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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.
Click on any image to bring up a larger version
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
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
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)
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.
Jan 13th 2011 - wet ground but dry and mild
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
Several possibilities for the failure with the earth electrodes:
Dec 1st 2010 - field testing in the snow
Nov 28th 2010 - sub-zero outside!
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
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
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
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.
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
(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
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
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 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
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
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!
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
June 21st warm and dry - DX DX!!! 3.6km range achieved (and more possible with same set-up)
MODS TO THE SYSTEM
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.
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.
This is the signal 0.75km from home.
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
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. http://m1gur.webs.com/1750hztoneburst.htm . This would give a very stable signal for QRSS.
June 3rd 2010
May 28th 2010
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
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:
May 25th 2010
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
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 http://www.calculatoredge.com/index.htm#electrical and http://www.66pacific.com/calculators/toroid_calc.aspx 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).
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 4W TX
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....
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.