Audio transmission through the ground

[dr pepper]

Not sure which subject this should be in but here goes.
When I was a kid I experimented sending audio through 3 ground probes, the audio came from a car radio with reasonable o/p power, through an mains trans connected in reverse to step up the volatge into the probes arranged in a triangle, the 'point' being signal and the rear 2 being 'ground'
I got around 50m range or so with plenty of mains hum.
I wondered recently whether I could improve this range to a few hundred meters, mainly for 2 applications wireless comms to an underground bund stores and another for a means of communication that is unlikely to be detected (only because it wouldnt be thought about by eaves droppers).
Reading about ground dipoles it appears that they are capacitive rather than inductive, and a lot of the signal is shorted by the low dc resistance of said ground.
I know subs use 76hz as a means of comms.

 

[4pyros]

Not sure which subject this should be in but here goes.
When I was a kid I experimented sending audio through 3 ground probes, the audio came from a car radio with reasonable o/p power, through an mains trans connected in reverse to step up the volatge into the probes arranged in a triangle, the 'point' being signal and the rear 2 being 'ground'
I got around 50m range or so with plenty of mains hum.
I wondered recently whether I could improve this range to a few hundred meters, mainly for 2 applications wireless comms to an underground bund stores and another for a means of communication that is unlikely to be detected (only because it wouldnt be thought about by eaves droppers).
Reading about ground dipoles it appears that they are capacitive rather than inductive, and a lot of the signal is shorted by the low dc resistance of said ground.
I know subs use 76hz as a means of comms.

How did you get "plenty of mains hum" running off a car radio?
"Reading about ground dipoles it appears that they are capacitive rather than inductive, and a lot of the signal is shorted by the low dc resistance of said ground."
Alot of it will depend on how wet the ground is. It may not work at all when the ground is dry.
"I know subs use 76hz as a means of comms."?
Subwoofers reproduce a range of low frequnces. like 40hz to 150hz maybe with a center of 76Hz.

 

[MikeMl]

The main hum comes from the mains, which uses the earth for lightning protection. Every pole has a copper plate on its bottom which is connected up the pole to a horizontal ground wire running from pole to pole.

To increase range, move the sending (and receiving) electrodes further apart. The receiving electrodes should be orthogonal to the transmitting electrodes.

 

[k7elp60]

I once did the same thing only I didnot use a mains transformer. I used a tube type output transformer to step up the voltage and I only used two prods in the ground. I also had a circuit that would allow me to make sure the earths impedance(distance between the two prods) matched the impedance of the transformer. The amplifier was battery powered. I don't recall the distance but it was significant.

 

[dr pepper]

Searching the net has shown that distance isnt very good with this you did well.
I think I'll do a test this weekend with a similar setup I did 30 years ago.
Yes mains hum comes from power lines, but I'm not sure if its electric field radiation from ground rods, or magnetic radiation from current carrying inductors, probably the former.
Wavelength is also an issue, the wavelength at audio frequencies stretches to the next county so even a few hundred M is going to be near field, a 1/4 wave dipole at 1kc would be 150km!

P.S. when I said subs I meant submarines, 76hz seems to be a chosen frequency for underwater comms.

 

[4pyros]

P.S. when I said subs I meant submarines, 76hz seems to be a chosen frequency for underwater comms.

Oh you had me going there.
I wonder why 76Hz?

 

[4pyros]

Aparently it has to do with the diameter of the earth.

Difficulties of ELF communication

One of the difficulties posed when broadcasting in the ELF frequency range is antenna size because the length of the antenna must be at least a substantial fraction of the length of the waves. Simply put, a 3 Hz (cycle per second) signal would have a wavelength equal to the distance EM waves travel through a given medium in one third of a second. Taking account of refractive index, ELF waves propagate slightly slower than the speed of light in a vacuum. As used in military applications, the wavelength is 299,792 km (186,282 mi) per second divided by 50–85 Hz, which equals around 3,500 to 6,000 km (2,200 to 3,700 mi) long. This is comparable to the Earth's diameter of around 12,742 km (7,918 mi). Because of this huge size requirement, to transmit internationally using ELF frequencies, the Earth itself forms a significant part of the antenna, and extremely long leads are necessary into the ground. Various means, such as electrical lengthening, are taken to construct practical radio stations with smaller sizes.

