Before my stroke I really enjoyed experimenting with both ends of the spectrum. I quite enjoyed seeing how far I could get on VLF earth-mode through the ground. This was a Facebook memory from 2011.
14 Aug 2023
10 Apr 2023
Earth-mode paper
Someone asked for access to this file. It is now open so if you have this link you should be able to read it. It was sent to me some years ago.
See https://drive.google.com/file/d/16vtYNznORclXJTfpaT-RC7Z3IV8RKqyi/view?usp=sharing .
27 Jun 2021
Earth-mode VLF
Earth-mode is a form of VLF communication that is open to all. With 5W I managed to cover 6km, although far further should be possible even with the same gear. I have updated my webpage to include a link to my earth-mode blog showing my experiments some years ago.
My understanding is this does not require a UK licence, but check with OFCOM as I cannot give legal advice.
See https://sites.google.com/view/g3xbm4/home/vlf/vlf-earth-mode .
5 Mar 2021
Earth-mode VLF blog
Earth-mode is a form of VLF experimentation that does not need high power or big antennas. It is at the frontier and is open to all.
My understanding is no licences are needed for this form of communications based on conversations with OFCOM many years ago. This is not a legal statement, just my interpretation.
For radiated VLF communications I think those in the UK need an NoV, whereas in many countries below 8.3kHz is a free for all. As very special techniques are needed for amateur VLF radiated reception, I think the chances of being intercepted are very small.
6 Jan 2019
Earth-mode VLF
An example of a typical earth-mode VLF transmitter is shown at the link.
22 Jun 2018
CIA 1960 report on earth current signalling
https://www.cia.gov/library/readingroom/docs/CIA-RDP78-03424A000400090001-9.pdf
24 Mar 2017
VLF (8.97kHz) TX
8.97kHz 5W TX |
See https://sites.google.com/site/g3xbmqrp3/vlf/8-97khz-earth-mode-transmitter
30 Aug 2015
Earth-mode VLF communications
"Hello Roger,How are you doing? I hope you are doing fine.Since this is my first mail adressed to you, please let me first introduce myself.My name Is Jan ( PE1AXM ) and I am a radio amateur since 1977 and live in the south-west part of the Netherlands, my age is 59.My radio interests are mainly: making home made equipment.About 1 year ago I discovered one of your articles that triggered my interest in 9kHz earth mode communication. So I started building. For Tx I use 2 earth rods, 2 meters long and separated 20 meters.Resistance between these rods is 16 Ohms.A home made amplifier is capable of delivering 1 Amp into these earth rods.For reception I use a small loop antenne that resonates at 9 kHz, followed by a low noise pre-amp and a PC with Spectran Plus software.To be honest, I thought that a range of 4 km's would be an easy job, but I soon found out that this was not true.My best dx so far is just over 1 km, but I realise that still a lot can be improved on my system.Roger, I thank you for your stimulating articles and I hope that you will make further recovery."
20 Apr 2015
VLF field test - a total failure!
Tuned VLF loop |
Deploying the VLF loop (Burwell in the background) |
Checking Spectran on the PC - nothing ! |
In summary, this VLF test was a total failure. I am at loss to explain things. Not only that, but the work has left me very exhausted. My brain is still foggy, but the results today were not due to any mistakes with the HW gear or PC Spectran settings.
I am not having much success today.
19 Apr 2015
VLF field test tomorrow
On the way I managed to blow up a TDA2003 IC, which I had to replace. The whole exercise was far more exhausting than I was expecting. It seems that after about 10-15 minutes of physical or mental effort I am done in. Before my cerebellum brain bleed everything today would have been trivial. Now trivial tasks feel like climbing mountains! Although I can see progress in my recovery there is still a long way to go. One of my aims (among many) is to be able to resume field tests as before, but time will tell if I am really up to this: it is quite hard when your brain is still foggy a lot of the time. Oddly, when sitting down at home or when driving things are fine. It is when I do something requiring real physical or mental effort that I get tired. I guess the radio work today was hard as I had not done this sort of thing for some time.
Anyway, the good news is that I hope to do a VLF field test tomorrow. Everything is ready and tested. It will not be until late afternoon as both my wife and I are busy before then. The XYL will be there at the test site if I get really tired. Setting up the gear will be especially tiring in my current state. The test site is not too far from home. I have soak tested the TX and it should be fine on QRSS3 using my loop and Spectran at the RX end. I shall report results tomorrow. This will be my first VLF field test in over 18 months. How I have looked forward to this. If the loop is successful I may try the E-field probe.
