Best results yet on 8.76kHz earth mode

8.76kHz earth mode signal in 45mHz BW at 6km

This morning I slightly changed my VLF low input impedance tuned preamp by increasing the tuned circuit Q and gain of the first stage by around 6dB. After lunch I did a test at 6km from home to see what effect this had on the S/N of my 5W beacon. In a 45mHz bandwidth (previously 11mHz) I was getting a signal around 18dB S/N using Spectrum Lab on my 80cm square loop on the ground. With an earth electrode antenna the signal was marginally weaker and the noise level higher.

My signal is the steady carrier (with a 2min ID break) at 8.7605kHz. The large wobbly line is interference, but who or what is the signal at 8.754kHz? This is rock steady and appears to key on and off every 5 minutes. It looks like some sort of control signal, or is someone else near Cambridge testing?

6km DX on 17.52kHz earth mode

My 5W QRP signal at 17.52kHz is clearly visible at 6km

Last evening I continued the earth mode tests at 17.52kHz by going out to the test site 6km from home and looking for the signal with a variety of antennas in QRSS3 and with an interrupted long carrier transmission.

No copy was achieved using QRSS3 and Spectran. However, using a continuous carrier with a 2 minute ID break I got extremely good copy (about 6dB S/N) with Spectrum Lab in 37mHz bandwidth with the horizontal 30t loop but no copy on the earth electrode antenna parallel to the road. Moving the loop 10m away from the road resulted in no copy, as did mounting the loop vertically or a few metres above the ground.

It would appear that at 17.52kHz it is easier to detect the signal at distance with a loop than with an earth electrode antenna whereas at 1.095 and 8.760kHz either scheme works. Perhaps someone who understands the theory will be able to explain this.

My next test is to repeat these tests at 137kHz to see to what extent 137kHz is aided locally by utilities assisted earth mode. At the test site 6km away I can see how strong the signal is with tight coupling into the ground at the roadside, and with an E-field probe some distance away from the road.

3.6km with 17.52kHz earth mode

This morning I did an initial test at 17.52kHz using my 5W QRSS3 TX and earth electrode TX "antenna".  A test at 3.5km from home showed the signal ~20dB over noise (0.73Hz BW QRSS3) using the 30t loop on the ground and with a 1t loop (about 20m sq) laying in the road (see attached Spectran screenshot). Signals were also copied with the earth electrodes parallel to the road but weaker than on the loop. They were also copied weaker still with the RX earth electrodes strung across the road. Signals are definitely coming through the ground/utilities with no evidence of any radiated signal and they are every bit as strong (or stronger) than at 8.76 or 1.095kHz.  However, at another test site 4km from home where I usually manage to copy 8.76kHz QRSS3 reasonably well there was no sign of my 17.52kHz signal either with the 30t loop or the earth electrode antenna. No idea why.

Later today I may try the 17.52kHz test at my 6km distant test site with a long duration carrier, although I suspect I may copy it in QRSS3, which I'll try first. I may also try a test using my 80m sq vertical loop as the antenna at the TX end, although I don't expect any great range by pure induction.

1.095kHz earth mode test at 6km

1.095kHz earth mode signal at 6km

Today I repeated the earth mode (through the ground) "long carrier" test of 17.7.11 this time at 1.095kHz rather than 8.760kHz. The same test location was used in the fens not far from the River Cam. This is now a very long way from home! As before, I got my wife to cut the carrier for periods of 1 or 2 minutes to positively identify my signal. I need to automate this so she can continue to do the gardening.  4 different receiving antenna arrangements were tested, using my low impedance MPF102 tuned preamp into the PC:

(a) an earth electrode pair parallel with the road,
(b) an earth electrode pair diagonally across the road,
(c) a single turn loop in the road, and
(d) a 30t 80cm diameter loop at the edge of the road. 

Best results were with (a) and (d). With (b) there was no detectable signal.  At 1.095kHz the signal was again around 10dB S/N in 11mHz bandwidth on (a) and (d) and marginally weaker on (c). This is a similar result to 8.76kHz although I was expecting the signal to be stronger at this lower frequency.  I'm now tempted to do an earth mode test at a higher frequency, possibly 17.52kHz which I can still see with Spectran and Spectrum Laboratory and I can derive with my 4060 divider circuit. Using earth mode I believe I'll still be legal as very little radiated signal can be present. The image shows the signal with the RX earth electrodes parallel to the road, i.e. set-up (a), at a distance of 6km from the home QTH where the TX was running 5W into 20m spaced earth electrodes.

