VLF earth mode

Many years ago I did quite a bit of experimenting below 10kHz. This was reception of G6ALB at 3km on 8.97kHz. I think he was running about 40W. This was 2011.

See https://sites.google.com/site/sub9khz/vlf-using-earth-mode.

VLF earth-mode experiments 2010

Way back in 2010 (when I was fitter!) I used to enjoy going out to see how far away I could detect my VLF earth-mode beacon running 5W. The furthermost I got was 6km, but much further would have been possible with more power and a larger baseline. Indeed, if the power is increased and the baseline larger I know of one station that has been received 100s of kilometers away. The RX station was able to detect the radiated component that was probably just microwatts. Earth-electrode "antennas" have been used successfully on  radiated tests at even lower frequencies with remarkable ranges being covered.

VLF earth-mode TX

Looking back over some of my old photos, I came across this photo of my 5W earth-mode VLF transmitter, that was last used over 10 years ago. 

The 3C90 toroid on the left is to match the low impedance output from the TDA2003 to the earth-electrode "antenna" in the ground. In the centre is the TDA2003 audio amplifier. On the right is the keyer IC that can produce QRSS or CW.

It would be good to do some more experiments in the future, if my health allows.

12 years ago

Earth-mode experiments in 2011

 
This was my earth-mode signal 4.5km away from home in 2011 at 1.095 kHz using Spectran software on the PC. Strictly speaking, I suppose this is ULF!

It is a long time since I have done any earth-mode experiments, but my health has got in the way. It would be good to do some more experiments.

VLF beacon TX

This was the beacon TX I built for earth-mode tests at 8-9kHz. It can send either CW or QRSS at about 5W. It is some years since it has been used. A range of 6km was the best I achieved by utilities assisted earth-mode. 

I don't think it has ever been used at this QTH and we moved here in 2013! My stroke rather stopped field work sadly.  I would like to do more, but fear this will not be possible. I am sure much further would be achievable. 

Interesting that earth-electrodes of sufficient size combined with plenty of power have been used in radiated sub-10kHz tests.

Earth-mode VLF

In days past, I managed a fair bit of success with VLF earth-mode. This is a form of VLF communication that many can enjoy. Gear can be simple and low power.

See https://sites.google.com/site/sub9khz/vlf-using-earth-mode/g3xbm-earth-mode-blog .

https://drive.google.com/file/d/1-ytg1yoyiezffsfSqdL2kWZW_Fx3sbru/view?usp=sharing .

10 years ago on 1.095kHz at 6km

Ten years ago I was on 1.095kHz earth-mode.

VLF earth-mode

It is many years now since my health allowed me to carry out VLF earth-mode experiments. Results obtained with 5W and simple kit amazed me. I got a range of 6km and far further would have been possible. I kept a blog of my results which can be accessed via my website and old website at https://sites.google.com/site/g3xbmqrp/Home/earthmode (do not follow the links on the left of this old website as these are very very old and may not work).

As I understand it, OFCOM is not interested in licencing earth-mode kit as long as no "harmful interference to wireless telegraphy" is caused. To be honest,  I think conduction modes are a total puzzle to them! Without (free) software ranges would be much shorter.

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

VLF earth-mode

Some years ago G6ALB did some earth-mode tests with me. He was running 40W and was a very strong signal with me 3km away in the next village as the photo shows. He is the QRSS signal, not the wobbly line! This was at the old QTH. This was at 8.7kHz.

VLF earth-mode blog.

Years ago one of my favourite activities was experimenting with VLF earth-mode communications through the ground.

I would put a beacon on at home and go out to see where it could be copied.  Ranges surprised me and far further would be possible. Sadly, I can no longer do these experiments, but this is a rich seam ripe for others to carry on. Certainly higher power and bigger baselines would help. At some point signals that are radiated would be detectable at great range. Very stable (GPS locked?) signals would allow very long integration times.

My blog is still there on my original website. Hopefully, this will give you some ideas.  This old website is not maintained, so please do not follow the links on the left as they may not work!

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

Field work?

My tiredness is still a problem but I am getting closer to restarting field work at VLF.  I bought a new soldering iron and have found a replacement bit for my old Weller iron (it needs a plug removing before I can use it though) so I should be able to match my 5W earth-mode VLF beacon to the earth-electrodes at this QTH. This will then allow me to do a trip to my first location to see if I can copy the signal. I hope to do all this in the next few days. It may need my XYL to help, but I am sure she will not mind! You may recall, I tested the RX side with Spectran in the lounge a few weeks ago and could copy the earth-electrode VLF beacon (running QRSS3 into a dummy load) using an E-field probe.

Proposed new 474.2kHz Earth-electrode baseline

The yellow line shows the approximate 15m baseline used currently on MF WSPR.

The red line shows the proposed new earth-electrode baseline which should place the northern electrode well clear of buried pipes and wires. It should be better, but you can never be sure until you try it! I (or a friend) need to hammer in some new ground rods first at either end of the baseline wire.

More VLF earth mode tests

Tomorrow morning, all being well, I hope to do a further test on VLF earth mode at 8.97kHz using my new Asus X101ch netbook using Spectran software and a simpler loop preamp on RX. Initially I'll test locally to check results are as expected before venturing further with the new (simpler and smaller) RX system.

Also, I want later to try TXing with a large, single turn, horizontal loop on the ground around the garden rather than the earth electrodes. On the basis that I am coupling into the ground and utilities, a horizontal loop might be as effective at coupling as the earth electrode antenna. A straight comparison of one against the other will be worth trying.

