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How a Metal Detector Really Works (UK Guide)

Updated: 3 October 2025 (UK English)


A metal detector transmits an alternating magnetic field into the ground. If that field encounters a conductive target (coin, artefact, jewellery), it induces eddy currents in the metal. Those eddies create their own secondary magnetic field that is captured by the detector’s receive circuit. Signal processing then estimates conductivity, size/shape, and time/frequency behaviour to identify a target and display a Target ID while suppressing responses from the ground and iron. Different technologies—VLF/induction-balance, pulse induction (PI) and simultaneous multi-frequency (SMF)—perform this trick in different ways.

If you remember one thing: detectors don’t “sense metal” directly—they sense how metal responds to changing magnetic fields.
Illustration of a metal detector with text "How a Metal Detector Really Works" over green hills. Website "DiggingHistory.co.uk" below.

The physics in plain English

Metal detectors exploit electromagnetic induction, first described by Michael Faraday in 1831. When we drive alternating current through a transmit (TX) coil, it generates a changing magnetic field B. If a conductive object is within that field, it experiences a changing magnetic flux, which by Faraday’s law induces eddy currents in the object. Lenz’s law says those currents flow in a direction that opposes the change, so they create their own magnetic field, our secondary field, which the receive (RX) coil senses.

Key physical ideas:

  • Eddy currents: tiny swirling currents that depend on the object’s conductivity, size, and shape; they rise and decay in time when the field changes.

  • Phase shift: the secondary field lags the primary field; the lag depends on material conductivity and permeability. VLF detectors measure this phase to infer Target ID.

  • Skin depth (δ): AC fields mostly penetrate to a limited depth δ ≈ √(2ρ/(ωμ)), where ρ is resistivity, ω=2πf is angular frequency, and μ is permeability. Higher frequency → smaller skin depth → better sensitivity to small/shallow targets, poorer depth on large/deep targets.

  • Magnetic susceptibility: soils rich in magnetite or other ferrimagnetic minerals exhibit strong background signals that must be balanced out.

Why this matters in the field: iron nails and hot rocks can mimic targets because they also generate secondary fields, but their phase/time signature differs from coins or jewellery. Modern detectors classify these differences.


Core detector architectures

VLF (Very-Low-Frequency / Induction-Balance)

How it works: A VLF detector drives the TX coil with a steady audio-range AC (typically 3–40 kHz). A physically separate RX coil picks up both the direct coupling from TX and the secondary field from targets. Clever coil geometry (concentric or Double-D) and electronics create an induction balance that cancels the TX component so the RX channel emphasises the tiny target response. The electronics compare the received signal’s phase and amplitude to the TX reference.

Strengths

  • Excellent discrimination using phase; stable Target IDs on coins and jewellery.

  • Efficient in low–moderate mineralisation (parks, pasture, arable fields).

  • Coil options and low power consumption.

Limitations

  • Depth and ID accuracy degrade in highly mineralised or salty ground unless ground balance and filtering are spot-on.

Where VLF shines: inland coins/relics, ancient sites with moderate iron, pasture and plough.

Pulse Induction (PI)

How it works: A PI detector sends brief, high-current pulses into a coil (often a single coil does both TX and RX). When the pulse switches off, the collapsing magnetic field induces eddy currents in nearby conductors. The detector samples the RX voltage during this decay window. Targets with high conductivity (silver, copper) and/or larger size produce longer, higher-amplitude decays.

Strengths

  • Much less sensitive to magnetic mineralisation and wet salt; superb for beaches, highly mineralised soils, and goldfields.

  • Very good raw depth on larger/deeper targets.

Limitations

  • Limited ferrous/non-ferrous discrimination compared to VLF (although timing-domain tricks help).

  • Higher power draw; heavier coils.

Where PI shines: surf/wet sand, severe black sand, highly mineralised ground, deep hoard hunting.

Simultaneous Multi-Frequency (SMF)

How it works: SMF detectors transmit and analyse multiple frequencies at once. Each frequency probes the target and the ground differently (because skin depth and phase depend on f). By fusing the information across frequencies, SMF improves ID stability and ground handling.

Strengths

  • More consistent Target IDs across variable ground (including wet salt).

  • Broader sensitivity spectrum, better small-gold response without sacrificing coin depth as much.

Limitations

  • Algorithm-dependent; behaviour varies by brand and model.

Where SMF shines: mixed terrain, beach to pasture in one session, uncertain or changing soils.

