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Easy to make Dual D coil

A

Anonymous

Guest
Anyone who has made a Dual D coil will attest to the fact that it is difficult to stop the top D winding from moving with a seesaw motion. One way around this is to use a thin, stiff piece of board such as PCB material (without the copper) or similar. One D winding is first glued to one side of the board.
Turn the board over and you have a nice flat surface. It is a simple matter to simply slide the second D coil into position and glue it down. I am planning to etch a winding on one side of a PCB. A second PCB coil is placed on the back of the PCB. sliding one board over the other nulls the receive coil.
The first time I made a Dual D this way, I used some thin, stiff, card from the art supply store. After the coils were in place, I placed the assembly into a coil cover from a commercial coil and filled it with spray foam. This made a very strong, very light weight, and well balanced search coil.
Don't forget to use a Faraday shield for both windings. I used a nickle spray coating for this. See the attached URL. Don't forget to leave a small gap in the shields. Dave. * * *
 
Do you spray on the inside of the coil cover Dave? Sounds like you actually spray the windings.
By the way,what do you think of this for a coil assembly idea? Double D(both used only as receive coils),plus a smaller transmit coil,centrally positioned over the null on the DD.
My idea is to be able to switch the transmit coil so that it becomes a conventional TX/RX for the purpose of pinpointing.Is this practical?
Regards.
Alan
 
Alan,
I take a piece of plywood and draw the shape of the "D" winding onto it. I then place pin nails (the ones with almost no head to them) around the pattern. This makes a great coil winding base.
I then wind the coils tying them with fine thread about every inch to keep them compressed. Next I remove the coil from the base and coat it with epoxy. After the coils are dry, I wrap a piece of paper tape cut down to a width of 3/16" or 5mm. through and around the coil, close to its connecting wires.
I now spray several coats of the nickel spray allowing each to dry. The paper tape is now removed leaving the gap which is the difference between a full shield and a Faraday shield.
The final step is to strip about an inch and a half or 4cm of insulation from the end of a length of wire wrap wire. Wrap this wire tightly through and around, next to one side of the gap. Spray this and apply a little glue to keep it tight. The wire wrap wire is used to connect the shield to ground.
As to your coil Idea, your best bet is to give it a try. You don't need a Faraday shield or even glued coils to test your idea on the bench. I tape crude coils to foam board with duct tape. The foam board is taped to a four foot piece of PVC water pipe which I mount verticaly in an old bench vice which sits on the floor. This keeps the coil/s away from metal and allows you to easily test your design.
 
Thanks for going into so much detail Dave.
Very informative and much appreciated.
Alan
 
This method has been proposed by Russian authors a few years ago (a chapter from the book is at http://metaldetector.bratsk-city.ru/shematic/IB/locmet_pcb.zip., Fig.4.
PCB seems rather elastic and the balance can be lostehen the times goes - my proposals are: a plain glass for the coil base (much more ridgid) and covering the balanced ciols with the sprayed urethane foam.
pp
 
Pawel,
It just goes to show you that there are very few things that are really new! Thanks for the input and the Russian URL. I also like your idea of using glass and urethane foam. Dave. * * *
 
There's only one problem - which glass will suit better - from the wife's photo or from my PhD diploma...
 
Fiber-reinforced phenolic sheet, which is widely available as a stock industrial material, is easy to worth with and has superb dimensional stability.
--Dave J.
 
I don't suggest that you use the wife photo... not if you want to go out a "play Metal Detective" this weekend.. LOL :0))
 
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