In comments following a recent blog (Remote Meter Reader Data Transmitter Power Supply), a commenter mentioned “sheet resistance.” This came up in a string of comments that concerned alternate modes of communication. The blog addressed methods for utility companies to remotely read your gas, electric, and water meters. We were postulating about techniques to pass communication signals through the water. No conclusions were reached, but it reminded me of a communications method that I played with as a kid.
The purpose of this communications method was to pass baseband signals (i.e., not modulated RF signals) from point A to point B through whatever conductivity there was in the ground. When I say pass signals through the ground, I don't mean that I would send some current through a long wire from A to B and then (to comply with Kirchhoff's Law) bring that current back through the ground's conductivity.
Instead, the signal current was injected into the ground at point A. OK, so this is not really a point, it's a line segment. There would be a couple of conventional ground rods (long steel stakes) that were driven into the ground as far apart as was practical along line segment A. Practical here means tens to hundreds of meters apart. To inject the current, a high-power audio or servo amplifier would be used. Preferably, this would be a balanced output amplifier.
To detect this signal, you would drive two more stakes into the ground along line segment B. To these, you would attach an amplifier with a balanced input. This amplifier's output would give you the desired signal (e.g., audio or data).
There are a few important factors to keep in mind. There is no separate conductor to act as a return path — all current flow is in the ground. There must be sufficient moisture in the soil or subsoil. Line segment A and line segment B are parallel.
Electrically, it looks like this:
A1 and A2 are the stakes for the drive signal (the input). B1 and B2 are the stakes that extract the signal (the output). R1 is the lateral or load resistance that the drive amplifier has to work into. It's shown as a single resistor, but it's actually the sheet resistance that manifests across the two points, A1 and A2. R4 is the lateral resistance presented to the receiving amplifier input port. Again, it's the sheet resistance that manifests across the two points, B1 and B2.
R2 and R3 are the longitudinal resistances from line segment A to line segment B. As above, these are just simplifications of the resistances for the sheet.
In practice, you can't send signals very far this way. Bandwidth is not very wide and signal to noise ratio is also not likely to be especially good. The attenuation is pretty severe unless your line segments (A and B) are pretty long. If A1 and A2 (and therefore B1 and B2) are that far apart, you'll need lots of wire to get to them. And at some point, you may just as well run the wire from A to B. Still, it's an interesting communications mode.
Have you ever used anything like this? Over what distance could you make it work?