Other/legacy: BFO, FBS/BBS, ZVT

  • BFO (Beat-Frequency Oscillator): early hobby tech; two oscillators (one in the coil) produce an audible beat that shifts near metal. Little/no discrimination; mostly obsolete.

  • BBS/FBS (Broad Band Spectrum / Full Band Spectrum): proprietary multi-frequency methods used historically (e.g., Minelab Explorer/E-Trac). Related in spirit to SMF.

  • ZVT (Zero Voltage Transmission): hybrid time-domain tech used in some gold machines to improve depth whilst retaining ground handling.


Coil designs and what they actually do

The search coil shapes the detector’s transmit field and receive footprint.

  • Concentric (VLF): TX outer loop, RX inner loop. Pros: tight pinpointing, good for low mineralisation. Cons: more affected by mineralised soils.

  • Double-D / DD (VLF & SMF): two overlapping D-shaped windings. Pros: blade-shaped detection zone for better ground coverage and mineral handling. Cons: slightly broader pinpoint.

  • Mono (PI): single winding used for both TX and RX. Pros: excellent depth, simple design. Cons: less discrimination; more susceptible to EMI without filtering.

  • Size trade-off: larger coils reach deeper and sweep more ground but are heavier and less sensitive to tiny targets; smaller coils separate targets better in iron and brush.

Shielding & resonance: Coils are tuned with capacitors to form resonant circuits that boost signal-to-noise at the working frequency (for VLF/SMF). Good shielding and cable routing reduce capacitive coupling and EMI.


From soil to screen: the signal chain

  1. Transmit – The control box drives the TX coil (continuous sine for VLF/SMF; pulses for PI).

  2. Coupling – The primary magnetic field enters the soil; conductive targets develop eddy currents and a secondary field.

  3. Receive – The RX coil outputs a microvolt-scale signal containing: residual TX leakage; ground response; target response; EMI.

  4. Front-end – Low-noise pre-amplifier, band-pass/low-pass filtering (VLF/SMF) or sample-and-hold timing windows (PI).

  5. Demodulation – VLF/SMF: in-phase (I) and quadrature (Q) mixers produce signals proportional to amplitude and phase. PI: late-time samples map to decay constants.

  6. Digital processing – Ground tracking, ferrous probability, target ID mapping, and notch discrimination. Some detectors use machine-learning-like classifiers trained on known targets.

  7. User interface – Audio tones and displays (VDI numbers, iron bias meters, depth gauges) summarise the classification with configurable discrimination and tones.


Target ID, phase, and discrimination

Target ID (TID/VDI) compresses complex signal behaviour into a number scale (often 0–99). Broadly:

  • Low numbers: iron/ferrous (negative or near-zero phase; short PI decays).

  • Mid numbers: foil, pull tabs, small gold.

  • High numbers: copper/silver coins, large brass/bronze.

Phase-based discrimination (VLF/SMF): Conductivity shifts the phase lag; iron also shows characteristic magnetic responses. Detectors map (I,Q) to bins and apply notch acceptance/rejection.

Timing-based discrimination (PI): The decay time of eddy currents differs across materials and sizes. Advanced PIs use multiple sample windows to estimate tau and suppress iron, but fine ID is limited compared with VLF.

Reality check: Orientation, corrosion, proximity to iron, and ground conditions can push a “good” target into a different bin. Target masking from nearby iron is a major reason for missed finds; smaller coils and faster recovery speeds help.


Ground effects: mineralisation, salt, and balancing

Mineralised soils contain iron oxides (e.g., magnetite) with high magnetic susceptibility. The ground itself behaves like a giant, slowly varying target. Ground balance sets a reference so the detector ignores the average ground response.

  • Manual GB: You pump the coil and adjust until the threshold hum remains steady.

  • Auto/Tracking GB: The detector measures ground phase on the fly and updates the balance.

  • Salt compensation: Wet salt behaves like a conductive sheet; SMF and PI cope better than single-frequency VLF. Special “Beach/Wet” modes shift frequency mixes and filters.

Why it matters: Incorrect GB reduces depth, destabilises ID, and increases falsing. On beaches, a good salt programme is essential for stable operation in the surf line.


Depth, frequency, and skin-depth trade-offs

  • Frequency: Lower kHz penetrates deeper and favours larger/high-conductivity targets; higher kHz is more sensitive to tiny/low-conductivity objects (small hammered silvers, fine gold). SMF blends the two.

  • Skin depth (δ): Shrinks with higher frequency and higher permeability; expands with higher resistivity. In simple terms, you won’t “see” deep large silver at 40 kHz as well as at 5–10 kHz.