I also want to try WSPR at VLF using both my PCs.

Earth-mode VLF RX kit updated

Today I've been updating my VLF receiving kit ahead of doing some further field tests, probably on Monday. I've now changed my RX loop so that the resonance can be adjusted in the field using a capacitance switch box to bring it to resonance.  The capacitance to bring it to resonance is around 240nF. The loop was about 1kHz off-tune so that the signal was around 4-6dB weaker than it should have been. Now I can tune the loop for maximum S/N in the field and can adjust the loop to a variety of test frequencies.  I've also peaked the 8.97kHz tuned circuit on my earth-electrode receiver input and fixed a faulty connection cable.

Whether these changes will allow me to get any further through the ground remains to be seen.

Earth-mode 8.97kHz VLF tests

Today I did the first tests using my new 5W 8.97kHz beacon looking for signals at my usual "close" test site 1.6km from the home QTH. Good copy on QRSS3 and even solid copy on 10wpm CW. Copy was using my 30t 80cm loop on RX although results were similar using a 5m spaced earth electrode pair. Tests at greater range over the weekend. This is a recording of the 10wpm 8.97kHz CW signal at 1.6km .

QRSS3 signal at 1.6km using a 30t 80cm loop on RX

10wpm signal at 1.6km using a 30t 80cm loop on RX

As the traces show, there are a lot of strong interfering signals around in this part of the spectrum. With an E-field probe there were lots of telemetry like signals audible which I think are being carried over the national grid overhead cables. These were about 0.5km away from the test site.

New VLF Beacon TX

Today I completed the build of my new VLF beacon transmitter. The beacon puts out 5W into a 50 ohm load, which is very close to the resistance of my 20m spaced earth electrodes. The beacon operates at either 8.970kHz or 4.485kHz with (a) continuous carrier, (b) 10wpm CW or (c) QRSS3.

VLF Earth-mode Beacon Transmitter

The main changes were to separate the keyer and the PA, mount the whole unit in a larger metal box and to use a 3C90 matching transformer with a fixed turns ratio having measured the ground resistance, which does not change greatly. On a continuous carrier soak test the case temperature only rises about 10 deg C, so frequency drift in the oscillator divider frequency source should be only around 0.1Hz at final frequency.

Having got the beacon finished, the next stage is to start work on some improved receiving kit and to see what sort of range can be achieved through the ground. My best DX so far with the original beacon was 6km, but this is certainly not the limit.

3C90 cores at 8.97kHz

The output of my TDA2003 based VLF earth-mode transmitter is around 5W into 4 ohms. The new semi-permanent earth mode "antenna" just installed measured at around 50-60 ohms resistive at 8.97kHz.  I used an AC potential divider technique to check this. Today I wound a small 3C90 based transformer using the advice I got from various people yesterday and it works very well, matching the TDA2003 perfectly to the earth electrode pair. I managed to destroy my K1EL message keyer's 5V regulator (and the keyer IC too!) so a rebuild is required before I go out into the field again to do some RX measurements. All being well these new tests will start in the next few days.

My SFH213 PIN photodiodes arrived today and my 12W Phlatlight LEDs are due next week. I'd better crack on with the new VLF earth-mode tests before these arrive as I will want to try some over-the-horizon 481THz tests with the more powerful optical transmitter and more sensitive detectors.

A return to earth-mode VLF experiments

This afternoon I installed a more permanent earth-mode ground system to use in forthcoming tests at VLF through to 500kHz. Instead of bringing the 2 earth connections into my upstairs shack, as I had done previously, I have now installed a couple of grounds and wires that come into my "designing" shack downstairs. This means I can now run a lot more tests using the test equipment at my disposal. It also means I do not tie up equipment in my "operating" shack upstairs when doing earth mode beaconing.

The diagram shows the current arrangement of the grounds and wire. At its highest point the wire is 1.5m above ground, running along the back garden fence. It is invisible.

Tomorrow I hope to get the ULF/VLF earth-mode beacon TX on-air initially on 8.97kHz and 1.147kHz in QRSS3 and QRSS30 and carry out my usual reception test at a point 1.6km from home where the signal is usually strong. Subject to satisfactory results with the new TX "antenna" I then intend to do a series of RX tests using new equipment out to around 10km from home.

8.971kHz VLF earthmode test

30t loop on the ground feeding E-field probe

An 8.971kHz earth mode (VLF through the ground) test was conducted today at 1.6km and 3.5km from home. The TX was 5W QRSS3 into 20m spaced baseline earth electrodes: one a 1m copper rod at the
far end of the garden and the other end grounded to my copper hot water tank in the house.

At the RX end I was using either (a) a 30t tuned loop feeding either the PA0RDT or G3XBM tuned E-field probe, and (b) the same E-field probes but fed with a 19 inch whip instead of the loop.

Good signals were received at both locations with the loop into both probes but there was no detectable signal when receiving on the 19 inch whip into the EFPs.

It was a struggle to see the difference between the PA0RDT and G3XBM E-field probe/preamps, but I think results with my tuned drain design may have been marginally better on this test.

VLF Talk - Cambridge Club Feb 10th

On Feb 10th I have been invited to talk to the Cambridge and District Amateur Radio Club on VLF through the ground although I intend to widen the scope to include all aspects of amateur radio communications experiments below 9kHz. In the last year or so this aspect of our hobby has come on leaps and bounds and what was once thought of as impossible being achieved. Surprisingly, this is a very accessible part of the spectrum in which to experiment. If you want to come along and are not a CDARC member I am sure you'd be made welcome by club members.