1 Apr 2015
First (restarted) VLF earth-mode test
5W VLF beacon TX |
TX frequency 8.976kHz initially with 10wpm CW, 300Hz bandwidth, sending my callsign and a dash. Nothing copied at first using a dummy load but over 70dB over noise using the earth-electrode "antenna" with no attempt to optimise match (fed via the same 3C90 step-up transformer used on 472kHz). With no probe at all (i.e.no RX antenna) the S/N was still some 40dB i.e. very good using the TX into the earth-electrodes.
QRSS3 signal received on Spectran (0.37Hz BW) |
E-field probe |
Honestly, this has really exhausted me!
25 Sept 2014
Experiments again - soon I hope
With a bit of effort, I should then be able to resume some VLF earth-mode and 136kHz experiments, looking for my signals from home when out and about in the car, parking up at various sites in the area.
Not many months ago this would have been out of the question, but I am getting better.
25 Mar 2014
One more day on 6m
Although I shall try again tomorrow, that will probably be my last day. A pity as I was quite hoping we might get an Es opening to link up to some TEP, but it looks unlikely now that I shall be in luck.
Later this week I may return to VLF earth-mode if I can optimise my TX earth-electrode match at 8.97kHz and re set-up my RX loop. My wife will have to drive me to my usual test sites as I am not allowed to drive yet.
5 Sept 2013
First 8.976kHz VLF earth-mode tests from the new QTH
Then I returned home and used the mysterious copper rod that goes into the ground just outside my shack as the local ground and still keeping the ground rod 12m away as the "far" electrode. I think this may have at one time been an oil pipe from a central heating oil tank. I have no idea where it goes. This earth-electrode pair has launched my 472kHz WSPR signal which has been copied in Belgium and Holland, but results have been disappointing, so I was not expecting much on 8.976kHz.
Rather than travel 2km, I parked outside my old QTH about 0.35km away in the village. My 8.976kHz signals were copied, but not very strongly. I suspect that this was true earth-mode without any real assistance from utilities as I do not believe either ground rod at the TX end is coupled to utilities grounds or copper pipes going into the road.
Weak signals over a 0.35km path |
Path tested this evening plotted with Google Earth |
27 Jun 2013
Earth-mode transmitter on my website
At the moment, my earth-mode tests are on hold. I am finding more and more of my time is being taken up preparing for our house move. At some point in the next few weeks it will be time to take down the V2000 VHF/UHF vertical and the other antennas. It is a long time since I've had no antennas up at all! With luck I'll soon have them back in place at the new QTH and I can start DXCC chasing (gently!) from scratch again. I wonder how long it will take to work my first 100 DXCC countries with modest antennas at the new QTH?
The new home has a totally different shape garden and I shall be intrigued to find out how well (or not) an earth-electrode antenna works on VLF, LF and MF. A 20m baseline spacing between electrodes should be possible by placing the remote electrode at the far corner of the garden with the feed wire diagonally across the garden.
20 Jun 2013
How much am I RADIATING at VLF?
The first thing is the effective area of the "loop in the ground" and based on a guestimate of 40 ohm metre soil resisitivity (could be somewhere between 10-100 ohm metres) my calculations give me an effective loop area of 600m sq at 8.97kHz - i.e. the signal current flows quite deeply into the ground.
The second figure is the current flowing in the loop (I) which I measure as 0.2A using a current transformer to sense the current.
Rrad = (31171 x Ae^2)/lambda^4 = 0.94 *10^-8
Lambda = 33km
Ae = 600 sq m
I = 0.2 amps
Rrad = 0.94 * 10^-8 ohms
ERP = I^2 x Rrad
So, plugging in the numbers:
ERP = 0.2 x 0.2 x 0.94 x 10^-8
ERP = 37nW
There may well be errors in my sums and in the assumptions made, but clearly 37nW is a tiny amount of radiated power and (almost) negligible. To get to a level where the radiated signal is detectable over 100km away, I would need to radiate around 4-8uW, i.e. several orders of magnitude more. Radiation resistance is proportional to the loop area squared so increasing the baseline by a factor of 10 increases the radiation resistance by 100 times. This could be helped with a much longer baseline (200m long rather than 20m) and increasing the power into the earth-electrode loop to 500W and elevating the loop part that feeds the far earth electrode with current. Such changes could result in a radiated power level of around 4-8uW based on the assumptions about soil/rock resistivity and skin depth. A 200m long piece of wire (e.g. along a field edge) and 500W of audio power are not that hard to envisage and a LOT easier than winding a huge loading coil and raising a kite supported antenna several hundreds of metres high.
