Rob Henke — A field guide to metal detecting: coil | robhenke.com
Coil size sets the ceiling on depth, and frequency decides how much of that ceiling you keep — here are the working numbers.
A 15 cm coil typically reaches 15-20 cm on good ground. That covers freshly lost coins, most jewellery and anything in the top layer of a ploughed field. It is the size most machines are sold with because it is light, quick and separates targets well in trashy ground.
A 25-30 cm coil reaches 25-35 cm on good ground. This is the answer for coins deeper than 20 cm: the target sits inside the coil's working range rather than at its edge, so the signal has body and the target ID has a chance of repeating across sweeps.
A 40 cm coil reaches 35-50 cm in mineralised soil, where smaller coils lose depth to the ground itself. It is heavy and slow over stubble, and it sees past small trash to the larger, deeper targets.
On narrow screens, swipe or scroll the plate sideways.
Frequency pairs with size. Lower frequencies go deeper: a single-frequency 5-15 kHz coil out-reaches an 18-38 kHz coil, though it separates small gold less well. A wide-band multi-frequency coil sweeping roughly 4-30 kHz finds silver coins at 20-30 cm while still catching gold at 10-15 cm.
Mineralisation changes the arithmetic. Above about 10-15% by volume, a single-frequency machine chatters and usually calls for 15-18 kHz — which is why the big-coil figures for difficult fields pair naturally with that band.
Before trusting any of these numbers, air test your own machine: a US quarter at 15 cm and a euro at 20 cm should return the same ID across 3-4 sweeps. If they do not, no depth chart means anything until the settings are fixed — see the target ID page for what a stable number looks like.
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