The US maintained two sites, in the Chequamegon-Nicolet National Forest, Wisconsin and the Escanaba River State Forest, Michigan (originally named Project Sanguine, then downsized and rechristened Project ELF prior to construction), until they were dismantled, beginning in late September 2004. Both sites used long power lines, so-called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometres (14.0 to 28 mi) long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system.

 

[Dean Huster]

When considering wavelength, remember a couple of things. The OP was shoving AUDIO into the ground, not RF. Audio uses the speed of sound against frequency to determine wavelength. RF (the ELF the U.S. Navy has for communicating with submarines to signal them to "come up for air" and receive traffic via the usual high-speed satellite link) of the same frequency uses the speed of light against the frequency to determine wavelength. The ELF transmissions were short in content but long in the sending time because of the frequency involved. There was no way to send the usual message content unless you wanted to wait weeks for the entire message to be sent.

 

[ronsimpson]

The OP was shoving AUDIO into the ground, not RF. Audio uses the speed of sound against frequency to determine wavelength.

"Audio" and the "seed of sound" only works when the sound is going through air.
"Audio" running down a wire has to do with the speed of electricity in copper.
"Audio" in the form of electricity passing through dirt travels at the speed of electricity in dirt. No sound waves.

I have a shortwave radio that go down to 100khz. I think years ago I monitored a band at 130khz.
Then there was a group that transmitted into the ground at about 30khz. (no antenna in the air) Some used a short ground rod as a "antenna" and a deep pipe like a well casing as "ground".

Band Frequency
ELF 3-30hz
SLF 30-300hz
ULF 300-3khz
VLF 3k to 30khz
LF 30k to 300khz

 

[dr pepper]

Maybe I'm missing something, 1khz is rf, when fed to a suitable aerial, and as far as I know when shoved into the ground works in the same way.
The russians have a vlf aerial that looks like a power distribution grid that operates on a few khz.
So a 1kz eh field radio wave will still have the same properties as any other, except for the wavelength.

As I understand it electrons flow slightly slower in a piece of rg58 than in air, thats why we make a dipole made out of coax slightly shorter than 1/4 wave.

 

[4pyros]

As I understand it electrons flow slightly slower in a piece of rg58 than in air, thats why we make a dipole made out of coax slightly shorter than 1/4 wave.

I dont know about that one, anyone else care to comment?

 

[dr pepper]

Heres a velocity factor chart, rg58 is 0.66, thats a lot different.

http://www.nr6ca.org/vf.html

 

[JimB]

As I understand it electrons flow slightly slower in a piece of rg58 than in air, thats why we make a dipole made out of coax slightly shorter than 1/4 wave.

I dont know about that one, anyone else care to comment?

OK, I will bite!

In a conductor, the electrons move quite slowly, just off the top of my head I do not know the speed, but it is quite pedestrian.

The speed of the "energy wave" in a transmission line such as coax or a twin wire line is significantly slower than the speed of light (electromagnetic wave in free space).

In the specifications for a coax cable is the term "Velocity Factor", the numerical value of the velocity factor is of the order of 0.6 to 0.95 depending on the physical construction of the cable.

The velocity factor is (wave velocity in cable)/(speed of light).

From memory, the wave velocity in a single uninsulated wire is (slighty) less than the speed of light.

As far a building a dipole using coax for the elements is concerned, the necessity for using a shorter length probably comes from fact that the coax is insulated with (usually) PVC which raises the capacitance of the cable, reducing the velocity of the wave on the cable.

JimB

 

[WTP Pepper]

Why not put the audio on a sub carrier? It will reduce mains hum.

