Further tests on 1kHz using conduction and induction

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.

Earth mode and induction testing at 1kHz

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....

Even with 20-30dB of 50Hz rejection it was still an issue in many places. Walking across the field behind my house, and well away from cables and pipes, the signal could be heard in the earpiece at RS53 to a distance of 0.35km. Mains hum and LW/MW interference (the RX is not in a screened box) rather than signal level  limited further range. This week I'll add more high pass filtering, put the RX in a screened box and make the whole thing more immune from BC breakthrough. As I can hear this signal by ear at 0.35km with essentially no narrow filtering, then I'm confident that twice as far will be possible with QRSS/WSPR and a PC based receiver. Not bad for 4W at 1kHz.

Thoughts on VLF Earth-Mode and Induction DX

Earth-mode (through the ground conduction) and induction communication (mutually coupled coils) have an inverse cubed attenuation with distance. Think about this: I reached 0.3km with earth mode for an audible CW signal with around 4W RF and a simple 5m base receiver and 10m base transmitter. To double this range, all other things being equal, means increasing power by 18dB to 250W. To double range again means raising the power to kWs. Conversely, improving the detectable sensitivity threshold by 18dB (increasing the electrode spacings at each end, using WSPR or QRSS, etc) reduces in power needed for a given range dramatically: my 4W signal could be detected at 0.6kms or a just a 63mW signal could be detected by earth mode at 0.3km. In reality, especially in urban areas, water pipes and cabling may help "propagation" and achievable ranges may be greater. DX is relative, especially with conduction and induction at VLF.

Useful American sub-9kHz experimental VLF site

VLFradio.com is a most interesting site. Among the stuff there are the results of some induction communications tests with a 90m circumference TX loop out in the desert away from man-made interference. Using just 0.5W a range of 1 mile (1.6kms) was achieved at 1.8kHz and with higher power up to 2 miles (3.2kms). These signals were copyable by ear, so further still would be possible with weak signal techniques. Also on the site lots of recordings of these tests as well as other VLF noises and man-made transmissions

Sub-9kHz induction comms resource

John F5VLF/G3PAI has brought a very useful website about induction mode communications below 9kHz to my attention. This site is mainly about cave location and beaconing systems, but there are some neat schematics for local coverage (thousands of feet range) TX and RX circuits that would be suitable for QRSS and similar experiments in the "Dreamers Band" as some have called it. See http://radiolocation.tripod.com/

Sub-9kHz Induction Field DX?

With all the discussion recently on the LF Reflector about how to radiate a signal on sub-9kHz I'm left wondrering how far, practically, one could expect to get simply with sub-9kHz electromagnetic induction using multi-turn loops at each end.

It's quite easy for anyone to generate 50-100W or so and feed this into a largish diameter multi-turn loop. Correctly aligned and with a similar loop at the far end and using decent weak signal processing just what range might be possible using modes like QRSS or WSPR? Has anyone tried this to see just what is practical, simply using induction fields?

The Cave Radio Group (CREG) may have some answers but I'm not a subscriber so can't read any of their documents.

LATER: Jim Moritz M0BMU did some sums (see my webpage on sub-9kHz radio) and reckons that about 4.4kms is the best one could do with 100W and a 10m diameter loop and purely inductive communications. More power and bigger (low loss) loops could get a bit further still.