{Metal detecting history}

Metal detecting history, myths and secrets.

Are you the proud owner of an all-singing, all-dancing ‘multi-frequency’, ‘digital’ metal detector sporting the very latest technology? If you believe this then you are perhaps sadly mistaken. What you actually have is an analogue, fixed frequency, induction balance metal detector – in essence, not much different from the units available 40 or so years ago. How can this be so you might ask?

The original need for metal detecting equipment arose from a military requirement to find explosive landmines buried in the ground. Early hobbyists realized that such equipment could perhaps be used to recover historic artefacts and treasures. Military units however were originally intended to find quite large steel clad mines rather than small coins and other relics. They were also heavy, unwieldy and used large external power supplies due to the need to be used in battlefield conditions.

There are four basic and practical operating principles that allow a metal detector to function. These are:
⦁ Beat Frequency Oscillation (BFO)
⦁ Pulse Induction (PI)
⦁ Transmitter Receiver (TR)
⦁ Induction Balance (IB)

The first of these was used for some of the very earliest detectors made for the hobby market (such as the M L Beach ‘Prospector’, first offered at £12/10/00 in around 1970 with wooden coil housing and a small transistor radio clipped to the stem – I had one of these from new in early 1971 and, although it worked, it was unstable, went crazy every time the housing touched anything and hated damp conditions). Without getting into the electronics involved, the BFO system did not prove to be effective and I believe that no modern hobby detector uses this system.

Pulse induction units were popular for a while and gave great depth penetration (C-Scope produced some models and other makes such as the C400 and Goldmaxx were around from the late 1970s on). The main problems were (and are) that effective discrimination was not available on early units and the detectors were extremely sensitive to iron targets – a major problem for users who had no interest in recovering iron targets so were mainly limited to beach use where iron signals are not usually a serious problem but where the extra depth penetration was good for recovering coins and jewellery. Although modern PI detectors claim to have effective discrimination circuits, this has had an effect on the depths attainable.

Transmitter Receiver units are usually a variant of the IB type and have, in recent times been limited really to the so-called ‘twin box’ / ‘two-box’ or ‘twin coil’ units which consist of two large coils or boxes carried on a horizontal shaft and used to locate large and deep targets or ore bodies (in mineral survey work).
Such units are still made by CScope, Whites and Fisher for use as ‘hoard’ hunters.

The induction balance principle, although very much ‘old school’ technology, has endured and is the one in use for almost all hobby detectors available today. The basics are that an electronic signal is transmitted from a wound wire coil and - rather like sonar or radar – this signal is reflected back from a target to be received by a second coil. The coils are tuned to a fixed radio frequency (there is no such thing as a ‘multi’ frequency detector; the only way to change the frequency is to exchange the coil for another that is tuned to a different frequency within the permitted range - the frequency operating range is strictly regulated and limited by government and international agreements; you would not want your heart pacemaker to be interfered with by transmissions from somebody using a metal detector near to you!) and the coils are electronically balanced together. Incidentally, coils are not all tuned exactly to the same frequency as, if this was so, they would all interfere with each other in use; slight variations, in combination with shielding to the upper coil faces, allow users to work in close proximity. Early Fisher detectors were notorious for interfering with other units; both Fisher and of other makes. At the same time, early Arado units were extremely sensitive to being interfered with by external electro-magnetic fields (such as generated by overhead power cables).
At its simplest level, all that is then needed is a power source and a method of displaying when interference in the transmitted signal has occurred for such a detector to function. This is what the control box provides - everything else is simply refining or amplifying the signal, filtering and displaying the result to the user.

In electronic terms, the most efficient and effective coil arrangement is to have transmission and receiving coils that are the same diameter and aligned side by side with a small overlap; such ‘double’ coils are sometimes seen today (the Nexus range, for example, only uses this configuration). There are practical problems with this. It is difficult to make, especially to mass-produce, a double overlapping coil structurally strong enough to keep the shape of the coils free from distortion (vitally important) without being heavy and cumbersome. Most manufacturers have adopted different, less efficient (but cheaper and easier) solutions by having a large transmission coil with a smaller receiving coil mounted inside it (concentric coils; this arrangement can clearly be seen on some coils with an openwork ‘spider’ design) or by having two D-shaped coils arranged side by side (a double-D coil).

