For a given object size there comes a point where detection range will fall as coil size is increased. This is illustrated by the accompanying chart. The coil diameter and detection range can be in any units i.e.inches, feet, centimetres provided both scales are the same. All the solid curves to the left of the 45 degree line show increases in range, while the dotted curves to the right show decreases in range. For example take an 11in coil (marked on Coil Diameter scale) and you find that you can detect a small object at just 5.5 inches (A on the Detection Range scale), the intersection falls directly on the 45 degree line and any increase in coil diameter would be on a decending curve giving less range. If, however, you can detect an object at just over 11.5in (B) with the 11in coil, then you can follow the curve up to get increased range with larger coils. A 20in diameter coil will give 14.2in and a 30in coil 15in range. You can see that as the curve approaches horizontal, a big increase in coil size only gives a small increase in range, the difference between a 20in and a 30in coil is less than an inch in range. Continuing along the curve, a 40in coil only gives the same range as a 20in and beyond that it will fall rapidly. These curves do not take into account the fact that larger coils will pick up more noise so that the full potential may not be realised. However they can give an idea as to what improvement is possible. Say you have an 8in coil and you can detect a coin at 12in. Find that intersection on a curve and follow the curve up. You can see that a 15in coil will gain you another 4in; quite significant. The curve maximum is always when the detection range equals the coil radius e.g. 5.5in for an 11in coil.
Eric.