You are right, you don't understand. What would your test show, nothing.
Set your Disc to 1 pass the coin and move it further and further away from the coil. Measure the distance. Increase the disc and repeat. It should see the coin at a greater distance.
A higher disc will allow you to detect coin size objects at greater depths. As the object is buried deeper its signal gets smaller. It also allows you to detect smaller objects becuse smaller objects have smaller signals. You can also set your Disc lower to avoid hearing smaller bits.
Set your Disc to 1 pass the coin and move it further and further away from the coil. Measure the distance. Increase the disc and repeat. It should see the coin at a greater distance.
A higher disc will allow you to detect coin size objects at greater depths. As the object is buried deeper its signal gets smaller. It also allows you to detect smaller objects becuse smaller objects have smaller signals. You can also set your Disc lower to avoid hearing smaller bits.