2011 VLF memories

 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.

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 .

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 .

Earth-mode VLF blog

On my old website I have a blog about my experiments many years ago with VLF "earth-mode"  communications using 5W. 

As the page is very old, I cannot guarantee links work. The links on the left of that page are very old especially and should not be used as this was my old website that is not maintained.

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.

See https://sites.google.com/site/g3xbmqrp/Home/earthmode . 

Earth-mode VLF

Yesterday, I mentioned amateur VLF experiments below 9kHz. A very accessible introduction is earth-mode communications, especially utilities assisted earth mode. Using the latter mode, 6km has been spanned,  with just 5W, although much further should be possible. With much higher power and larger baselines at the TX end, I expect the small radiated power will be detectable hundreds of km away, although this is not going to be earth-mode.

An example of a typical earth-mode VLF transmitter is shown at the link.

CIA 1960 report on earth current signalling

An amateur in Germany, DL3IKE, found this recently:

https://www.cia.gov/library/readingroom/docs/CIA-RDP78-03424A000400090001-9.pdf

VLF (8.97kHz) TX

8.97kHz 5W TX

Before my stroke in 2013 I did a fair bit of experimentation at VLF, successfully receiving quite a few European amateurs on VLF doing radiated tests. I did some "through-the-ground" utilities assisted earth-mode tests out to 6km using just a 5W transmitter. At some point I'd like to do more.

See https://sites.google.com/site/g3xbmqrp3/vlf/8-97khz-earth-mode-transmitter

Earth-mode VLF communications

Today I received a nice email from a station in The Netherlands on the subject of earth-mode communications. Apparently he had been inspired by my work and was still hoping to achieve better results. Earth-mode requires simple gear and simple test gear, but results can be unpredictable. It is an area where true experimentation is still possible. From my limited tests when I was fit (before my stroke) I am convinced there is a lot more to learn.

"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."

VLF field test - a total failure!

Well, I did my first VLF earth-mode field test today since my brain bleed in Sept 2013. Sadly, it was a total failure. I am at a loss to explain the results.

Tuned VLF loop

I put the 5W, 8.976kHz, beacon on into the same earth electrodes used successfully on 472kHz (best DX on TX over 1000km) and went about 1.6km to a car park where I always had strong signals using a tuned loop on RX and earth-electrodes on TX from the old QTH in the same village. Nothing at all copied today - absolutely nothing. I waited in case I was in a pause between QRSS3 callsign transmissions, but no absolutely nothing at all. Not even the tiniest hint of a signal.

Deploying the VLF loop (Burwell in the background)

So I parked in the road about 0.3km from the home QTH and again nothing at all. At this point we drove home. What a dismal failure this was turning out to be. I am sure it is telling me something but I cannot work it out.  Sure enough, everything was still working at the TX end.  I am using one remote ground and the mains ground at the shack end, exactly as for 472kHz.  I did check this mains ground was grounded to real ground.

Checking Spectran on the PC - nothing !

Whatever utilities assistance I was getting before at the old QTH is not the same at this newer QTH. All very very odd. I was expecting to see something. In the past I received transmissions from the old QTH at the new QTH (about 0.3km away) using just an E-field probe on the car. From the old QTH I spanned 6km!

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.

VLF field test tomorrow

Today I adjusted the turns ratio on my 8.976kHz VLF QRSS3 and 10wpm CW 5W beacon to better match my earth electrodes. At this QTH they look much greater than 50 ohms, which I measured at the "old" QTH and optimised for.

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.

First (restarted) VLF earth-mode test

5W VLF beacon TX

OK, I was only testing from the shack to the lounge, but this is a start.  TX is 5W from my TDA2003 beacon.

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)

Later with QRSS3, a 50 ohm TX dummy load, E-field probe at the RX, I got the signal at 10-20dB S/N.  At the RX end I was using my simple E-field probe into a USB soundcard into my Windows 8.1 laptop. 

E-field probe

The blue photo shows the signal at the RX. The first part shows the TX signal on the earth-electrodes and the second part (weaker) was the signal on the EFP with the TX into a dummy load.

Honestly, this has really exhausted me!

Experiments again - soon I hope

Before too long now, I hope to be able to resume driving again, following my brain bleed over a year ago.

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.

