Earth mode means communication between a TX and RX primarily through a conducting medium such as soil. rock or water. VLF or ULF signals are injected between two earth rods, or an earth rod and a water pipe, setting up a current flow between them. Current will flow in an infinite number of expanding paths between the two points because the medium has resistance. At the distant RX, a couple of electrodes may be inserted and the (weak) potential difference between these detected. Signals thus propagated attenuate rapidly with distance (inverse cube attenuation rate) so pure earth mode ranges can be quite limited.
Buried pipes and cables may help conduction in some directions so attenuation may be less than expected. An associated induction field is created and this can be detected with a loop antenna close to the ground or sea in which the signal is being propagated. The electrostatic field around a pipe carrying an earth mode signal can be detected with an E-field probe. Where buried pipes and cables are believed to be assisting propagation the mode is called utilities assisted earth mode. In reality, in any urban or suburban location this is the most probable way in which signals through the ground are propagated to any distance.
From recent correspondence with OFCOM in the UK it would appear that they are not interested in licences for earth mode. I interpret this to mean that, as long as no undue interference is caused, people are able to experiment with earth mode communication without any need for special permits. This is my interpretation and not a legal statement of fact.
Unlikea high powered transmission with a large Marconi type vertical, the radiated part of an earth mode signal is vanishingly small and to all intents and purposes non-existent. For this reason earth mode has been investigated by military users to provide highly secure communications between buried bunkers. It is not known if such systems are in use today.
The first earth mode TX with matching transformer
In the last year or so I have been experimenting with earth mode from my QTH in Burwell Cambridgeshire in the UK. Experiments started with 4W from a TDA2002 audio IC at 0.838kHz being fed between an earth rod at the bottom of the garden and the water pipes in the house, which connect to the pipes in the road. Separation is around 20m. Initial reception tests were done with a pair of electrodes (meat skewers!) and a small preamp connected to a pair of headphones. Range achieved was up to around 0.3km. Later, the tests were modified and an 80cm square loop used with the receiver. Results were better and the system was more portable. Best DX by ear using 10wpm CW was around 0.5km.
The RX loop antenna (80cm 30t) is shown right
Following these early tests, the transmitter was modified so that a QRSS3 and QRSS30 (very slow morse) beacon message could be transmitted. At the receiving end the pre-amps were progressively optimised to reduce mains hum and broadcast breakthrough and the outputs coupled to a PC sound card. With the PC running Spectran software the range was extended considerably and signals could be detected at 0.838kHz out to a range of 5.6km across the surrounding countryside. Signals appeared to propagate along buried water pipes, with signals disappearing where there was no evidence of pipes in the road. In some places signals could be detected with the loop at up to 20m away from the road and signals even at more than 5 km were sometimes so strong that 10wpm CW was copied. Distances and signal levels exceeded expectations!
The ultimate range of "utilities assisted" earth mode is not clear, although even with the 4-5W signals used the indications are that at least 10km would be possible with favourable utilities geometries in the ground. Another experiment that needs to be carried out is along a sea coast with one electrode in the sea and the other around 100m separated on land. My expectation is that signals at VLF will propagate a long way along a coast. This needs to be tried.
The Spectran screen showing the 8.76kHz earth mode signal at 5.1km range
In November 2010 I received an NoV to transmit between 8.7-9.1kHz, so I continued the earth mode tests but at a higher frequency of 8.760kHz. The frequency was produced by dividing an HF crystal down using a 4060 divider. Stability was very good and tests at QRSS30 should be possible later. The TX was changed to a TDA2003 with an output of 5W. With QRSS3 tests were made at the same locations as the tests earlier in the year at 0.838kHz and results compared. Signals were received at most of the locations but at reduced strength, with initial indications that signals at limit of range were some 6dB weaker at 8.76kHz than at 0.838kHz.
The image show the 5W transmitter for 8.760kHz based on the TDA2003 AF IC.
Frequency reference is provided by an HF crystal which is divided down in a 4060 divider. The beacon keying and content is provided by a K1EL beacon chip which can be set to beacon in 10wpm CW, QRSS3 and QRSS30 speeds. The large coil shown is a toroid matching transformer that allows loads of between 10 and 160 ohms to be matched. My own earth electrode arrangement looks like around 40-60 ohms at VLF.
The schematic of the present VLF earth mode beacon transmitter is shown below. Click schematic for a full sized view.
During the spring and summer of 2011 tests will be continued. Further tests will include some very long carrier transmissions to see if any signals can be detected very far away from buried pipes. Although the amount of energy radiated will be tiny, it is possible that the network of buried pipes in which the signal is being conducted acts as an inefficient VLF radiator and very very low level signals may be detected way beyond the range of purely earth mode propagation. For these tests, the crystal and divider may have to be replaced with a GPS locked source or similar.
The map shows local test locations out around 8km where earth mode reception was and was not possible
In February 2011 G6ALB, who is 3km west of me, transmitted an earth mode signal using 40W to 20m spaced earth electrodes in the garden using
QRSS3 on 8.76kHz. He was easily copied here in my upstairs indoor shack using just an E-field probe held vertically in the shack (see image right). I believe this is picking up the electrostatic field around pipes in the house. He was also copied in the street outside my QTH using a loop on the ground. Well away from pipes there was no detectable signal, at least in QRSS3 bandwidths. A further test later this year will be with both the G6ALB and G3XBM beacons running when range with both will be tested. More fun for sunny spring days!
A friend holding my E-field probe which was receiving G6ALB's
earth mode signal in my shack 3km away from his QTH.
Samual Morse tried earth mode as a means of getting his telegraphy to work across a river in the early 1840s. First serious use of earth mode goes back to WW1. This interesting piece was found at the IEEE site
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.
Amateur interest started, I believe in the 1920s. There was some interest in the USA in the WW2 years when more conventional radio transmissions were stopped because of wartime restrictions. Earth mode was covered frequently in QST in the 1940s war years. In the mid 1960s Practical Wireless ran a couple of articles about "ground communications" and a lot of interest was created at the time. Articles also appeared in US magazines on the subject in the early 1960s.
(G0AKN) have also tried "earth mode" at 73 and 136kHz with some success, although in reality propagation was more likely radiated. A copy of G0AKN's paper on earth mode communications experiments is attached at the bottom of this page as a link. Cavers also use earth mode to communicate with buried chambers underground. It has also been used in mines for emergency communication.