Earth Mode and Induction VLF testing

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This page is to record my experiments (and those by others) below 9kHz using earth mode and induction communications (most recent entries are shown first). This is a bit like a blog, so you'll have to scroll down to the very bottom to see how this all started.

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Feb 4th 2011

With my 5W QRSS3 beacon on 8.76kHz I went out looking for my signal at the local National Trust car park 1.6km from home. On the 80cm loop, signals were as expected, but on the E-field probe disappointing when compared with the results with G6ALB's signal a few evenings ago. Although I could detect my earth-mode signal using the EFP with it flat on the ground, there was no copy with the probe hand-held in the air. I was going to try the test at my test sites 4.6 and 5.3km from home, but I aborted this. G6ALB was hoping to get his beacon running on 8.755kHz at the same time but problems at his end prevented any transmissions this morning. It would have been good to see two VLF signals on the RX screen!

I'm beginning to think that the E-field probe detects the E-field signal well close to pipes, but maybe the H-field loop works better in some situations. Results are inconclusive at this stage.

Feb 2nd 2011

This evening during further tests looking for G6ALB's signal on 8.76kHz I discovered that best results receiving at my QTH 3km from Andrew were with a small E-field probe rather than a loop or earth electrode pair. With the EFP signals could be detected in my shack with just a 19inch whip! This suggests that the electrostatic component is the dominant one rather than magnetic or simple potential difference measurable between the 2 earth electrodes. A repeat test trying to pick up Andrew's signal with my earth electrode pair (one side connected to copper pipes going to the road) resulted, again, in absolutely zero copy. The E-field probe is the best antenna at my QTH for receiving 8.76kHz earth-mode signals.

The picture shows a friend of mine (Peter Branson, not a radio amateur) holding the E-field probe in the bedroom and detecting G6ALB's QRSS3 signal 3km away which is displayed on the PC screen.

Click picture for a larger, clearer image.

Moving the E-field probe around the room made little or no difference to the S/N of the QRSS3 signal received.  
The schematic of the E-field probe used (the small metal blue box Peter is holding) is shown here. It is powered by a small PP3 9V battery within the unit.

Feb 1st 2011

This evening
G6ALB fired up his 8.760kHz earth-mode TX running QRSS3 and 40W into earth electrodes spaced 25m apart. We started outside his QTH with a colossal signal, moving to the next village (Reach - about 2km) where the signal was still good copy. Finally we parked up outside my QTH in Burwell exactly 3km from Andrew's TX. Signals picked up in the road with my 80cm portable loop into the tuned preamp were still strong - at least 20dB S/N on QRSS3 (see attached).  For some reason I was unable to copy Andrew's signals on my own earth electrode pair: I may have had a fault and need to check why, as I was expecting this to give the best result. Andrew's signal was so strong on the 80cm loop that 12 wpm CW would have been copied.

This is the first time I've received another amateur by earth-mode (conduction). The result suggests much further is possible. Experiments continue!

Feb 1st 2011

Looking and listening for G6ALB 3km away by earth-mode (conduction through the ground)

Following the recent OFCOM statement about earth-mode not needing an NoV, Andrew G6ALB has decided to have a go and is setting up to send me a test transmission from the next village 3km away using 100W into 25m spaced earth electrodes using QRSS and CW.

In preparation, I've just done some measurements of the noise floor, the level of the 11.905kHz Russian Alpha beacon and the 9kHz broadcast intermod line at 9kHz using my 80 square metre vertical wire loop and with the 20m spaced earth electrodes.

The noise floor is higher with the earth electrodes but the level of the Alpha beacon and the 9kHz broadcast intermod line (produced by intermod between MW/LW broadcast stations 9kHz apart) little changed.
These were the levels measured using a small preamp tuned to 8-9kHz into Spectran:

11.905kHz Alpha beacon level            -50dB (earth electrodes)             -50dB (80 sq metre diam. wire loop)
Noise floor around 8.76kHz                 -60 to -70dB (earth electrodes)   -70 to -80dB (80 sq metre diam. wire loop)
9kHz broadcast intermod signal          -38dB (earth electrodes)            -40dB (80 sq metre diam. wire loop)

For earth-mode conduction, I believe the 20m spaced earth electrodes will pick up more signal, so that is what I will be using to look for Andrew's signal.

Jan 18th 2011

NoVs not required in the UK for non-radiating earth-mode systems?

Wanting to check earth mode propagation behaviour at a series of frequencies below 9kHz to see how results compared - earlier tests at 0.838kHz seemed around 10dB better than at 8.76kHz - I asked OFCOM if I needed an NoV extension.

Today I received this letter from OFCOM, clarifying the legality of so called "earth mode" (conduction/induction via the ground) experiments in the UK. Note the sentence, "We would not normally issue NoVs for communications using non-wireless telegraphy apparatus/stations i.e.  where you rely only on conducted emissions and where there are no intentional radiated emissions."One could argue that such non-radiating tests at frequencies above 9kHz would also be legal as long as they don't cause any "Undue Interference to any wireless telegraphy" (words from the WT Act 2006) and one was not intending to radiate.  This sounds a wholly pragmatic answer from OFCOM. Please note this is my interpretation and not a legal position.


