There will inevitably be those people who still insist on building their own coils, despite the warnings given previously. This may simply be because Tesoro coils are not available locally, coils prices are outside the project budget, or they just want to see how it's done. Whatever the reason, here is a set of step-by-step instructions that you can follow to build a DD-coil from scratch using low-cost materials.
First acquire a solid coil cover. Either purchase one new, or go to your local metal detector shop, as they will often have a “bucket of bits” where you can find one for a cheap price. You will need one that is capable of holding a 20cm diameter (slightly under 8”) coil. The one used in this example was 21cm (see Figure 10-19). The man-ufacturer is unknown.
Next cut a wooden circle from a sheet of 5mm thick 3-ply that will fit tightly inside the coil cover. This will form the top section of the search coil. You could sub-stitute 5mm thick MDF (medium density fiberboard), if you prefer, as it may be less susceptible to twisting.
Using a separate wooden base, draw a circle of 20cm diameter, and mark out the D-shape according to Figure 10-20. Using either nails, right-angled cup hooks or wooden doweling, wind the coil into the D-shape using 114 turns of 0.2mm enam-elled wire. This first coil will be the TX.
Fig. 10-19: Coil Cover
Secure the windings tightly in several places using electrical insulation tape. See Figure 10-21.
With reference to Figure 10-22, wrap the wire tightly with small strips of self-adhesive aluminum tape. Make sure to leave a gap, otherwise the aluminum tape will act as a shorted turn instead of an elec-trostatic shield.
Loosely spiral wind the coil (see Fig-ure 10-23) with tinned copper wire. The wire used in this example was 22SWG (0.71mm). Again, make sure there are no loops created during the winding process.
Leave a 2cm tail of wire at the end, as it will eventually need to be connected to the screen (0V). This is known as the drain wire.
Finally wrap the entire coil with Gaffa (Gaffer) / Duct tape. This is a strong, tough, fabric-based sticky tape. See Figure 10-24.
Repeat this operation for the RX coil, using 119 turns of 0.2mm enamelled wire.
Fig. 10-20: Coil Details
20cm 7.5mm
10cm
Fig. 10-21: Coil wound with
electrical tape Fig. 10-22: Coil wound with electrostatic shield
Hopefully you should now have two D-shaped coils. Mark each one so that you can identify which one is which. Then construct the mounting bracket, that connects to the detector stem, according to Figure 10-25. These should be made from 5mm thick MDF.
Fig. 10-23: Detail of drain wire Fig. 10-24: Coil wound with final layer of Gaffa/Duct tape
Fig. 10-25: Mounting bracket base and lug (x2), 5mm MDF
20mm 60mm
15mm 15mm
60mm
5mm
40mm
30mm
20mm 12mm
Other mechanical parts required are (see Figure 10-26):
• 1x plastic bolt, washer and wing nut
• 1x cable entry sleeve
• 2x rubber washers
• 4x coil cover spacers
Depending on the internal depth of the coil cover used, either find or make 4 spacers to support the plywood top. For the example coil shown here, it was found that a small plastic shelf support was the exact size required. These were glued into the base of the coil cover. See Figure 10-27.
Assemble the mounting bracket and glue it onto the plywood circle, as shown in Figure 10-27. Finally, paint the whole assembly to match the coil cover.
Now the mechanical construction is complete, it's time to fix the TX and RX coils in position. Using a hot glue gun, fix the TX coil first. Once the glue has set, sol-der the connecting cable to both coils and connect to the detector. Leave a small loop of wire at the end of the RX coil to allow some final adjustment prior to sealing the search head. Remember that both the TX and RX coils share a common connection, as described in Chapter 9, and shown in Figure 9-2. Remember to connect the drain wires to the screen.
Then follow this procedure:
1.With the TX and RX coils slightly overlapping, monitor both the TX signal (TP1) and the RX pre-amp output (TP3) using an oscilloscope.
Fig. 10-26: Mechanical parts required for search head
2.Gradually move the RX coil towards the TX coil until the RX signal reduces to a minimum.
3.Continue to move the coils together until the RX signal has an initial phase-shift of 20 degrees (approximately 6s). This is clearly shown in Figure 10-4.
If you cannot achieve a 20 degree phase-shift try reversing the RX leads.
4.Fix the RX coil in this position using hot glue.
5.Solder the drain wires to the screen, as shown in Figure 10-28. At the 20 degree position the residual voltage should be somewhere between 200mV and 400mV peak-to-peak when measured at TP3, but it may go as high as 1V, depending on the actual coil shapes.
6.Follow the directions near the end of Chapter 9 to calibrate the detector for use with your coil.
7.If you wish, you can fix the coils more firmly in position by using epoxy glue, but hot glue was found to be adequate for the purpose. Figure 10-29 shows both the TX and RX coils fixed in their final positions.
8.Once you have successfully calibrated the detector, attach the coil cover and make it waterproof using a silicon sealer.
Fig. 10-27: Coil cover with spacers Fig. 10-27: Mounting bracket complete
Fig. 10-28: Search head wiring
Cable to detector
Orange (5)
Violet (4)
Grey (2) Screen (3) TX Coil
RX Coil
The fully assembled search head is shown in Figure 10-30, attached to the detector stem.
The internal glued coils might not look very pretty, but the result was a stable search head that pro-vided some reasonable results. It was found that our DD-coil had a slightly reduced performance when compared to the Tesoro 9x8 web coil. It was also noted that the Tes-oro concentric was better at pin-pointing than the DD, and was the preferred coil to use in trashy and iron-infested areas.
You may also like to experi-ment by changing the shape of the overlap between the TX and RX coils. With the parallel overlap, shown in this example, the sensi-tive area is a thin strip between the front and rear of the search head.
One advantage of this approach is that you do not need to overlap consecutive sweeps as you must do with the concentric. The main dis-advantage is a less than perfect pin-pointing capability.
It takes a lot of time, energy and nerves to construct your own coils, but the result can be quite sat-isfying. As we mentioned earlier, the problems usually start when both the coil and electronics are unproven. If you still feel up to the challenge, then good luck with your experiments.
Fig. 10-29: Coils glued in position
Fig. 10-30: Search head mounted on stem
CHAPTER 11