VLF beacon transmitter schematic

G3XBM's VLF 5W Earth Mode Beacon

This is the schematic of my TDA2003 5W ULF/VLF beacon transmitter used in recent earth mode tests. The frequency is derived from a 4.48MHz crystal that is divided down to either 8.760 or 1.095kHz using a 4060 IC. This is keyed in either 10wpm CW, QRSS3 or QRSS30 with a K1EL beacon keyer IC. The output of the TDA2003 is matched to the resistance presented by two earth electrodes using a tapped toroid transformer. Best DX is now 6km across the fenland near me.

6km DX by VLF earth-mode today

8.7605kHz earth-mode reception at 6km across the fens

My personal DX record for VLF earth-mode (through ground conduction/induction) at 8.7605kHz has been pushed up to 6km today using a long carrier transmission with periodic ID breaks and Spectrum Laboratory software with bandwidth of 11mHz on receive. Transmit power was 5W from a TDA2003 transmitter.

See https://sites.google.com/site/sub9khz/earthmode/vlf-xbm-blog

More VLF earth-mode tests

8.76kHz QRSS3 signal at 3.8km

Recently the water company repaired some pipes right outside our property in the road. I was informed the pipes are/were NOT metallic.

Today, just as a check, I tried 8.760kHz earth-mode running my usual 5W into 20m spaced earth electrodes just to see if the repair work made any difference to signal levels some kilometres away.

At my favourite roadside test site 3.8km from home I set up my RX and tried 2 different loops on the ground (a) my 30t 80cm square loop and (b) a large single turn loop.  Using QRSS3 the signal was copied on both antennas at the usual strength. This makes me think any "utilities assistance" is NOT purely from metal water pipes in the road.

My next planned test is to see how well an earth-mode signal propagates along one of the many fenland water channels using one TX electrode actually in the water and one on dry land. For RX I'll either use another earth electrode pair (one in and one out of the water) or a loop or E-field probe. The next couple of weeks are pretty busy with family matters, so this fenland river test may not take place until sometime in August.

Earth mode VLF test at 1.095kHz

When testing with utilities assisted earth mode in recent months I seemed to be getting weaker signals at 8.76kHz compared with my tests at 0.838kHz last summer. So, today I decided to go back and recheck results at a lower frequency at test sites 1.6, 2.6, 4.5 and 5.1km away from the 5W QRSS3 transmitter at home. Test locations were rural fenland roadsides, a National Trust car park and 2 roads in the next villages.

I used the same TX as at 8.76kHz, but divided down by a further x8 to 1.095kHz using the 4060 divider. The receiver was a horizontally orientated 30turn, 80cm across, series tuned loop into a low impedance MPF102 FET preamp with a drain circuit peaked at around 1.1kHz and emitter follower into PC running Spectran.

My initial test at 1.6km was a total failure - no signal visible at all in Spectran at 0.73Hz bandwidth. Then I realised that I had BOTH the garden earth electrodes in use at the TX (neither connected to pipes). A phone call to my wife and one wire was connected to the pipework in the house with the other still connected to the earth electrode at the bottom of the garden some 20m from the house. The signal then appeared at good strength.

Signals were received at all test locations indicated, but there was no signal visible (in QRSS3 bandwidths at least) when moving away from the roads into nearby fields or at some other test locations where pipes were absent (or plastic). At one location I rested the loop within 2m of one of the fenland water courses, but nothing was detected. The image attached  is the signal at 4.5km, which is strong in 0.79Hz bandwidth. Even stronger signals are possible with more optimum positioning, but I keep having to move my loop to avoid it being crushed by oncoming farm tractors! I do get some odd looks too.

My conclusions from the tests to day are:

• This was definitely utilities assisted earth mode. The initial (accidental) test today with 2 earth electrodes not connected to water pipes failed.
• Signals are strong close to where the metal pipes are, but rapidly disappear where no (metal) pipes are close by.
• Signals at 1.095kHz are definitely stronger than at 8.76kHz in the 3-5km range.