You should seek you own legal advice as Ofcom is unable to give legal advice, however, we would issue NoVs, where appropriate, for wireless telegraphy apparatus/stations.

In the case of the 9 kHz band NoV we currently stipulate the frequency band only around 9 kHz as discussed with the Met Office and we do not intend to issue NoVs for lower frequencies in the band. We would not normally issue NoVs for communications using non-wireless telegraphy apparatus/stations i.e.  where you rely only on conducted emissions and where there are no intentional radiated emissions.

I would also draw your attention to other relevant legislation (e.g. Wireless Telegraphy Act 2006) and the Terms of the Amateur Licence and in particular to clause 7(3); “Notwithstanding any other terms of this Licence, the Licensee shall ensure that the Radio Equipment is designed, constructed, maintained and used so that its use does not cause any Undue Interference to any wireless telegraphy”.

I am also copying in Ash Gohil in the Ofcom Licensing Centre who is in the team that Rod worked in. Please address any further queries to him in the first instance.

I hope this helps.

Kind regards,


:: Paul Fonseka
Spectrum Policy Group-Business Radio

Jan 13th 2011 - wet ground but dry and mild

Today being mild and dry (for the first time in almost 6 weeks) I decided to do a field test with my improved 8-9kHz tuned
preamp and loop antenna. A 5W QRSS3 "earth mode" (through the ground) test signal was emitted using my 20m spaced earth electrode pair on 8.760kHz from the Burwell QTH. I ventured out into the countryside armed with the 80cm loop and preamp to see where the
signal could be copied. In addition to my usual test sites at 1.6 and 5.1km I tried other directions and sites today. The map attached shows where signals were received and where no copy was achieved. The signal does seem to travel along the metal water pipes, which must be absent to the east where I assume they must be plastic, at least in places.

Reception is definitely not as good at 8.76kHz as it was at 0.838kHz but this could be because the ground was saturated with rain from the last few days. Ideally I'd like to repeat the tests in dry weather on 8.76kHz and on frequencies around 6, 4, 2 and 1 kHz (avoiding mains harmonics) to see how signal levels compare. Last summer the signal at >5km was so strong on 0.838kHz that 10wpm CW was copyable!

Dec 15th 2010 - 4 deg C and wet

Long duration transmission test

Today I ran my 5W 8.7608kHz beacon into the earth electrode antenna on a quasi-continuous basis with transmissions of 30-60 minutes interspersed with 30-60 minute breaks. The tests lasted from 0900-2200z. Several stations between 25-70km were looking for my signals and Paul Nicholson up in Yorkshire has still to analyse his recorded files. So far, no station has detected any sign of the signal, which was not surprising considering the radiated ERP would have been in the pW level.  I was hoping that a few stations 10-20km  from JO02dg might have been looking as these would have had some chance of copying something. So, an unsuccessful test this time.

Paul Nicholson (Todmorden) advises me to use a GPS locked source so that the TX frequency is known with great precision. Then he, and others, can look for statistically significant weak signals at precisely the times and frequency of transmission. My HF crystal and divider source would not have been sufficiently stable to allow this sort of "digging in the noise" over any great time period.  For QRSS3, the current stability is fine. For the record, these were my transmission times: 0840-1000z, 1029-1229z, 1330-1500z,1531-1600z, 1630-1800z, 1830-1900z,1930-2207z on 15.12.10.

Dec 10th 2010 - wet soil after a week of sub-zero temperatures

Today I did a test at 5.1km to compare reception of my 8.76kHz 5W QRSS3 transmission using (a) an 80cm loop antenna and (b) a small earth electrode antenna (14m spacing) using a variety of preamps.  Although quite solid reception (17dB S/N in 0.18Hz BW) was achieved using the loop laying flat on the ground, there was absolutely nothing detected using the earth electrodes, which was both surprising and disappointing. I used the very same preamp in both cases and also tried the lo-Z input tuned preamp with the earth electrodes that successfully copied DK7FC at 648km last weekend.

Several possibilities for the failure with the earth electrodes:
  • The electrode spacing was far too small
  • The barbecue skewers used as electrodes were too short and not making decent soil contact
  • The soil was too wet, so the pick-up was lower than with dry, low conductivity soil. The test site is in the fens where the soil is a dark peat.
Next step is to repeat the test up on the chalk uplands to the south of the QTH where soil conductivity will be lower and to try a larger earth electrode spacing on receive.

Dec 1st 2010 - field testing in the snow

With a temperature around 0 deg C I did a field trip today TXing on 8.76kHz VLF to test two things: (1) to check the rebuilt 4-5W TX based on the TDA2003 could be copied at moderate range and (b) to try out the new tuned preamp with earth electrode receive antennas and see how these compared with a loop. The usual first test site, a National Trust car park about 1.5km from home was chosen. Today it was totally deserted with more sensible people curled up in front of warm fires. With the loop and with the earth electrode antenna (electrodes about 13m apart across the car park) decent
copy was achieved in QRSS3 using Spectrum Lab this time.  For the earth electrodes I just used barbecue skewers pushed in about 10cm into the very cold ground. Reception was even possible with the two earth electrodes just 2m apart. The first picture shows the signal received with a loop. the second one is with the earth electrode antenna on receive. The ground was covered in a thin layer of wet snow and results when the soil and soil surface are dry should be better on earth mode conduction using earth electrodes at each end. The roads in the Fens were too icy to try reception at a more distant point 5-6km away. Also, I need a much longer baseline earth electrode pair in the car in case this is needed at greater range.

