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Ground Communication via Sheet Resistance

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?

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15 comments on “Ground Communication via Sheet Resistance

  1. Gregst
    August 3, 2013

    Brad, essentially you are trying to communicate not through the “ground”, but through the soil.

    The factors to take into consideration:

                       1) Chemical composition of soil – is it conductive, what are its signal absorption characteristics ?

                       2) Water saturation: its characteristics after the rain would be very different from a dry condition

                       3) Interfering ground currents – selectivity would be dismal.

    So, why would we be wasting energy for heating soil ?

    Why not to use the RF ? You know all the troubles of communicating through water, why to add another level of complication by using the unpredictable, dense and highly absorbing soil ?

  2. Brad_Albing
    August 3, 2013

    @Gregst – I had suggested it just as an interesting experiment. But I acknowledge all the points you've made that would make it impractical.

  3. Netcrawl
    August 3, 2013

    @Brad that was great! communication method like this seem to be the right stuff for smart metering companies, where links are made through the grounds, the article clearly shows how things are done and its application.  I think smart companies are doing this already, the only problem here is in bandwidth there's some little problem.  

  4. Davidled
    August 4, 2013

    Signal such as audio might be highly distorted unless suppressing noise in the transmission line. For example, human voice has  many different types amplitude with frequency. I am wondering baseband signal covers all human audio. Main point is that this communication might be worked depending on how to design the Amp and filtering circuit in both transmitter and receiver.

  5. samicksha
    August 5, 2013

    I believe, if term sheet resistance is used, it is implied that the current flow is along the plane of the sheet, not perpendicular to it.

  6. Brad_Albing
    August 6, 2013

    @samicksha – Actually, you would get curent flow in all directions. Some would be flowing directly from A1 to A2. But a little would flow outward and show up across B1 and B2.

  7. jkvasan
    August 13, 2013

    @Brad,

    Can we safely assume that for a proper transmision/reception, R1 and R4 (load) needs to be high as necessary and R2 and R3 (series) , low. In a ground, can the signal distinguish between the series impedance and load impedance?

    I assume this kind of transmission is available already yet not able to understand how this works.

  8. Brad_Albing
    August 13, 2013

    @JK – Yes. That would give the best results, In actual situations, you don't have that much control over the resistance values (except to the extent noted in my blog).

    The signal can't distinguish. At the input side (across A1 and A2), all the various resistors just appear aa a single resistor (Thevinen equivalent). Again, it's a simplification – actually an impedance, with inductance and capacitance scattered thru the sheet.

    I don't know if anyone is actually using this technique in a practical, modern-day communications system.

  9. Rcurl
    August 14, 2013

    I remember experimenting with this as a freshman in high school.  I used a 50 watt public address amplifier and connected the 70 volt line output to a couple of pieces of rebar driven into the ground about 50 feet apart. About 100 feet away I had a second set of rebar “electrodes” connected to a 600-ohm headset. I remember hearing very faint audio with lots of hum.  I rotated the receiving electrodes so they were at a right angle to the transmitting electrodes and the transmitted signal disappeared entirely- as you might expect.

    Interesting stuff!

     

     

  10. Brad_Albing
    August 14, 2013

    @Rcurl – yep – that's the version of the experiment I recall from my youth. It could of course be improved with an amplifier + a 60Hz notch filter.

  11. WKetel
    August 18, 2013

    Communication through the ground is a thying that many of us have played with. The principle is that a voltage gradient is established between the driving terminals and some small voltage is induced between the detection terminals. Like a resistance grid except for the 60 Hz field due to the neutral connections of the power distribution system, and other random connections. It does work and sometimes it works well, but usually it does not.

  12. Brad_Albing
    August 19, 2013

    @Wketel – admittedly, reliability is low. So maybe confine the medium to low speed data com links.

  13. WKetel
    August 19, 2013

    Actually the reliability is not bad, but the signal to noise ratio is very poor.

  14. Brad_Albing
    August 19, 2013

    @WKetel – hence, perhaps more suitable for low-speed data transmission, rather than a voice channel.

  15. Herbertorium
    August 12, 2014

    I remember reading that this form of communication was widely used between trenches during WWI. Of course there would have been less interference from power distribution systems at that time.

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