Lockdown Challenge: Mole radio

So, our final experiment from Neil Downie looks at how radio can work underground – a problem as radio waves won’t travel underground. This is an experiment with real-life applications, as emergency communications in mines need to cater for broken wires. So how do you send signals through the ground without (underground) wires?

Lockdown Challenge #46: Radio Mole goes underground

You may not have hammered large nails into your lawn, but strangely this is the secret to unlocking underground communication. Now radio waves don’t go through solid rock. But that doesn’t mean that you can’t communicate underground. Electric currents will flow through ground, and currents can carry speech or music. Here is how you can have your very own Mole Radio channel for you and the animals who live under your lawn.

For the transmitter, you’ll need a radio or CD player or something with an audio output and an earphone socket with matching plug. You’ll also need a receiver: for example, an amplifier-speaker combo, such as those sometimes used on computers. And, of course, large (10-15cm) nails and lots of wire.

First push (or hammer) in your transmitter nails on opposite sides of the lawn. You can put them 15-20m or more apart. Maybe pull the nails out after putting them in and use a bucket of water around and in the hole before putting them back. This will help make a connection between the nails and the soil. Now hook up wires from the nails to the earth and the output signal of the earphone plug and your Mole Radio channel is ready to transmit.

Now for the receiver. Push the two nails into the ground 2m apart on the lawn, in a line roughly parallel with the line between the transmitter nails. Now hook up a metre or so of wire to the signal and earth wires of the amplifier and clip these onto the nails. Try different positions to see how they affect the receiver output.

Some amplifiers will not work with DC on their front end. Adding a capacitor (50nF-500nF) in series with the input will fix that. Also, if you have an amplifier with a lower gain, you could add a single transistor amplifier to boost the gain (see drawing). Or just use earphones on the output from the amplifier.

If the lawn isn’t too dry then one nail may be enough for the receiver. Hold the other wire in your hand and your feet work as the other ground contact.

The current between nails in the ground flows closer to the surface if the surface is porous soil with a high humidity level. The same applies to wet sand on a beach. The way the current goes in the ground depends on what is down there. In fact, archaeologists use that to look for ancient buildings, as do prospectors searching for valuable minerals. Finding gold in your garden is probably difficult though!

In the simplest case of a uniform volume of ground, the voltage at any point will follow a simple 1/r law from each nail, so voltage to receiver ΔV = Vo (1/r₁ – 1/r₂). If conduction is mainly near the surface, then the voltage from each point will follow a 1/log_e(r) law from each nail. You can see from the two plots that surface conduction usually gives more ΔV to the receiver.

Deep conduction (uniform conductivity):

Near-surface conduction:

Mole radio has been tested for real, for use in mines, in emergency situations where underground phone wires are broken. It may well come into general use. Finally, if one channel of underground communication isn’t enough, you could add another channel. By adding nails on the other edges of the lawn and feeding it with another signal, you may find that you can position the receiving nails to ‘null out’ the Mole Radio NorthSouth signal, while still getting Mole Radio EastWest.

If you liked this, you will find lots more fun science stuff in Neil Downie’s books, like ‘The Ultimate Book of Saturday Science’ from Princeton University, and for lots of other things (and a free copy of the ‘Exploding Disk Cannons’ book), visit www.saturdayscience.org. In line with this experiment, Neil’s current work includes developing a new ventilator system to support people with breathing difficulties – get more information on this great project here: Exovent.org. 

There is the back catalogue of Lockdown Challenges from the past year to choose from if you are looking for more options. The IET also has a host of resources that adults can use to engage children with the world of STEM.