Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 32 Number 1: 11-26 - January 1970
A publication of the National Speleological Society
Optimum Frequencies for
Underground Radio Communication
Nevin W. Davis
The radiating properties of loop and wire antennas through conducting rock are analyzed. Along with measured curves of atmospheric noise vs. frequency, this enables calculation of the signat-to-noise ratio (a measurement of received signal quality) for frequencies between 1 kHz and 10 MHz. For induciton surveying equipment, the optimum frequency is about 3.5 kHz. This gives an 8-db improvement in the received signal-to-noise ratio and about a 30% increase in range over the much-used 2 kHz.
For voice communication, the optimum frequencies lie near the 160-meter amateur band. The signal path attenuation is demonstrated to be not as severe as some people surmised. At a distance of 500 m from a buried 1-watt, 1-MHz transmitter (buried 100 m), using a 25-m wire for an antenna, the calculated signal-to-noise ratio at the receiver would be 70:1 during the day.
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