So, having established the basics, what next? Everything else is rather a case of ‘smoke and mirrors’! Taking the first aspect, the control box circuitry must be tuned to the same frequency as that of the coil for optimum performance (so if you do use several coils, you will not get optimum performance from each unless they are all tuned to the same frequency). In mass produced units, there is an allowed tolerance for tuning and this can explain why two apparently identical detectors of the same make and model may have different performance characteristics and capabilities. There are two approaches for detectors: motion and non-motion detecting. Most units today have relegated non-motion responses to pinpointing, often via a trigger toggle switch; to obtain a response in normal use the coil must be actually moving when passing over a target.

Then there is the matter of ‘ground balance’, where the control box circuitry allows a fixed or variable filter to be utilised to screen out all responses below a certain base level. All soils and minerals have a different level of electronic conductance; the capacity to conduct electricity or to respond to radio frequencies vary hugely. This is often referred to as ‘ground mineralisation’ in detecting terminology and can vary dramatically across a single field.
The degree of such mineralisation is infinitely variable. A ground balance setting that proves effective on one site (or part of one site) will be near useless on another. The higher the setting needed to screen out unwanted background interference, the greater the adverse effect upon depth penetration and response to small or faint signal desired targets.

Discrimination or the ability to screen out ‘unwanted’ (usually ferrous, iron-based) targets is another filter commonly used today. Early detectors had no ‘discrimination’ capability at all – the only way to find out what a signal might be was to dig it! Experience in using the detector gave some assistance but digging was the only real answer (and to this day I have many interesting iron finds from detecting; from foot patterns to keys to spearheads to cannon balls). Even today, with quite sophisticated discrimination filters (including such features as ‘notch’ – popular some years ago time with beach users who wished to ignore aluminium ring-pulls and similar junk), many iron targets will still show as positive signals; some of this is due to electronic characteristics – anyone who has detected will be aware of the phenomenon where pieces of iron that have holes, such as washers, give positive signals. The ability to discriminate at all takes advantage of the differences in electrical properties of different metals and alloys. In general, the more ‘metallic’ the target is, the stronger the response. The higher a discrimination filter is set, the more targets that will be ‘rejected’ or ignored – hence why serious detectorists searching a good site will work in ‘all metal’ mode to recover every possible target.

The response of a detector to any given signal can be presented in several ways. The first is audio, through a loudspeaker/headphones and most detectors use this as the primary method. An audio response, whether against a ‘silent’ background or a rising tone response from a background base tone will indicate a potential target. Once again, whether factory set or user-selected, the base tone is achieved by filtering ‘unwanted’ responses below a set base threshold level. Some detectors have the ability to give a variable tone response – typically where non-ferrous responses give a higher pitched audio response, above that of the base tone and ferrous signals a correspondingly lower-pitched response. Many things will give a potential response, from ‘hot’ rocks to fired clay to iron pyrite nodules; all contain iron or other metals to some degree; some find these background so-called ‘false’ signals irritating but most can be eliminated by being non-repeatable. Filtering them all out risks missing small and deep responses.

Responses can also be displayed visually and most detectors do this as a secondary response or ‘target/signal analysis’ and it is here that most units apply the secondary filters such as discrimination. These displays can be ‘analogue’ (by a swinging needle meter) or ‘digital’ by using liquid crystal display screens (LCD), light emitting diodes (LEDs) or other means. The type of display is the nearest any detector can get to being a ‘digital’ unit – ALL metal detectors actually operate on an analogue system and, as said above, most on the induction balance principle. Any target analysis displayed, however it is presented to the user, relies on the electronic filters (preset or user-set).
Some are simply presented; others have all sorts of ‘bells and whistles’ with flashing lights, multiple colours, multiple LCD bars, etcetera, etcetera, but essentially they all do exactly the same job.

Power supply is another vital area. In general terms, the more ‘powerful’ the signal generated, the greater the depth it is possible to obtain a target response from. Once again, government regulations restrict the signal power in order that interference with other equipment will be avoided. The signals sent from the transmission coil will attenuate the further they extend from the coil (the same thing happens with sonar and radar). Soil will absorb or smother the signal to the point where any possible response from a target is too weak or distant to be picked up by the receiver coil.

The size (diameter) of the coils is also of great importance. In very general terms, the larger the coil diameter, the deeper the penetration of signal into the ground that is possible. However, again in general terms, there is a relationship between coil diameter and range of target sizes detectable. Broadly speaking, a large coil will be better at finding large targets deeply buried while a small coil will do better at finding small targets but with more limited depth. It is for this reason that the higher end detector makers often provide a range of additional coils of various types and sizes that can be fitted.

