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Basket Coil for Pulse Induction

This coil  allows you to get more depth, but it is more difficult to manufacture.
And so – we take a board or a piece of chipboard of the right size and draw two circles on it. The diameter of the first circle is 11cm, the diameter of the second is 18cm. Then we divide each circle into 21 equal parts. For this we have to remember the school knowledge of geometry. We drive 21 nodes into each circle. Pre-wear a piece of PVC pipe on each node. And we start to wind. Wire 0.7-0.9mm. The order of winding the coils and the relative position of the nodes are shown in Fig. one.

We shake eight turns in the specified order. Then we repeat it three more times (a coil should contain 32 turns). We get approximately the following picture .

Then carefully fill the interturn space with construction foam. It is important not to fuss and not to overdo it with the amount. It is better to squeeze the foam in small portions and push it evenly with a stick. Small voids foam eventually fill itself through expansion. Wait until the foam hardens .

Next, extract the nodes. PVC pipes at the same time remain in the foam. Then carefully with the help of a knife we separate this “bagel” from the board. This operation can be avoided if you put a piece of film or even paper on the board before driving nodes. Then using a knife cut off all unnecessary. We do this carefully so as not to damage the insulation of the wire.

Such a coil  made of reinforced foam turns out to be surprisingly tough and easy. In principle, you can simply wrap it with tape, install the mount and use. Although we still recommend placing this coil in some durable housing and sealed.
The described coil allows to obtain the detection range of a 5-kopeck USSR coin up to 26-27 cm (against 21-23 cm for the “classical” coil).
An even greater detection range can be obtained from a larger diameter coil. True, such a coil will be worse to feel smaller coins, and it is less convenient to handle when searching. For example, a coil with an external diameter of 28 cm and an internal diameter of 20 cm (contains 24 turns, the winding order is the same as for the previous coil) allows you to get a range of a five-kopeck coin up to 30-32 cm.Such a sensor made of reinforced foam turns out to be surprisingly tough and easy. In principle, you can simply wrap it with tape, install the mount and use. Although we still recommend placing this sensor in some durable housing and sealed.