Underground Signals Defy Expectations: The Sub-9kHz Earth-Mode Phenomenon

For years, conventional electromagnetic theory suggested that signals injected into the ground below 9kHz should suffer catastrophic attenuation over distance—especially at power levels feasible for amateur experimentation. Yet meticulous logs from radio amateur Roger G3XBM reveal a different reality: consistent reception of 5W continuous-wave (CW) and QRSS3 signals at distances up to 6km using earth electrodes.

G3XBM's beacon transmitter setup driving earth electrodes spaced 20 meters apart.

The Experiment That Challenged Assumptions

Between 2010 and 2012, G3XBM systematically tested earth-mode propagation across Cambridgeshire's varied terrain. Key findings emerged:

  1. Utilities Assistance Dominates: Signals traveled significantly farther (>5km) near roads with likely buried metal pipes/cables versus open fields (<0.5km). Orientation independence at distance strongly hinted at conduction via infrastructure rather than pure ground-wave or induction.

  2. Frequency Matters: Lower frequencies (1-2kHz) consistently outperformed higher ones (8-9kHz), with signals at ~1kHz showing approximately 10dB greater strength at 4-5km ranges. This challenges simple inverse-cube attenuation models.

  3. Receiver Innovation: Optimized receiving loops (e.g., 30-turn, 80cm diameter, series-tuned) outperformed E-field probes and electrode pairs at distance. A tuned-drain JFET preamplifier provided critical sensitivity gains.

  4. QRSS Unlocks Range: Slow modulation modes like QRSS3 (3-second dot length) enabled reception where even 10wpm CW failed, leveraging extreme receiver bandwidths down to 420 microhertz to dig signals out of noise.

# Simplified QRSS3 Beacon Transmission Pattern (Conceptual)
def generate_qrss3_message(callsign):
    # Converts characters to extended Morse code with 3-second dots
    morse_code = {'G': '--*', '3': '***--', 'X': '-**-', 'B': '-***', 'M': '--'}
    message = ''
    for char in callsign:
        message += morse_code.get(char, '') + ' '
    return message  # e.g., "--* ***-- -**- -*** --" for "G3XBM"

QRSS3's slow symbol rate allows ultra-narrow bandwidth reception, crucial for weak-signal VLF work.

Implications for Subsurface Communications

These empirical results have tangible implications:

  • Emergency Comms Potential: Low-power earth-mode systems could provide localized communication resilience where traditional RF fails, leveraging existing buried infrastructure unintentionally.
  • Sensor Network Design: Understanding coupling mechanisms could inform low-data-rate subsurface IoT networks in agriculture or geology.
  • Propagation Physics: The consistent discrepancy with theoretical models (inverse cube vs observed inverse square?) suggests complex interactions between geological layers and human-made conductors warranting further study.

"With 100W power (+13dB on 5W), reducing RX bandwidth to 420uHz... the range could be increased by a factor of 4 i.e., to 24km for the same S/N... assuming inverse cubed attenuation rate." — G3XBM, speculating on scaling potential after 6km success.

The Unanswered Questions

Despite breakthroughs, mysteries persist:

  • What specific infrastructure (water pipes? telecom cables?) enables the observed "utilities assistance," and what are its electrical characteristics at VLF?
  • Can true non-assisted earth-mode propagation be optimized for practical ranges beyond a few hundred meters?
  • How do soil composition and moisture (tested from frozen peat to saturated fens) quantitatively impact path loss?

G3XBM’s methodology—rigorous field testing, component optimization, and open documentation—provides a blueprint for further investigation. As regulatory bodies like OFCOM clarified earth-mode’s legal status during these tests ("non-wireless telegraphy apparatus"), the door remains open for continued amateur exploration of this spectral frontier. The 6km milestone isn’t just a distance record; it’s evidence that beneath our feet lies a propagation medium far more complex—and potentially useful—than equations alone predict.

Source: G3XBM Earth Mode Blog