Modern metal detectors have just about reached the limits of what induction balance is capable of. There may well be minor improvements and tweaks in control box circuitry that might be developed but unless there is a fundamental discovery of a new principle, there will never be much more than can be attained today. Indeed, in some areas, modern detectors are not as good as their forebears. I have been detecting since 1970 and, over the years, I have used many different units but I well remember my early CScope IB300 and TR400 detectors being easily capable of finding an old penny at 10” plus depth. Many modern and popular units today would struggle at 6”. For tone discrimination, the CScope Promet and Metadec detectors were well ahead of their time – I still use a Metadec II with 4” coil today and, on some sites, it is still unbeatable. Similarly, I still use an Arado 130 (where there are no overhead cables or other electrical fields!) and an Arado 95 on clay beaches – Lord alone knows what
I will do it when they finally die for good!

Finally, all metal detectors have an element of compromise in them. The best will offer a range of coils that will make a detector more versatile but no one detector is the perfect unit to use on all types of sites or for seeking all types of targets; some will have greater ground balance stability on some soil types; others will be especially sensitive to small hammered silver coins.

So, if your budget limits you to one detector only, how do you choose which is the ‘best’ for you? All I can advise is that you try and handle as many different units as possible (detector rallies where dealers will have a range of units on display are good) and talk to as many people who use the unit you are thinking about as you can; but do remember that detectorists are rather like fishermen and most like to think that they have the ‘best’ equipment – especially if they have paid a lot of money for it! A comment such as “I got this over 12 inches down in really contaminated ground” while showing you a tiny hammered farthing that actually only gave a whisper signal while lying on the surface is human nature! If you want a detector that will perform reasonably well in most conditions then almost any of the mass-produced intermediate level detectors will do (around the £500-£700 level) and it will be a choice of features, availability of other coils, weight and balance in use or perhaps what one has the most flashing lights or the most complex set-up options - if you want to spend an hour tapping through multiple screens selecting every possible variable......and then wondering whether maybe altering just one setting out of the dozens available will magically turn your detector into a super-detector that will just as easily find that huge bronze age hoard 4 feet down as that Saxon gold thrymsa worth thousands.....this way madness lies!

Having made your choice, take the time to learn how to properly use it! You have invested what can be a large sum of money and to write it off after only a few hours trial is foolish. Read the manual from cover to cover and try it out on a wide variety of site types. Test the various settings and persist. It can take months of regular use to become really familiar with the foibles of a modern detector and to get the maximum performance from it. If all you want is the occasional outing then choose a detector with the minimum of variable controls. Preset ground balance and discrimination settings will work reasonably well in most places. Manual ground balancing is more of an art than a science as there are so many variations in site conditions.

Detectors that operate at the limit of IB capabilities will take considerable time and experience to get the most out of them and are not the best choice for beginners or casual users. Sadly, there are those who are willing to criticise such units after only the briefest of test periods. I would not contemplate writing (let alone publishing) a test report on any but the most basic of detectors unless I had spent a bare minimum of a week of near full-time use with it.
Setting up a detector manually where there is significant choice in settings and controls is not easy or simple; it takes considerable experience to get things right and achieve the best possible performance; this cannot be achieved with only a few hours of use. To give up and condemn after only a few hours use indicates a very amateur approach and one not to be taken seriously.

If you don’t want to spend effort setting your detector up properly (and constantly tweaking it as conditions vary) or digging lots of deep holes, I suggest that a deep seeking, manual setup detector might not be your best choice. The same comment applies if your sites have large amounts of modern non-ferrous rubbish (303 rifle cartridges soon lose their appeal if you have to dig 18” down to retrieve them!). Most top range detectors do have some element of auto-setting that can be useful for practice; a few allow a fully automated mode to be used but relying on these will never achieve the maximum potential.

Regardless of the detector chosen, luck will still play an important part. It does not matter what detector you have, you will not find anything unless you pass the coil directly over something that is located within its detection range – and then make the decision to dig it up! There will always be the first time user, often with a ‘cheap’ detector, who will stumble by chance on a find that any experienced detectorist would give his right arm for! Isn’t life fun?

Written by _Christopher Wren (Published 2012)