There is one more test to do at some point that needs some assistance locally (I think within 10km to work). I want to try a long stable carrier test with someone looking in the very narrow bandwidths we've been using for the radiated tests recently. What I'm wondering is just how far from the pipes does an earth mode signal "leak"? Using Spectrum Lab with the very weak signal bandwidths like 0.4-2mHz, the weak remnants of a signal might just be visible some way from the "easy" utilities assisted coverage area.

The remaining tests I want to do still with earth mode are:

• River bank or sea coast test - just how far can one get?
• True earth mode in open country without any utilities assistance
• Earth mode at higher frequency e.g. around 70kHz, which I can manage with my present set-up. Based on recent OFCOM correspondence I believe such tests would be legal as long as there is no intentional radiated signal or interference to others.

This class of experiment is not in the same league as those being done by the radiating VLF DXers, but fun nonetheless, especially as anyone can do this sort of test without NoV or special permits needed.

Earth mode VLF tests with E-field probe RX on car

As an experiment today I rigged up an E-field probe on the car to try to do something similar to LA3EQ's earth mode tests of the last few days. Instead of WSPR I used QRSS3 on 8.76kHz putting my usual 5W into the earth electrodes (one connection to the copper pipe ground and one at the bottom of the garden 20m away). The E-field probe consisted of my 1.5m long whip into an MPF102 source follower feeding a 2N3904 emitter follower with roughly the same arrangement as the PA0RDT E-field probe. The probe was attached at the top of a 2m long PVC pipe poking out of the rear window vertically. The actual E-field probe was powered by an internal 9V battery and was mounted about 1m above the car roof.

With the laptop PC running Spectran on the front passenger seat I was able to drive around and glance at the screen whilst moving. In the village there was a reasonable copy of the signal but outside of the village I only managed to get a couple of places 1km apart where the signal was strong enough to copy. One was at 2km and another at 2.1km. Compared with a loop on the ground at the same spots last year the signal was at least 10-20dB weaker. Although I am getting reasonable copy of the 11905Hz Alpha beacon the signal level is around 10dB down on the level on the 80m sq loop at home. So, maybe the E-field probe can be improved still.

In summary, with utilities assisted earth mode, for that is what the propagation is, the car mounted E-field probe is, as yet, nothing like as effective as a loop for RX.

I'd like to rig up some way of driving along with a horizontal loop attached to the rear of the car about 0.5m above the ground. This way I could drive around the local area and log where copy is possible. Presently I have to get out of the car with the loop to take a measurement when doing earth mode tests.

So, a useful test even though the results were not outstanding, or even good :-)

VLF earth-mode success: G6ALB copied at 3km on 8.76kHz

Spectran trace of G6ALB on 8.76kHz at 3km

This evening G6ALB fired up his 8.760kHz earth-mode TX at his home QTH running QRSS3 and 40W into earth electrodes spaced 25m apart. We started listening 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. His keyer and 8.760kHz signal were generated using "Direct Digital Synthesis" from a 25 MHz clock using my Digilent Spartan S3 FPGA board.

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

VLF earth-mode tests legal without NoV

Today I received a letter from OFCOM, which clarifies the legality of operating so called "earth mode" (conduction/induction via the ground) experiments at frequencies below 9kHz in the UK. In it, the official from OFCOM stated, "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.".

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 to be legal. One could reasonably also argue that such through the ground conduction tests at frequencies above 9kHz would also be legal as long as they don't cause any "Undue Interference to any wireless telegraphy" and one was not intending to radiate.

This sounds a wholly pragmatic answer from a government body.

Further 8.76kHz VLF test transmissions today

8.76kHz earth mode tests - best DX 5.1km and 4.75km

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 "earth mode" (through the ground) test signal was emitted on 8.760kHz from the Burwell QTH and 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.

How far is possible with earth mode on the Dreamer's Band?

A further observation from the test today: with 17dB above the noise signals at 5.1km using QRSS3 there is plenty of (theoretical) range still to be had with utilities assisted earth mode and just 5W.