Nov 28th 2010 - sub-zero outside!

Today I completed the design of a new "front-end" for my tests on 8.76kHz "earth mode" when using earth electrodes as the RX antenna. Up to now I've used an 80cm loop/preamp, usually with the loop laying on the ground to get maximum signal pick-up. Now I want to compare results with an electrode pair antenna at the RX. The impedance of these is about 30-100 ohms, so I needed a low input impedance. I decided on a grounded gate FET amplifier with a simple LC tank circuit in the drain tuned to 8.76kHz feeding via an emitter follower into the sound card of the PC. This arrangement has sufficient gain as well as selectivity to reject 50Hz and lower harmonics and reject the stronger VLF and LF stations that could cause intermod and overload. I've also added a small LC lowpass filter on the input to the FET to help with BC band rejection. This arrangement will be field tested at a site 5.1kms from the home QTH as soon as the weather improves: it is currently below freezing day and night!

If my theory is correct, that signals reach this distance with assistence from buried pipes and cables, then an earth electrode pair stretched across a road will form a coupling loop within the ground around whatever is in the road and give good coupling into the receiver. It will be fascinating to see how a 30t 80cm loop and an earth electrode pair compare. I really have no idea what the resul will be. This is what amateur radio at VLF is all about: "try it and see".

Nov 25th 2010

Just received a special build of Spectran from Alberto I2PHD that keeps the same filename for captured grabber images. This means I can now use Spectran with my VLF grabber. This is fired up from time to time when people are doing VLF tests. It will also allow me to look for my own signal at home when I am out transmitting in the Fens locally some kms from home. I find Spectran easier to use than Spectrum Lab, which is a clever program but complicated for someone like me to configure.

I have also rebuilt the 4-5W beacon TX on 8.76kHz and given it a 1 hour continuous soak test. There is a video about this on my YouTube channel (see left).

My next job is to work on a permanent VLF E-field probe outdoors and to make improvements to the receiving system for field use. I need a better preamp/filter that can also be used with earth electrode receiving antennas as well as the loop and E-field whips.

Nov 5th 2010 - very wet

Having blown up my 4W transmitter yesterday I built a smaller QRSS3 beacon TX today that puts out just 320mW into 50 ohms. I connected this to the earth electrode antenna and went out with the loop and E-field probe and PC to see how it performed. The signal was copied weakly, but solidly, at 1.5km away at the local National Trust car park (see picture from Spectran showing the XBM part of may callsign). An attempt at 5.1km was unsuccessful although faint traces of the line at 8.760kHz may have been visible. A picture of the complete 320mW beacon TX is on the left.

Nov 5th 2010 (Bonfire Night!!)

Managed to blow up my 8.76kHz TX last night by putting the supply the wrong way round: took out the 4060, 5-Vreg and several electrolytics.The TDA2002 was also damaged. Rebuilt the keyer and freq gen part OK but now need to source a new audio PA IC or module. Now I have a 1N2007 across the pins! You learn the hard way.

In the meantime I may try even lower power with my trusty 2N3904/3906 1W PA and see if this can be seen at the NT carpark (1.5km) and at Lord's Ground Farm (5.1km). If this is earth mode conduction then reducing the power by 6dB will not make such a huge difference in range. Also, as this PA is efficient and runs cold for ever I could try 1 day on, 1 day off, QRSSSSS and see if more distant people could copy it using very narrow bandwidth reception as per DK7FC's tests.

November 3rd 2010     dry/mild, but soil wet

This morning 0915-1115 GMT I transmitted a QRSS3 beacon signal on 8.760kHz under the terms of my NoV recently received.  I think this may be a first in the UK, legally at least.

TX was 4W from a TDA2002 audio IC matched into earth electrodes 20m apart (see picture right). The beacon message (callsign and QTH locator in QRSS3)  was provided by a K1EL keyer chip and the
frequency reference was an HF crystal divided down by 512 times in a 4060 divider IC. Clear reception was possible 5.1km away from the transmitter location (see attached screen shot from Spectran), detecting the signal with an 80cm loop fed into an E-field probe (Hi-Z input) into Spectran software. Marginal reception was just possible at 5.3km. Best reception was always with the loop flat on the ground suggesting the main mode of propagation is utilities assisted earth mode, as was the case at 838Hz back in the summer.

Comparing results on 8.76kHz with those at 838Hz earlier in the summer in several locations from 1.5km out to 5.4km, my first impressions are that signal levels are at least 6dB weaker on 8.76kHz, but more careful tests will be needed. When I tried to look for any sign of radiated signals by aligning the loop vertically end-on to the TX location, no signals were detected although with QRSS3 and receiving in the bandwidth used this would have been hopeful with 4W from the transmitter into earth electrodes.