VLF earth-mode "propagation" variations
The recent tests were to see how signal levels varied with frequency, but I am finding VERY large differences in signal level day-to-day. The 8.97kHz signal was around 10dB S/N today at 1.6km whereas it was around 20dB S/N a week or so ago. I was unable to copy a signal at 1kHz and 8.97kHz at 6km at all when I tried a few days ago yet the 8.97kHz signal was quite decent a few weeks ago at the very same spot.
Weaker 8.97kHz received signal at 1.6km today |
Weak 4.485kHz received signal at 1.6km today |
Conclusions so far? Signal strengths between 1-17kHz at 6km range don't vary that much over the frequency range, but signal levels can be up to 20dB different day-to-day as a result of other (as yet not understood) variables in the system or path.
18 Jun 2013
Earth-mode at 1kHz and 18kHz today
Results were interesting: although signals were copied at 1.6km and 3.6km on both test frequencies, nothing at all was copied at 6km, where very good signals were copied on 8.97kHz a few weeks ago. Ground conductivity and weather conditions were identical on both test days: dry for the last several days, so conductivity likely to be lower than when the fen soil is saturated.
I have no real idea why 8.97kHz should appear to be a "sweet spot" in frequency. It is possible that other frequencies lower than 8.97kHz and higher than 1.1kHz may be even better. In all cases the RX loop was resonated and positioned on the ground varied for best signal.
1.12kHz earth-mode signal at 1.6km |
17.952kHz earth-mode signal at 1.6km |
1.12kHz earth-mode signal at 3.6km |
17.952kHz earth-mode signal at 3.6km (note MSK signals close by) |
6 Jun 2013
Good results with new VLF earth-mode RX system
The new preamp is just a single MPF102 impedance converter feeding into a small,low cost, external USB sound card (£3) that feeds my baby Asus X101CH netbook (£172) running Spectran software to display the received QRSS3 signal. The external soundcard was only needed as I didn't have a 4-pin jack to use the audio-in socket on the Asus.
Tests were carried out at the usual test sites at 1.6, 3.6 and 6km as well as one further (unsuccessful) test at 6.2km.
In all locations, decent signals were copied, even at 6km. The little Asus netbook has an SD card running Readyboost and behaved perfectly well during the tests. Now, the whole system is very simple to deploy in the field to take measurements. The preamp is only tuned by the loop and its resonating capacitors (a capacitance decade box), so the very same preamp can be used right across the VLF and LF spectrum with a suitable receiver.
Today the weather was dry and has been for around a week now. I believe signals by earth-mode are stronger after a period of dry weather as the soil conductivity is lower. Results are as good, or better, than I have ever experienced before with earth-mode at 8.976kHz.
The next test will be to repeat this test at around 1kHz and possibly lower frequencies.I also want to try WSPR and repeat the tests at much higher LF frequencies around 72-73kHz. For the latter test, I need to build a simple down converter to audio.
6 Mar 2013
Frustrating day at both ends of the spectrum
Failure 1 at VLF: whereas in the past I've had a decent signal at my test site at Tubney Fen 3.5km from home on the loop antenna on 8.977kHz, today I could copy nothing. There was some strong interference and I thought that may have been the problem, so I moved on to a second test site 5km away where again I usually get a good signal. Again nothing. In the past this second site has given me signals so strong that I could copy 10wpm CW from home on 1kHz by earth mode. Now around the village and in nearby Swaffham Prior the signal levels were (as far as I can recall) similar to past levels, so what has changed? One theory is that the wet winter has saturated the fenland soil so conductivity is much higher, resulting in much higher attenuation at VLF. Another possibility is some utility has changed - a change of pipe type or a re-routing of an electricity cable? My TX and RX equipment is largely the same as in previous tests.
Optical QRM on the 820Hz sub-carrier frequency |
My next immediate priority is to change the optical beacon TX so that I can use a range of different sub-carrier frequencies all derived from an HF crystal divided down with a 4060 divider. This will also allow me to run a continuous signal which will help with alignment and I can avoid QRM by moving the HF crystal frequency if needed. I could also arrange DFCW modulation by FSK keying the HF crystal: this will allow a continuous signal for audible alignment yet be detectable with software packages like Spectran in QRSS3.
As regards the VLF earth-mode tests, for now I am going to draw these to a halt and will try again in a month or so when I hope the fen soil conductivity has reduced. If things are unchanged then I suspect that something in the utilities metalwork out in the fens has (permanently) changed.