One more day on 6m

For the last few days I have been patiently trying to find some TEP-Es on 6m WSPR, to no avail. Although 6m TEP (trans-equatorial propagation) is being reported in Israel, Italy, the Balearics and Morocco, here in the UK all remains quiet. I have only spotted a couple of G stations at around 70km - absolutely nothing else!

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.

First 8.976kHz VLF earth-mode tests from the new QTH

This evening, I did a couple of tests on 8.976kHz earth-mode from the new QTH with my 5W beacon TX. Initially I used the far earth rod about 12m from the house paired with a mains ground and set out on my travels with my loop to my usual first test site just outside the village. This is a car park for the Devil's Dyke walk. Unfortunately the car park has been invaded by gypsies, so I decided to move on to my next test site in the village of Reach (2km). Nothing at all was copied of my beacon.

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

Conclusions so far are that both for VLF earth-mode work and for 472 and 136kHz radiated work I need to much improve the earth-electrode arrangement in my garden to get credible results. I need to do some sort of mapping of the garden to see where would result in a good solid connection to copper water pipes going into the road (probably at the outside copper tap) and the best place to locate the "far" electrode as far away from the house (and other houses) and buried pipes as possible.

Earth-mode transmitter on my website

I have added a page on my main website showing the VLF earth-mode beacon transmitter currently being used. In the next few days I'll add the article that appeared in RadCom in 2011 which shows the schematic as well as some more details of the recent tests.

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.

How much am I RADIATING at VLF?

As a matter of interest, this afternoon I worked out how much power I am actually radiating when carrying out my earth-mode tests. The main transmission mode is conduction through the soil/rocks and buried utilities, but an earth-electrode antenna will produce some very very small amount of radiation too.

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

In the last few weeks I have been doing QRSS3 (slow CW) tests at approximately 1kHz, 4.48kHz, 8.97kHz and 18kHz transmitting 5W into 20m spaced earth electrodes (1 electrode at the bottom of the garden and the other connection to house copper pipes) and checking signal strengths at 1.6, 3.6 and 6km away from the home QTH using my portable loop antenna, preamp and a small netbook PC running Spectran software. Propagation is by utilities assisted earth-mode i.e. the main means of signal propagation is (I believe) conduction through buried pipes and cables with the induction field at the RX point being picked up with the loop. I have also used an E-field probe to detect the E field signal at some distance.

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

There will be some dBs variation depending on the exact positioning of the RX loop on the ground but the variations seem suggest something else. Today there were a lot of static crashes and I don't know if this upsets Spectran's DSP processing? The other variation could be soil conductivity: today was wet (raining) whereas the best results seem to be with dry settled conditions when the soil has dried out a bit.

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.

Earth-mode at 1kHz and 18kHz today

As a follow-up to my recent tests at 8.97kHz, today I repeated the QRSS3 earth-mode (through the ground) tests with my latest receiving equipment but this time at around 18kHz and 1.1kHz.  TXing at 18kHz is legal as there is no appreciable radiation. As before, the TX was 5W into earth electrodes 20m apart back at home. I wanted to see how signal levels varied compared with 8.97kHz at my 3 usual test sites out to 6km distance from home.

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)

Good results with new VLF earth-mode RX system

Today I carried out a test with my new simpler RX system for VLF/LF earth-mode "through the ground" communications, testing with a 5W TX into 20m spaced earth electrodes at 8.976kHz.

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.

Frustrating day at both ends of the spectrum

Today I tried two experiments. Firstly a continuation of my tests with the loop and E-field probe out in the fens on 8.977kHz and then tonight, another over-the-horizon test on 481THz.

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

Failure 2 at 481THz: I tried a third attempt at my over-the-horizon QRSS3 reception of my signal on a 820Hz sub-carrier. This has been successful in the past. The 100mm lens TX was carefully aligned just to the right of our local windmill on "the hill" and I traveled to what should have been a direct line path (but over the horizon so the signal has to be scattered) about 3.5km away. Optical conditions looked good with clear visibility of street lights in Burwell. Well, I am pretty certain that I was able to hear my signal by ear in the headphones keying away slowly, but because of a lead failure (later fixed) and then a strong interfering signal very close to the 820Hz sub-carrier, I failed to see my signal with Spectran and make a screen capture. I think this QRM signal is related to new street lighting as panning the horizon brought up this interference on most street lights at around 3km range. The solution will be to choose a different sub-carrier tone frequency. BTW, it is fun to hear the strobe lights from aircraft: these are very strong and can be copied well off the direct path by scattering.

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.