In the next couple of days I want to try the same set of tests using the 70m square vertical TX loop used on 500kHz and 136kHz. If my theory is correct and this is utilities assisted earth mode then I would expect results to be far worse with the loop as there will be less strong coupling into the ground.

As I said some days ago, these tests are not in the same class as Stefan's experiments, but they are fun to do and I'm learning all the time.  At some point in the near future I will do some extremely slow QRSS tests and let people know before hand when the transmissions will take place. There is some remote chance that slightly more distant stations may be able to detect the signal. 

UK dreaming has started, if only on a small scale.

October 28th 2010

As requested in my NoV, I have officially asked the Met Office for permission to transmit in the 8.7 - 9.1kHz band next week Tuesday - Friday 0800-1800 GMT. They've confirmed this will be OK. My initial tests will be a repeat of the earth mode tests done at 838Hz earlier in the summer using the same kit (4W, 20m spaced earth electrodes, plus also testing with the 70m sq loop antenna used on 136 and 500kHz. The most likely frequency will be 8.750kHz as I should be able to obtain a stable signal at this frequency by division of an available HF crystal. I will find a means of getting on 8.970kHz later. Modes will be 12wpm CW and QRSS3 initially. These local tests are not in the same league at all as Stefan's and I'm only expecting a few kms at best. However, it is a start and it allows me to test TX and RX equipment and PC packages. Initially today I modified my TX from 838Hz to 8.97kHz using a twin-tee oscillator but the short-term drift was appalling. Using a 4.480kHz crystal divided down in a 4060 IC to 8.750kHz should be very stable.

October 27th 2010

Plans for (near) 8.97kHz operation
At the moment I'm getting kit together for operation on, or close to, 8.97kHz now that my NoV is here. My first steps will be to repeat the tests done in the summer on 838Hz but on the higher (!) frequency. Initially I'll use the 20m spaced earth electrode antenna and 4W and see whether the coverage locally is similar. Another test will be to TX using the 70m square vertical wire loop as used on 136 and 500kHz suitably resonated. With a very stable signal source (an HF crystal divided down to the Dreamer's Band) modes like QRSS100 or slower become a possibility, opening up the chance of reception beyond the 5.6km previously achieved. Whether any signal will be radiated with this set-up is doubtful. As DK7FC has shown, much larger earth electrode antennas and powers around 100W are capable of getting a signal radiated and detectable in the far field.  First 33km band tests in QRSS3 are likely late next week.

October 22nd 2010

My NoV to legally operate on 8.7kHz - 9.1kHz has been received today from OFCOM (see RHS). There are restrictions on operation: the Met Office has to approve before a test transmission and logs must be kept to allow the Met Office to see if interference to their systems was being caused. All being well I hope to start some tests in this band in the next few weeks, probably WSPR and QRSS.

Sept 26th

In a few weeks time I hope to test my TX "in the air loop" (currently being used on 137kHz and 500kHz WSPR) on 838Hz. This should allow me to gauge the range possible purely by induction rather than by utilities assisted earth mode conduction. I've little idea how far I'll achieve with this 80 square metre single turn loop fed with 4W RF, but I would expect this to be less than 1km with QRSS3.

July 17th 2010 (warm and dry)
  DX increased to 5.6km

Today I resumed my VLF earth mode tests using 4W into 20m spaced earth electrodes. The aim of the test today was to determine the absolute maximum range possible by utilities assisted earth mode. Best result was 5.6km along Commissioner's Fen to the west of Burwell, the home QTH. Signals we just copied at this distance using the 80cm, 30 turn, loop and preamp into the PC running Spectran. A test a 6km resulted in no detectable signal. At a point about 5km from the QTH the signal was so strong that I could copy it when some 20m away from the road (presumably with buried pipes) or when holding the loop high in the air above my head. Another possible clue to the mode of propagation was fenland drainage water channels. Signals always seemed to be strongest when close to these. So, conclusions today are:

(1) The propagation is definitely "utilities assisted" conduction apart from the last few metres of induction into the loop. Even so, I am amazed that the signal can be seen so clearly on Spectran when running QRSS3 with such low power (4W) to such a small earth electrode antenna with just 20m spacing.
(2) Even at 2.4kms from the home QTH, the signal induced in the ground via buried pipes was stronger than any detected by pure induction.
(3) There is weak evidence that at the more distant points some assistance may be being given from water channels criss-crossing the wet fenland.
(4) There is no evidence that overhead electricity cables are helping: when orienting the loop for best coupling into these the signal actually decreased.

July 13th 2010
Just back from holiday in France and intend to do some more earth mode tests this week. SM6LKM has a very neat sound card based VLF receiver ideal for listening to sub-9kHz ham signals and SAQ on 17.2kHz. On his website you can download the files and instructions. This will be very useful for audio reception in the field all the way up to 9kHz. The picture here shows it tuned to receive around 8.97kHz.

June 27th 2010

Horst DO1KHS has been doing some earth mode experiments but has had disappointing results so far. He believes this is due to the high conductivity soil in his area. He is planning further tests on July 23-25 with an 800m earth electrode spacing based in JO30ON (Erpeler Ley). Distance to Stefan DK7FC's grabber is 160km and to DF6NM's 300km. He's hoping for some far field reception.

