Search coils explained: DD vs concentric, size, and the overlap rule
The search coil is the only part of your detector that touches the problem. Everything else — processor, tones, target ID — interprets what the coil's field brings back, and that field has a shape and a size that decide in advance what your machine can and cannot do on a given patch of ground. Coil marketing reduces this to two slogans (bigger is deeper, DD is better), both half-true and both capable of costing you finds. This guide gives the fuller picture the specialist literature teaches: the field as a volume of ground, the honest version of DD versus concentric, why width — not length — sets depth, and the sweep-overlap rule that follows from all of it.
The coil is a volume of ground, not a magic eye
Think of the transmitted field as encompassing a volume of soil, minerals included. An 11-inch concentric coil's field takes in a few gallons of dirt; step down to an 8-inch coil and the volume falls to about one gallon, cutting the ground-mineral effect several times over (s1). That single idea explains most coil behaviour. The larger the coil, the more mineralization its receive winding grabs, and the harder the circuitry must work to separate a target's signal from ground noise — which is why a big coil can lose depth in bad ground instead of gaining it (s1). It also explains the Double-D's reputation: an 11-inch DD's narrow field volume is smaller than even the 8-inch concentric's, so it suffers less ground effect while covering more ground per sweep — the combination that has made DD the standard configuration on most modern detectors, valued for depth, ground balance in adverse conditions, and separation in trash (s1). Configuration, in other words, is not about the coil's outline — the winding arrangement inside defines the type, and a coil that looks like a DD may be wound concentric (s4). The named configurations run from concentric and DD through monoloop (the pulse-induction arrangement) to imaging and 2-box designs (s4).
DD vs concentric, honestly — including the myth
| Concentric | Double-D | |
|---|---|---|
| Field shape | Cone — widest at the coil, narrowing with depth | Blade-to-football — a lengthwise field, more uniform front to back |
| Home ground | Clean, mildly mineralized soil | Mineralized soil, salt beaches, iron-laden sites |
| Strength | Pinpointing precision and discrimination; ~12-inch central depth in clean ground | Stable depth across the sweep (~6–8 in cited in comparisons), vertical separation in layered trash |
| Weakness | False signals and lost depth as mineralization rises | Less central depth than an equal concentric in clean ground |
Figures from published coil comparisons, checked October 23, 2026 (s2). Now the corrective every beginner deserves. The knife-edged blade in DD advertising is, in the words of specialist coil literature, largely myth: electromagnetic fields radiate and cannot be focused into beams. The same source sketches the truer shapes — a round concentric's field like a soccer ball cut in half, a comparable DD's like a football cut lengthwise — and warns that targets under a DD's edge produce real signals that can mask centreline targets to a surprising degree (s3). Practical consequence: a DD does not make the ground beside your target disappear. It narrows the sensitive volume and steadies the machine in bad soil; separation still comes from your sweep discipline and your ears. In mild ground the verdict flips, and the specialists say so plainly: concentrics are the better-behaved coil there, and DD earns its keep as the bad-ground and prospecting choice (s3).
Size: depth follows width, and trash sets the ceiling
Two size rules, both under-taught. First: depth tracks coil width more than length. A 5×10-inch elliptical behaves like a stretched 5-inch coil, not a skinny 10 — marketers list the long number first; compare coils by width, or by total area, for an honest depth expectation (s3). Second: size is a trade against separation. Large coils — 12 inches and up — penetrate deeper and suit open ground and relic hunting, but they see more targets at once and more mineralized volume; small coils give up depth and gain the precision that trashy parks reward, isolating one target from its junk neighbours (s1, s4). The stock mid-size coil most machines ship with is the compromise between those poles, which is why the classic second-coil purchase is a small sniper coil for hunted-out trashy sites or a large coil for open fields — bought for a named site type, not as a general upgrade.
The overlap rule your coil actually needs
Guides quote conflicting overlap figures — 25–50%, 30%, even 60% — because the right number depends on the field's shape at target depth. A concentric's cone is wide at the surface and narrow at depth, so deep coverage has gaps unless sweeps overlap heavily; published guidance puts concentric overlap at 25–50%, with one comparison going to 60% (s2, s4). A DD's field runs more uniformly along its length, holding steadier depth across the sweep, so roughly 30% overlap covers the ground (s4). Reconciled into one field rule: overlap by half with a concentric, by a third with a DD, and slow down instead of widening the gaps — because an un-overlapped strip is not searched ground at all, whatever the coil cost. Notice this is a technique fix, not a purchase: most 'my coil lacks depth' complaints are uncovered wedges between sweeps.
Choose a coil in five steps
- Grade your ground: if the machine chatters and ground-balances at extremes (salt wet sand, iron-rich soil), mineralization is your constraint — DD territory (s1, s2).
- Grade your trash: dense modern trash argues small; open historic ground argues large. Count targets per sweep on your real sites, not imagined ones (s4).
- Compare by width: shortlist coils by their smaller dimension and total area, ignoring headline length (s3).
- Match configuration to steps 1–2: concentric only where ground is mild and pinpointing precision is the prize; DD as the default everywhere else (s2, s3).
- Re-learn the overlap for the new coil on a buried test target at working depth, and set your sweep rhythm there before the next real hunt.
What success looks like: the machine runs quieter at usable sensitivity, targets separate instead of blending, and a gridded test patch gives up its planted coins on the first pass — coverage and configuration agreeing with each other.
FAQ
Is DD always better? No. In clean, mild ground a concentric pinpoints more precisely and can out-depth an equal DD at the centre; DD's advantages are mineralization handling and sweep consistency. The ground chooses (s2, s3).
Will a bigger coil make my detector deeper? In mild ground over open sites, generally yes; in mineralized ground, the extra soil volume can cost the depth it promised. Width, not headline size, is the number to compare (s1, s3).
Can I tell a coil's type by looking at it? Not reliably — configuration is the internal winding arrangement, and lookalike shells house different windings. Check the manufacturer's specification for your exact coil (s4).
General information only. Depth and field figures are from published comparisons and specialist literature checked October 23, 2026 and vary by machine, settings, and ground.