  • Coil height & sweep: Keep the coil low and level. Raising the coil reduces coupling dramatically.


EMI and interference management

Detectors are radios. Nearby power lines, phone masts, dog fences, and other detectors can inject noise.

  • Noise cancel / frequency shift (VLF/SMF): choose a cleaner channel.

  • Lower sensitivity: trade a little depth for stability.

  • Spacing & sweep discipline at group digs.


Practical setup recipes

General pasture/park (VLF/SMF)

  • Sensitivity: run as high as you can while keeping stable audio.

  • Recovery speed: medium-fast for iron-ridden areas; slower for cleaner ground to gain depth.

  • Ground balance: perform a proper pump-GB at the start and when soil changes.

  • Iron volume: keep some iron audio; it preserves context and reduces digging big iron.

  • Coil control: low, level, overlap swings by ~30%.

Wet salt beach (SMF/PI)

  • Choose Beach/Wet mode or PI with suitable delay.

  • Reduce sensitivity near the surf if falsing; let the algorithm handle salts.

  • Use larger coils to cover sand flats; expect deeper holes.

Highly mineralised inland soils (PI or SMF with tracking)

  • If VLF insists on falsing after GB, switch to SMF/PI.

  • Smaller DD coil to improve separation and reduce ground signal footprint.

Hunted-out sites (unmasking)

  • Use a small DD coil (5–6") and higher recovery speed.

  • Approach mapped hot spots from multiple directions; rotate on iffier signals.


Care for finds & legal essentials in the UK

Even though this article is technical, responsible detecting is part of “how detectors work” in real life.

  • Follow the Code of Practice for Responsible Metal Detecting in England and Wales (2017). It covers permission, protected sites, and reporting finds.

  • Treasure Act 1996: certain finds must be reported to the coroner within 14 days. Scotland operates under Treasure Trove (different process).

  • Record your finds with the Portable Antiquities Scheme (PAS) to help archaeological research.

  • Beaches & foreshore: The Crown Estate allows detecting on much of the foreshore (check local exclusions). The River Thames foreshore requires a separate Port of London Authority permit.

Conservation basics

  • Keep artefacts in breathable bags; avoid scrubbing silver or bronze in the field.

  • Photograph in situ if possible; record GPS or field references.


Frequently asked technical questions

Why do small hammered silvers “sound bigger” at high kHz? Higher frequency increases induced eddy currents in small/high-resistivity items within the limited skin depth, improving signal-to-noise.

Can VLF truly reject iron? It can probabilistically reject most iron via phase, but flat iron and crown caps can mimic non-ferrous IDs. Keeping partial iron audio and cross-checking from multiple angles helps.

Why do Target IDs jump? Orientation, corrosion, mixed alloys, adjacent iron, and shallow targets under the coil edge all disturb the phase/timing signature.

What is “masking”? A strong signal (often iron) saturates or dominates the receive channel and processing window, hiding weaker adjacent targets. Smaller coils, faster recovery, and different angles reduce masking.

What’s the real-world depth? Depth is governed by coil size, frequency, soil conditions, EMI, and target size/orientation—not just a spec sheet. A quality 11" DD in mild ground may hit typical hammered at 6–8"; larger silver or bronze may go 10–12"; tiny gold may be 2–4" in mineralised soils.


Glossary (fast reference)

  • Eddy currents: Circulating currents induced in conductors by changing magnetic fields.

  • Ground balance (GB): Calibration to cancel average ground response.

  • Mineralisation: Magnetic/ conductive properties of soil causing background signals.

  • Phase (VLF/SMF): Angle between TX reference and RX target signal; used for ID.

  • Pulse delay (PI): Time after TX pulse before sampling; longer delay suppresses small/low-conductivity targets and some ground noise.

  • Skin depth (δ): Depth at which AC field amplitude falls to ~37% of surface value.

  • Target ID (TID/VDI): Numeric representation of likely target class.


Further reading

Physics & fundamentals:

Detector engineering & operations:

UK law & best practice:


Conclusion: How Do Metal Detectors Work in Practice

Metal detectors work through the fascinating interplay of electromagnetic fields, eddy currents, and signal processing. Understanding how VLF, PI, and SMF systems operate, as well as how coils, ground balance, and discrimination interact, empowers detectorists to get the best from their machines. By respecting the legal framework of the UK and recording finds responsibly, hobbyists not only enjoy the thrill of discovery but also contribute to national heritage.

In short: your detector is not a “metal sensor” but a physics engine, listening to the whisper of eddy currents beneath your feet. Master its language, and the ground may give up its secrets.


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