June 26th 2010

was transmitting today (0800-1100z) on 8.97KHz. His signals were visible on his 8.97kHz grabber . Stefan is testing further improvements to his earth electrode antenna. The message is very clearly "73" sent in DFCW 600.

Stefan can improve his ground system still further and can run a lot more power, so we have not yet seen the ultimate performance and distance using this antenna rather than his kite supported Marconi vertical.

Markus DF6NM has been studying the theory involved and this has led to discussion on the LF Reflector discussion group about the ultimate limits of such systems using amateur power levels and antennas of manageable size.

June 24th 2010    DK7FC's Earth Mode 8.97kHz tests in Germany

Stefan DK7FC has been doing some experiments with around 50W into a 300m spaced grounded earth electrode "antenna"  in a remote location in Germany. His experiments were successful with his transmission of 30 minutes clearly received on his E-field probe receiver and grabber located in the city some 5.2km away.

Stefan can improve the electrode arrangement and increase current into the ground as well as run up to 500W.  He is hoping to see the signal on the DF6NM VLF grabber located 174km away. If successful, this would be the first reported amateur radiated VLF DX using earth electrode antennas. Stefan found that multiple earth rods helped reduce losses and increase the current injected into the ground. Because of the remoteness, it is less likely the 5.2km path was "utilities assisted" by buried pipes as has been the case in my own experiments.

June 24th 2010   (warm and dry)   DX increased to 5.25km

This evening I repeated my visit out in the fens with my loop seeing how far I could copy my 4W 838Hz beacon. After getting good QRSS3 copy at the same location 5.1km out, I then tried going further out along the same fenland drove road. Although I managed to copy the signal at 5.25km this evening, no copy was achieved at any greater range despite travelling along several different lanes in many different directions. I still would like to know what it is in the roads that is helping - I assume water pipes, but it could be gas pipes or buried cables. What is clear now is that without these, the signal is absent more than 0.5km from home. This time I tried looking for the signal in far more places about 4-6km to the north of the village, but there was no trace of signal in this direction. Best directions are west (fenland peat) and south (chalk uplands). I've still to test to the east.

Another future test will be to try receiving the signal with a second pair of earth electrodes instead of the loop at some distance from home as one might expect stronger signals. This test will have to wait until the weekend though. If similar to results close to home, the 50Hz noise will be much worse though and this may prevent reception unless very quiet spots can be found.

June 22nd 2010 (warm and dry)
    DX increased to 5.1km

Further tests today to work out how far a signal on 838Hz will propagate through the ground or air and how this happens....

Screen shot of 5.1km reception

First thing I tried was
to go along the same country lane to the south of the village as last evening at a distance of 4.2km (0.6km further than last night). Nothing, zero, ziltch, and disappointment.

So, I then returned to the very same spot as last night and sure enough the QRSS3 signal was clearly visible on the Spectran display. This was with the loop pointing home across the fields. Then the crunch point: I put the loop on the ground flat on the road and the signal went up. So, this was pretty conclusive proof that the signal was being aided by pipework in the road. To check this, I moved 50m into a field and away from the road and tried listening/looking for the QRSS signal with the loop both flat on the ground and facing home - nothing at all. This was conclusive proof that, at this distance at least, conduction through pipes was helping the signal reach this far.

Next test was to go to a different roadside location some 4km west of the QTH and try the same test again. Nothing copied at all, either with the loop vertical or horizontally on the ground. I repeated the same test on the way back towards home about 0.5km west of G6ALB's QTH, but again nothing seen.

Before coming home, slightly disappointed, I ventured up along a fenland road where the soil is really dark peat. To my surprise, the signal was copied again at good strength on the roadside 4.1km west of the QTH with the loop facing the QTH. Again, I repeated the earlier test and placed the loop horizontally on the ground. The signal was SO strong that the Spectran trace turned red and I could clearly hear the QRSS signal by ear! 

A drive up a road called Lord's Ground Drove resulted in the signal still being copied well just beyond Lord's Ground Farm at a distance of 5.1km. At this point I phoned my wife at home and asked her to switch the TX over to 10wpm CW and the signal was recorded with Spectran and clearly audible with the narrow filters in. Using the crystal earpiece instead of the PC I was not able to hear the CW in the 50Hz noise. Listen to this recording at 5.1km from the TX but use narrow filters around 838Hz to hear the 10wpm beacon signal clearly.

Coverage map showing locations tested


  1. At local distances (up to 0.5km) the signal propagates by both conduction through the ground (aided by pipes in the ground) and by induction (proved by the orientation of the loop when well away from roads and pipes)..
  2. At greater distances (3-5kms) the signal reaches its destination by conduction through the ground aided by utilities in the roads and with inductive coupling into the loop at the RX.
  3. There is no evidence of induction playing a real part in propagation at 5km distance.
  4. When the signal is being "utility coupled" the direction with respect to the TX earth electrode pair (and loop formed in the ground) is not important. What matters more is whether there are metal pipes between the TX and RX locations; if these are not present then the signal will not be received, I think, at any great distance. There is some possibility that coupling may also be possible as a result of overhead cables, but this has not been tested.
  5. Even allowing for the conduction aided by pipes, I am amazed that just a 4W TX into a 20m baseline earth electrode pair can be copied at 5.1km (and probably considerably further), and even more surprised that 10wpm CW copy was possible at this range.
  6. "Utilities assisted" VLF communications could be very viable over a range of at least 5kms.
  1. Try an E-field probe at 5km distance. It should reject 50Hz noise more and may work as well as the loop?
  2. Try to establish the limit of true induction communications unaided by ground conduction paths.