Assuming 18dB loss every time distance is doubled (an inverse cubed attenuation rate for induction/conduction - is this right?) then almost 10km should be just possible, assuming the utilities that are aiding propagation are still there in the ground. Going from QRSS3 to QRSS30 should give another 8-10dB (see http://www.qsl.net/on7yd/136narro.htm#QRSS). Increase power from 5W to 100W gains another 13dB, so QRSS30 and 100W could give a range of some 20km by utilities assisted earth mode in favourable locations.

So, in some locations using the utilities buried in the ground all around us could offer some quite decent ranges with an earth electrode "antenna" at the TX end that requires minimal matching and no large loading coils, just a step-up transformer.

What is less clear to me is what exactly is the signal being carried along? Water pipes (often these are plastic now), overhead mains cables, gas pipes (usually non-metallic), phone lines, rivers even?

8.76kHz tests with loop + earth electrodes on RX

Today I did a test at 5.1km to compare reception of my 8.76kHz 5W 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.

New front-end for 8.76kHz VLF earth mode tests

Today I completed the design of a new "front-end" for my tests on 8.76kHz when using earth electrodes as the RX antenna. Up to now I've used an 80cm loop/preamp at the far end of the test, usually with the loop on the ground for maximum signal pick-up. Now I want to compare results with an electrode pair antenna at the RX, laid across the road to form a pickup loop within the ground around any buried pipework and cables. The electrode pair 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 fed via an emitter follower into the PC sound card. It has sufficient selectivity to reject 50Hz and lower harmonics and reject the stronger VLF and LF stations which could cause intermod and overload. I've also added a small LC lowpass filter on the input to the FET. This will be field tested at a site 5.3kms from the home QTH as soon as the weather improves: it is currently below freezing day and night!

UK first transmission on the Dreamer's Band (8.760kHz)

Signal received at 5.1km from home QTH on 8.760kHz VLF

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 is a first in the UK, legally at least.

8.76kHz TX

TX was 4W from a TDA2002 audio IC matched into earth electrodes 20m apart. The beacon signal was provided by a K1EL keyer chip and the frequency reference by dividing down an HF crystal by 512 times in a 4060 divider IC. Reception was possible 5.1km away from the transmitter location, detecting the signal with an 80cm loop fed into an E-field probe (Hi-Z input) into Spectran running on a WinXP PC. Best reception was always with the loop flat on the ground suggesting the main mode of propagation is utilities assisted earth mode. Comparing results on 8.76kHz with those at 838Hz earlier in the summer, my first impressions are that signal levels are some 6dB weaker, but more careful tests will be needed.

136kHz with QRP and earth electrode antenna

Today I increased the power from my QRSS3 136.93kHz beacon to 2W out of the PA into the 20m spaced earth electrodes. Another "ride about" test locally with the deaf FT817 and E-field probe RX antenna gave the following results:

At 2.4km good copy on the Spectran display

At 4.6km clear copy on the Spectran display.

At 8.6km the signal was detectable, but weak (see picture attached from Spectran screen).

These reports are roughly "end on" to the electrodes and I've yet to try other directions such as orthogonal to them. No attempt was made to match the PA output to the impedance presented by the earth electrodes which are around 40-60 ohms below 10kHz; I've not measured it at 136kHz yet.

Even with true QRP it looks like a range of at least 10km should be possible on QRSS3 (and probably WSPR) with a half decent receiver (better than my crude set-up) using just a 20m earth electrode TX antenna on 136kHz.  I'll leave the QRSS3 beacon running over the weekend on 136.93kHz from my QTH in Burwell, Cambs (JO02dg) and any reports and screen shots would be much appreciated.

New VLF DX record today: 5.6km on 838Hz earth mode

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. Signals were 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.

VLF Earth Mode: 5.25km DX today with 4W

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 as 2 evenings ago, 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'd 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.

Earth Mode DX map

This map shows the places round and about my QTH in Burwell (JO02DG) where I was, or was not, able to receive my 4W 838Hz earth mode signal in tests on June 21 and 22nd. Red dots here show attempts that failed, i.e. no signals detected, and green dots show where signals were successfully received and recorded. Best DX was 5.1km, but further is certainly achievable on QRRS3 with "utilities assisted" earth mode i.e. where ground coupling is enhanced by buried pipes and cables. Best DX with 4W into the 20m spaced earth electrodes using purely induction seems to be around 0.5km.