SEARCH-HEADS

There are many different shaped search-heads on the market, but they are all essentially similar and the coils inside fall into 2 categories, concentric , where the one coil is laid out within the circumference of the other and double-D , where the transmit and receive coils are identical and 'D' shaped laid back-to-back so that the oblong area where the 2 coils overlap is the area of greatest sensitivity. In a pulse induction search-head there is usually one coil performing both transmit and receive functions.

Metal detectors are carefully tuned electronic instruments. Just like racing car engines, the higher the level of tuning, the better the performance and the higher the detector price. There is a relationship between search-head size, depth of detection, ground effect, and the minimum size of object which can be detected. This relationship is not proportional and all the search-head design characteristics have to be compromised to reach the ideal search-head for the individual machine. The bigger the search-head, the deeper it will go, BUT big search-heads are unwieldy, have reduced sensitivity to small objects, and are more susceptible to ground effects. If you fit a search-head twice the size, it won't go twice as deep, more like about 20% deeper, and if you fit a search-head five times as big, you'll be trying to manage a search-head that's a whopping 50 inches in diameter and you'll be lucky if it goes 25% deeper than the standard 10 inch search-head. In addition, such a big search-head will have no sensitivity to small objects and will be subject to severe ground effect, that is a tendency for the machine to detect the ground itself, giving many false signals.

Very small search heads, around 4 to 6 inches in diameter can be very useful for getting into tight spots, down into the furrows on ploughed land for example. They are less susceptible to ground effects so they can be good on a beach or mining area. It takes longer to cover the ground area with a small search-head and the depth penetration is slightly less. The optimum search-head size for a general purpose treasure hunting detector is between 8 and 12 inches in diameter. Hoard hunters or large object finders are metal detectors designed to find large objects at great depth. Some of these detectors have extremely large search heads, 2 metres in diameter and more, requiring 2 people in their operation. It is also possible to gain similar depth performance by arranging the transmit coil some distance away from the receive coil, and in a different plane, as in the C.Scope CS9000HPX. Remember that the physical properties of large search-heads make them insensitive to anything smaller than a large biscuit tin. There will also be a lot of false ground signals, and you have to consider the engineering practicalities of digging holes 3 metres deep or more!