Luis,

Thank you for the underground mapping example; nice work by the speleo team.

“As this is the near field it is supposed to work only at magnetic field due to the small size of the antenna ? So only magnetic field for underground signals at this distances ?”

At 38kHz the impedance of the transmitting loop’s field at 100 meters is ~ 25 ohms, which puts an optimized E-field (electric probe) receiver at perhaps a 45dB disadvantage, but your near-field intensity through 70 meters of average-conductivity rock and 30 meters of air may be (based on your schematic and description) as much as 0.1 pT, so the E-field (electric probe) receiver will work well at 100 meters.    

“Can we expect to receive the signal with an electric probe antenna in the near field or the only chance is to be further away ?  And then losing the signal due to the distance ….”

Based on your schematic and description: only in the near field.

At 1 km (and 38 kHz) and farther, any disadvantage of the E-field (electric probe) receiver is small.

At 1 km the near-field strength for the configuration that you described could be as much as ~ 1fT, so the near field component of the signal at 1 km would be detectable (with significant integration time and/or low noise), using an electric probe receiving antenna or a loop receiving antenna.

But due primarily to the small aperture of the loop transmitting antenna that you described, the radiated field component at 1 km is far below the threshold of detectability (perhaps 0.01 attotesla at 1km), so at distances greater than a kilometer, the near field component fades below detectability, and the far-field component of the signal fades even farther below the level of detectability.

For most practical purposes, to be detected well into the far field, a VLF loop transmitting antenna needs to be physically very large and driven by more power than batteries can practically provide.

Note: the electric-probe antenna efficiently detects the radiated component of the signal, and the radiated component of the signal decreases from the transmitting antenna through the far field, so in outdoor above-ground (“open field”) VLF signal reception, moving an electric-probe receiving antenna from a magnetic-loop transmitter’s near field to its far field never improves reception by a receiver with electric-probe antenna.

“What would be the radiated power of such antenna ?”

Based on your schematic and description, probably less than 100 pW.

Portable ULF and VLF loop transmitters are great for hundreds of meters through rock or air, but detection of ULF/VLF loop-transmitter signals in the far field requires transmitting-antenna apertures that are mechanically and electrically challenging in many independent ways

Thank you for the underground mapping example; and best wishes to the speleo team. I suppose it would be too complicated, but I wonder if they could correct their maps for angle-effects of air/rock conductivity variations, if they monitored phase in addition to angle of arrival.

73,

Jim AA5BW