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The top shell is shielded in the same manner with a drain wire of its own. It‟s conductive! This drain wire will then be soldered to the cable shield. I‟m going to suggest doing it anyway.Īn effective method of attaching a drain wire to the graphite shield is to mix a spot of DAP rubber contact cement with powdered graphite into a slurry. Depending on how conductive the coating is, aīreak may be required to stop any eddy currents in the shield. In this case, I am using a break just because it is a best practice. I‟m not really sure if this is required as I have built shells with and without a break using graphite and both methods work. In this project, I masked off a small strip of tape that when removed, leaves a small gap in the conductive coating. When applied, it leaves a conductive coating that has a resistance of about 35k Ohms from one end of the shell to the other. This is a dry spray lube that can be picked up at a “Home Depot” hardware store for under $5.00. A cheap source of graphite spray is a hardware store. For this project, I chose to spray the inside of the shells. The cardboard can be sprayed with conductive graphite or the shells can be sprayed. There are two easy ways to accomplish this – either by building a shield on the inside of the shells or by building a shield directly on the cardboard. In order for the coil to be stable and not react to the effects of ground capacitance, it must be shielded. This makes the whole assembly very rigid. After tuning and nulling, the inner ring is filled with slow cure epoxy.įinally, a second sheet of cardbard is glued to the exposed side of the foam “donut”. The Rx coil must be the wire closest to the bucking coil – ” Rx end” (P8 pins 3 & 4).įor the tricky part – When the bucking coil is wound around the Rx coil opposite direction from the Tx coil, the frequency goes up by about 60Hz, so a turn or 2 will have to be added to the Tx coil to bring the frequency back down to 6.59kHz. For the best null, the hot end of the Tx coil must be connected to “Tx start” (P8 pin 1).
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The bucking coil is wound opposite the direction of the Tx coil – directly on top of the Rx coil. NOTE: The above specifications are not to be solely relied on! Your specific build needs to be measured and verified or the phasing will most likely incorrect! The IDX project is not so tolerant to the coil Tx coil – 7.5″ diameter – 35t for 22 AWG wire (583uH) wound counter clockwise + 1 uF cap.īucking coil – about 11 turns wound on top of Rx coil wound clockwise. Rx coil – 3.25″ diameter – “about” 440t for 32 AWG wire (35 mH) wound counter clockwise + 33nF cap. I have done it both ways, and they both work. Then this coil is simply glued into a channel cut into the foam.Īlternately, it can be wound in a ¼” x ¼” channel around the outside of the “donut” foam core. The Tx coil is made on a separate former and held together with Cyan glue. The foam “donut” is then glued to a sheet of stiff cardboardand the Rx spool is glued into the center. It is then cut into a donut shape to accept a spool for the Rx coil.The Rx coil is wound on a homemade spool of cardboard with a foamcenter. I used “large cell ”foam that is relatively stiff and brittle from a craft store.įor illustration purposes – start with a sheet of 1/2 inch foam boardfrom a craft store – It’s “large cell” foam, that is somewhat brittlebut also stiff. To begin with, let us take look at our desired specifications for our coil:įoam board – 1 2 ” (11mm) thick, 12 ” X 12 ”. Some of these advantages include cost of construction, lighter weight, easier pinpointing, greater detection distance for a given diameter, better discrimination and iron rejection.Even though constructing the Concentric coil can be a little tricky, it will be demonstrated that it can be done successfully! In this example we will be building a coil to match a White ‟s coil for the IDX project.While the specifications for a White ‟s coil are presented here, similar construction methods can be used for a multitude of coils. The Concentric coil configuration does however offer some advantages over the DD coil.
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While it is targeted toward the home hobby builder it is not as simple as the more popular DD coil configuration. This document is intended to demonstrate one method of constructing Concentric Co-planer coils.