June 21st warm and dry - DX DX!!! 3.6km range achieved (and more possible with same set-up)


The screenshot at 3.6km away from the TX
(click to enlarge)

Some significant steps taken with my local earth mode / induction tests today, which I'll summarise here. 
  • Firstly, having installed the K1EL keyer chip, I decided to use its internal 800Hz sidetone oscillator, after a bit of LP filtering, to drive my 4W PA. This arrangement now allows me to run either QRSS3 or 10wpm CW beacons on 800Hz at the flick of a switch. It is more stable than the external twin-tee oscillator I was using and is perfectly suitable for QRSS3 at audio frequencies.
  • Secondly, I modified my loop/preamp to have marginally better sensitivity at around 700-900Hz.
  • A quick "by ear" test with the switch in the 10wpm position and the 0.5km range I'd managed before was achieved using the 80cm loop and the 20m baseline earth electrodes.


Now here comes the real fun. At this point I decided to take my loop, preamp and laptop equipped with Spectran out into the field to see what I could see/hear.
  • My first location was a field 0.75km south of the QTH just out of the village. Although nothing could be heard, the QRSS trace was solid on the screen.
  • Next step was to proceed further south along the Heath Road out of Burwell (QTH JO02dg) to a point 2.5km away from the QTH. Again, with Spectran, a good signal was visible on the screen. At one point when rotating the loop I could hear the signal too. Listen to this signal here. Play it with Spectran with a narrow BPF centred on 838Hz and the signal is clearly audible, whereas without you'll hear plenty of 50Hz buzz!
  • The next test was to go out of the village on a different road (towards Cambridge) to a car park adjacent to the Devils Dyke. This was 1.4km away from the TX but about 40 degrees further to the west than the earlier test locations. Immediately the loop was set up (leaning against the back of the car) a good clear signal was visible, again no audible signal though.
  • At this point I decided to "go for broke" and drove down another rural lane to a roadside spot 3.6km away from home, but in the direction of fire of the TX earth electrode loop. Setting up beside the country lane I could see the QRSS signal on the screen!! Best orientation was with the loop orthogonal to the village, which by now was too far away to see over the slightly rising ground. Listen to this signal here. If listening/viewing this clip with Spectran, look for the clip of "BM" in QRSS3 at 838Hz (part of "G3XBM JO02DG") before my wife back home switches off the TX!
    • At this point my laptop batteries were dying and I had to get some food for my wife for tea, so I packed up and came home, very happy.
This is the signal 0.75km from home.

  • Going from 10wpm CW to QRSS3 has made an amazing difference: signals that cannot be heard are visible, and at decent distances. This is a new experience for me.
  • The free running sidetone oscillator in the K1EL keyer is fine for QRSS3, certainly for tests of about 1-2 hours long.
  • I suspect some "utility coupling" is going on, although at the furtherest point loop orientation did line up with the village. Is the signal propagating along the water pipes to some extent?
  • With "utility coupling" I suspect that getting a report from G6ALB in the next village will already be possible on VLF as I've already heard my signal at a greater distance than he is from me.
  • At VLF, I am wondering just how far is possible in semi-rural areas with manageable 50Hz noise levels (it is well filtered in my RX) and with some degree of "utility coupling" which is inevitable in today's world where pipes and wires criss-cross everywhere.
  • Even with just 4W and QRSS, remarkable distances can be achieved. Clearly I could get further than 3.6km already with the same equipment at each end.
  • I need a location right out in the wilds (a bit hard to find in SE England) for further tests, to see what performance is like without any coupling into water pipes etc..
  • Increase the area of the TX loop part that is in the air as this could add another 3dB to signal level.
  • More power? Loops at both ends? ....?

June 18th

Here is a sketch of my earth electrode arrangement at the TX end of the earth mode station. In the last couple of days I tried transmitting with this same earth electrode "antenna" on 500kHz using WSPR and managed to be copied at a range of 210km. I'm now trying to work out how big the effective "loop in the ground" is both on 500kHz and sub-9kHz. From measurements on 500kHz the loop within the ground does not seem to be that large, although this will be larger on sub-9kHz as the skin depth is greater.

