I maintained a VLF through the earth transmitter for a while, it was a 15 km loop running at 470Hz and it could go about 500 metres through solid rock to a receiver about the size of a small lunchbox that had a ferrite coil tuned to match.
Loop ran at about 5 to 6 amps at 250 volts (~1200 watts) and was extended now and then, so there was a rack drawer full of matching circuitry to help get it to resonate nicely.
Used FSK at +/- a couple of Hz to very slowly encode text messages, used to take a minute to encode 16 characters, but there were a few “standard messages” for emergencies that could be sent in just a few seconds of coding.
The 470Hz tone got coupled into everything - you could hear it on phone lines, audio on network cameras, anything with a microphone and amp within a hundred meters of the loop would pick it up and you could hear the FSK shifting to a different tone every second or so.
The loop had quite a bit of capacitance. When there were loop faults you’d disconnect the amp and matching circuit and test the loop with a megger at 500 volts. It would take a few seconds to charge and stabilise the reading compared to near instant readings when meggering normal electrical circuits.
When there were no ground faults it would store that charge and give you quite the belt for some time afterwards if you didn’t drain it. So after the first couple of times you learned to short the loop after testing!
Ah yes, I know some of those words.
Mining?
Also, how did you spread your loop? Was it continuous or did you have splices? What gauge was it? Was it just lying on the ground?
Yep, mining.
It ran throughout the mine and the mine layout was such that it was basically a vertical loop that was pinched in the middle with two lobes either side. Basically it ran up in the top corner of tunnels, and just hung vertically in ventilation shafts and boreholes when needed.
Gauge was about
6 or 8 mm2edit: 10mm2 (8 AWG), from memory the overall loop resistance was about 20 ohms.Plenty of splices due to damage from heavy equipment, and there were a few spots where we could break the loop and test which part was shorted or open.
You could use just standard electrical wire connections like BPs as it wasn’t particularly high voltage or current and there was no attempt to make it radiate “nicely”, plenty of spots where we used twin core cabling to extend the loop to some section and basically the opposing currents would cancel the field on that length of twin.
Used for underground messaging and blasting. If you search for " MST PED system" you might find some info, it’s pretty obsolete now that most places use Wi-Fi and vhf uhf radio on leaky feeder underground.
From what i can calculate 5000m dipole gives a frequency output of 27,056.269 Hz
