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I'm interested in developing a bespoke through-the-earth (TTE) radio transceiver for a measurement instrument in a limestone cave system.

The issue I have is that conventional RF signals cannot penetrate the surface due to the conductivity of the permeable material so I've turned to frequencies in the 30kHz to around 130kHz range (potentially higher, not sure how effective this would be as per current research).

The plan is to use ground spikes to inject current into the earth and for the conducting medium to serve as a channel for the signals to reach the transceiver underground, and vice versa.

My question is: As I aim to make this project as cost effective as possible, what do I need to consider when designing this hardware? Are there any existing RF modules (preferably for Arduino) that can produce signals at these frequencies directly through a home-made antenna across the ground, or do I have to build one myself? If so, how would I go about designing such a module?

I'm not interested in transmitting audio or video from the cave, only .CSV files via MySQL. I've already identified the need for a GSM module to transmit the data from the surface to an online server for remote data collection purposes - the main issue is the data transmission between the transceivers. Attached is a diagram summarising a similar system given to me by a developer of a TTE radio system.

Any help would be greatly appreciated, even if just in the form of a nudge towards some appropriate sources.

enter image description here

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  • \$\begingroup\$ Stay below 153kHz or you'll be into broadcast bands. \$\endgroup\$ – Brian Drummond Dec 20 '20 at 23:33
  • \$\begingroup\$ Noted, I don't want anyone knocking at my door unexpectedly! \$\endgroup\$ – K. Millar Dec 20 '20 at 23:44
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    \$\begingroup\$ Soil minerals and moisture being more conductive that air thus lower impedance, I would advise you use H field control more than E field and thus use loop antenna , lower the frequency and data rates with appropriate Q in the audio band such as 2400 baud modem technology with QPSK and spectral balancing with a DSP Rx after training just like a modem. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 21 '20 at 2:13
  • \$\begingroup\$ Here’s an ancient but interesting paper, reporting success at various frequencies, depths and geologic strata. core.ac.uk/download/pdf/17354936.pdf \$\endgroup\$ – Mark Leavitt Dec 21 '20 at 5:56
  • \$\begingroup\$ Through the earth mine communications (wiki article). Read it. \$\endgroup\$ – Andy aka Dec 21 '20 at 10:30

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