June 16th 2010 dry and warm

Today I modified my VLF TX so it could send either a 10 wpm or QRSS3 beacon signal. I also went out into the fields with my loop/preamp and a PC with Spectran to receive it. For the first attempt with a PC I went 0.3km away from home where the 4W signal from the earth electrode "antenna" could clearly be received by ear. With Spectran I could read the 10wpm signal with 20dB S/N in a 12Hz bandwidth, if my understanding of the Spectran settings is right. This is a recording of the signal. This suggests that with QRSS3 and really screwing the bandwidth down I should be able to do much better. My problem is knowing how to calibrate the soundcard etc and clearly I need a more stable source than my free running twin-tee audio oscillator. One possibility is to use the 800Hz sidetone oscillator within the keyer. Another is to use a 4060 IC as an oscillator and divider e.g. . This would give a very stable signal for QRSS.

June 3rd 2010

I have completed a further "by ear" range test using my TDA2003 4W PA into a 20m base electrode pair "antenna" walking around the village in many different directions using the 80cm loop RX at around 1.5kHz. I improved the 50Hz rejection this week by adding another Sallen-Key HPF. When walking over metal pipework in the roads the signal is strong up to at least 0.25km in almost any direction and limited by 50Hz noise. When walking across fields well away from pipework the loop orientation exactly lines up with the base electrode "loop in the ground" at distances up to 0.5km.  I have not managed to hear the signal (by ear that is) at any further distance in any direction. Next test will be after June 14th when I shall try PC based reception in QRSS and with a TX multi-turn "in air" loop.

May 28th 2010

Here is a picture of my latest version of the VLF 1-2kHz receiver for use with my 80cm loop. The version before this (Wednesday) had one less Sallen-Key active high pass filter and was able to copy my 4W grounded electrode transmissions "by ear" at 0.5kms. This latest version now has 2 sections of 50Hz active high pass filtering so should offer better S/N at the limit of range. The audio above 2kHz is attenuated by the passive LPF. All transistors are 2N3904 and not an op-amp in sight.

I can't test this receiver in the field yet as I have my grandson staying and using the shack as a bedroom, so TXing not possible.

May 27th 2010 (dry after rain last night)

Today I made some
more slight improvements to my loop and audio receiver and did a further walk test listening by ear. The receiver is now a feedback biased amp followed by a Sallen-Key HPF (50Hz rejection) followed by a passive LPF, followed by 2 further stages of feedback biassed amps into the crystal earpiece. This time I managed to hear the signal just at 0.5kms. At 0.4kms it was a very solid signal. This was in the middle of a field well away from cables and pipe work. The TX is still the TDA2002 with 4W into the 20m spaced electrodes. At 0.4km where the signal was good still I tried placing the loop on the ground to see what happened. The signal was about the same level as when orientating the loop for best signal when held vertically. At the limit of range the limiting factor was 50Hz mains hum. I will try another Sallen-Key HPF in circuit to take this down another 20-30dB, which should be enough.

For the first time I also went listening for my signal in the other direction towards the centre of the village. This was much more difficult with an absolute fog of 50Hz mains hum. The best I could achieve was 0.35kms into the village, although at this distance I was orientating the loop to null the 50Hz hum in order to hear the signal. With better 50Hz rejection the signal would have been usable.  Moving up to 8.97kHz should also help - when I have got my up-converter working better or when I've built a M0BMU VLF direct conversion RX.

I've a friend with a long back garden (no pipes or cables well away from the house) who is almost exactly 1km away from my QTH and he is in my best direction. As soon as I can (grandchildren permitting) I may take my laptop and set this up in his garden to look for the signal. I may also try listening/looking on electrode pairs from the same location. There are so many things to try still but I just don't have enough time in the next week.

May 26th 2010

Several things tried today:
  • Changed the audio receiver to "preamp-HPF-gain-gain-gain" and achieved about the same range as before. It was no better on sensitivity despite mains hum being low enough: the signal just dies at 0.42km.
  • Then tried a small preamp in front of an up-converter with the FT817 - disappointing performance and not sure why not more sensitive. This needs more attention as this should be the best scheme for sensitivity/selectivity.
  • Then tried to better match the TX to the ground electrodes by putting a small current meter in line and optimising the series capacitance from the output transformer to the feed - it was best with no capacitance, suggesting it looks like a resistive load.  I see about 25V p-p across the feed to the ground electrodes.

May 25th 2010

Today I've been having a go at resonating my receiving loop antenna. I did this with a single turn loop around the main loop which was coupled to an audio signal generator which was swept to find the peak across the loop and a parallel capacitor. Using this technique I was able to find a range of C values to resonate the loop 220n puts me spot on 8.97kHz, but I've got around 1400n to get me down to around around 3.5kHz.  I am going to try a walk-about receive test on this frequency this evening. Working back, the 30t 80cm loop has an inductance of around 1.5mH.

LATER: Well, I tried it and it was really no better. Then I improved the Sallen-Key active high pass (50Hz) filter adding another couple of poles which got the mains hum down some more. Then I removed one audio gain stage in the RX just leaving 2 common emitter stages with 2N3904 transistors. With this and the loop resonated around 2kHz I did a further walk-about test. Range was 0.42kms at the limit of range but S/N was better and 50Hz hum was not a problem. The RX needs more gain, but peaked at the TX frequency, so I will put back the last stage of gain and play some more tomorrow.....

