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!
That part’s easy, you just mine a tunnel, and then you mine a parallel tunnel 500m away from the first one. Ta-da , 500 metres of solid rock now exists between the two.
Note: mining a tunnel through solid rock can be somewhat difficult if you don’t have the right equipment, YMMV.
But seriously, the amount of dicking around maintaining the loop and the limited abilities of VLF meant that you would quite quickly have to run leaky feeder and bidirectional amps for two way radio, or you’d build a network using fiber and use Wi-Fi. We primarily used the system for remote blasting, and then once technology progressed a bit the initiators were connected via the network instead.
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 mm2 edit: 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.
You could normally blame your GSM phone for that, it would be woken up by a cell tower to do a bit of high powered chatter before actually ringing or receiving a SMS.
Very slow frequency modulation. Slow enough you could hear the tone change slightly as it shifted modulation every few seconds.
Frequency Shift Keying is the more accurate term for this, as it’s not a continuous shift up and down of the frequency like you’d get with FM radio, it’s stepped.
If you imagine a higher tone being decoded a “1” and a lower tone being a “0”, basically it was something like that. Listen long enough and you’d get enough 1s and 0s to build up a message.
i just had an image of an early nondigital version of this. some unwashed musician you hired sitting at the bottom of the shaft with an earpiece jumps up ITS AN EFFF SHARP EVERYBODY RUN
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!
My first thought was “dude, where’d you get 500 meters of solid rock?”
The stuff’s everywhere, it’s actually pretty hard to get away from it haha.
Well I got plenty of sand
Okay, where’d you get only 500 meters of solid rock?
That part’s easy, you just mine a tunnel, and then you mine a parallel tunnel 500m away from the first one. Ta-da , 500 metres of solid rock now exists between the two.
Note: mining a tunnel through solid rock can be somewhat difficult if you don’t have the right equipment, YMMV.
But seriously, the amount of dicking around maintaining the loop and the limited abilities of VLF meant that you would quite quickly have to run leaky feeder and bidirectional amps for two way radio, or you’d build a network using fiber and use Wi-Fi. We primarily used the system for remote blasting, and then once technology progressed a bit the initiators were connected via the network instead.
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.
I was about to blame you for the sounds my pc speakers used to make, but I didn’t live near any mines.
You could normally blame your GSM phone for that, it would be woken up by a cell tower to do a bit of high powered chatter before actually ringing or receiving a SMS.
TIL, Dave is cool
amplitude modification or frequency modification?
Very slow frequency modulation. Slow enough you could hear the tone change slightly as it shifted modulation every few seconds.
Frequency Shift Keying is the more accurate term for this, as it’s not a continuous shift up and down of the frequency like you’d get with FM radio, it’s stepped.
If you imagine a higher tone being decoded a “1” and a lower tone being a “0”, basically it was something like that. Listen long enough and you’d get enough 1s and 0s to build up a message.
i just had an image of an early nondigital version of this. some unwashed musician you hired sitting at the bottom of the shaft with an earpiece jumps up ITS AN EFFF SHARP EVERYBODY RUN