International
(Parts 1 and 2) April, May 1991
LF Experimenter's Source Book (2nd. Edition)
from Radio Society of Great Britain
Herts EN6 3JE