May 24th 2010

This evening I extended the TX electrode spacings by connecting one end to the copper pipes in the house, almost doubling the effective TX baseline to about 20m. With this, my "earth mode" signals were much stronger and I was able to cover further than before in a "by ear" walk-about test around the fields.  TX frequency was 1kHz. Ultimate range tonight was 0.4kms using 4W (probably less as the TDA2002 was hot and probably turning down the power) and receiving on an 80cm loop. active HPF, small AF amp and crystal earpiece.  Range was limited by 50Hz mains hum and background sferic noise. I must try this set-up with my HF up-converter and FT817 this week which has good rejection of 50Hz  (and its harmonics) mains hum and a narrow CW filter. On the map the green blobs are where the signal was strong and the orange blobs where the signal was just copyable. I only tried listening to the south and south east of my QTH which is in the "line of fire" of the earth electrode antenna.

May 13th for the next week...

I am not going to be at home, so sub-9kHz tests on hold until a week's time. Will use the time to read up about low noise receivers, loop theory, voltage probes, earth mode, filter design, etc.

May 12th 2010

Today I continued "by ear" testing local induction and conduction communication using my 4W PA at 1kHz with 10m spaced grounded electrodes. Using the 80cm loop, HPF and audio amp I was able to achieve 0.3kms today, slightly less than on Monday. Everything else is unchanged, so I'm wondering if the effective TX loop area formed by the 2 earth rods in the ground changes with soil moisture? I.e. when the soil is damp is the loop smaller? Logically this makes sense. Copy using my 80cm loop and VLF up-converter was disappointing: I was unable to get further than 0.2km. This system needs further work as this should be every bit as good or better than the audio amp receiver.

Also today I checked the resistance of the TX grounded electrode system using an audio oscillator and potential divider method: it measures approximately 50-60 ohms from 1-10kHz, which is not too bad. I also tried receiving my signal 0.2km away using another pair of grounded electrodes spaced by 10m. Although copy was just possible, the signal was buried in mains hum. Mains hum pick-up on the loop is much lower. This again suggests that the main means of communication is induction, not conduction through the soil.

May 11th 2010

Started on the build of an 8.97kHz to 20MHz up-converter using a dual gate MOSFET. The idea is to use a 82mH choke and 4n7 +/- cap as a tuned circuit on gate 1 (resonates around 8kHz). This converter would be used with either the loop antenna or ground electrodes for CW, QRSS and WSPR tests using the FT817 as the main RX. Have been calculating values using some of the on-line calculators at and  to work out resonance, coil inductance and turns needed. Sensitivity should be fine and the dynamic range manageable as long as the converter gain is kept very low (can adjust by keeping the drain load low).

LATER: Despite several attempts I couldn't make the MOSFET based up-converter as sensitive as the SBL1 based up-converter elsewhere on this site. Not sure why as I tried to optimise everything I could and tried both 3N211 and BF981 MOSFETs. There was no sign of instability so I'm puzzled why this should be so deaf. The only thing I wondered about was oscillator noise at 20MHz causing some sort of blocking as it is only 7-9kHz away from the wanted signal on the MOSFET output.

Having basically failed with the MOSFET up-converter I shall "go walkabout" later this week with the FT817, SBL1 up-converter and the 80cm loop to see how well it performs as a receiving system on sub-9kHz using the 4W earth-mode TX system from home. It will be interesting to see if the range with the up-converter (which should have much better 50Hz rejection and selectivity) will be better than the basic audio frequency receiver used yesterday. I may need to reduce the cut-off frequency of the low pass filter to reject some of the powerful VLF military stations when out in the field.

May 10th 2010

Today I restarted my earth-mode and induction communication tests at VLF using with my 4W transmitter and 10m separated earth electrodes. After some false starts, I ended up by field testing a simple receiver consisting of 30t of wire 80cms square into an active high pass filter followed by 3 stages of audio gain into a crystal earpiece. 2N3904 transistors were used at each stage. With the loop over my shoulder I set the TX sending 1 second "beeps" at 1kHz and went walking....

Even with 20-30dB of 50Hz rejection, mains hum was still an issue in many places. Walking across the field behind my house, and well away from cables and pipes, the signal could be heard in the earpiece at RS53 to a distance of 0.35km. Mains hum and LW/MW interference (the RX is not in a screened box) rather than signal level  limited further range. This week I'll add more high pass filtering, put the RX in a screened box and make the whole thing more immune from BC breakthrough. As I can hear this signal by ear at 0.35km with essentially no narrow filtering, then I'm confident that twice as far will be possible with QRSS/WSPR and a PC based receiver. Not bad for 4W at 1kHz.

I'm assuming the main mode of communication is induction with the earth electrode pair at the TX end forming a loop within the ground. I will have to try RX tests at the same spots using earth electrode pairs to see how the signal and hum levels compare. Orientation of the loops is consistent with induction, although when mains hum was high the best orientation was to null the mains hum. This reduced signal level but not as much as it reduced the 50Hz hum, so S/N was better.

Roger Lapthorn,
Jun 16, 2010, 12:42 PM
Roger Lapthorn,
Jun 21, 2010, 11:41 PM
Roger Lapthorn,
Jun 21, 2010, 11:36 PM
Roger Lapthorn,
Jun 22, 2010, 2:08 PM