 

[Dean Huster]

"Audio" and the "seed of sound" only works when the sound is going through air.
"Audio" running down a wire has to do with the speed of electricity in copper.
"Audio" in the form of electricity passing through dirt travels at the speed of electricity in dirt. No sound waves.

Horsehockey!!

The speed of sound in air is around 1100 feet per second. The speed of a radio signal in air is pretty much the speed of light.

"Audio running down a wire" has two definitions: (1) an electrical signal much like the audio in a piece of RG-58C/U coaxial cable coming from an audio oscillator output, which happens to be pretty much the same as the RF output from an RF signal generator and either one of which is travelling down the cable at the speed of light -- actually a bit less than the speed of light which is where the velocity factor cited by drpepper above comes in, necessary for the proper operation of a time-domain reflectometer; and (2) an acoustic signal that would be made by tapping on a wire. A better illustration of this would be tapping on a length of pipe or I-beam. In this case, the SOUND travels faster than the speed of sound in air (I'll try to find those tables later). The speed of sound in water is of special concern for the sonar technicians aboard a submarine. It's also faster than the speed of sound in air. In general, the speed of sound in any material is measured in feet per second as opposed to miles per second which is how we'd measure a radio wave or light.

In general, we're confusing this term "audio" to mean any frequency within the audio range of human hearing. When one uses a transmitter and antenna to transmit a signal at 15KHz, we're not talking audio -- we're talking radio. Audio travels in air at 1100 ft/s; radio, even at the ridiculously-low frequency of 15 KHz travels at about 186,000 MILES/s.


https://www.engineeringtoolbox.com/sound-speed-solids-d_713.html
https://www.engineeringtoolbox.com/sound-speed-liquids-d_715.html

 

[ronsimpson]

The OP was shoving AUDIO into the ground, not RF. Audio uses the speed of sound against frequency to determine wavelength.

Horsehockey!!

So if I get this right; You said audio in ground (using electricity) travels at the speed of sound. Then you turn around and "Horsehockey" that comment.

I think that we agree that audio and RF travel very fast in the form of electricity. Audio of the form of sound waves travel slow.

 

[k7elp60]

Even a single conductor of wire has a velocity factor. As a rule of thumb it is 0.95. In simple terms an antenna matches the impedance of the transmission line to the impedance of air, which happens to be about 377Ω. If one is to feed audio into the earth and they match the impedance of earth then the earth becomes the medium. If you reread my first post I was able to achieve this. It was not for communications between two units, but it was used in helping to find precious metals. All substances have a frequency that they resonate at. Gold, silver and other prescious metals have them too....I don't recall what they are. I do know this the ground is very lossy when used as a medium for transmission of AC waves. Going one step further pure water is a insulator, so what makes water a conductor is impurities in the water. If the earth from point A to point b were damp and had the same chemical process between the two points one would get maximum transmission between the points.
The reason it takes time for RF to travel the length of coax is that the capacitance of the two conductors has to charge and discharge as the frequency travels from one end to the other.

 

[4pyros]

Its all still waves. HA HA

 

[Dean Huster]

I think that we agree that audio and RF travel very fast in the form of electricity. Audio of the form of sound waves travel slow.[

I do agree with that summary. I think that a lot of the problem is with some bizarre terminology.

There's even a difference, albeit small difference, between the speed of radio waves travelling in air vs. a vacuum. TDRs have to have that velocity factor set in so that they can accurately measure distances to faults. Who wants to dig up 800 feet of phone line when you can find the fault within 30 feet?

 

[unclejed613]

if you really want to look at some serious work that has been done in this field, look online for a magazine called "Speleonics". since spelunkers are often called upon to help when people are trapped underground, they have done a lot of research into ways to use the ground as a communications medium, starting with devices known as "trench phones" (and i did a google search for that term and came up with zippo for some reason..... maybe there's another name for it i can't think of right at this moment) and also 1khz-3khz radio waves magnetically coupled through the ground.