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Bad coil, testing a coil, building a coil
Posted by: Charles (Upstate NY)
Date: February 19, 2017 01:00PM
A forum member asked me about a bad coil and I ended up writing more than I planned to, here's a copy for anyone who might find this useful...

Lets talk coils so you are armed with information and understand why a coil might be bad from the factory, how to test one, even fix one or build one.

Inside an Explorer DD coil there are two windings of copper wire, that's all its not complicated. The transmit winding TX transmits the signal inducing signals into targets, the receive winding RX listens for or receives the signal back from the targets.

When you build a DD coil the two windings of copper wire have to be precisely positioned to be in "balance". This is where the term balanced coil comes from, the positioning of or balancing of these two windings of copper wire. You balance a coil using an oscilloscope. You connect one channel of the scope to TX to monitor the signal the detector is transmitting, in the case of the Explorer a square wave. And you connect the other channel of the scope to the RX receive winding, you don't want to see any signal on the RX receive winding, it should be a flat line.

Now if the two windings of copper wire are not positioned correctly, RX will not be a flat line. You will see a small amount of the square wave being transmitted out the TX winding on RX, not good. The further out of position or balance the two windings of copper wire are, the stronger the signal will be on RX, screwing up the coil. With signal on RX that should not be there I think this hurts depth and tends to swamp small target signals.

So here's the thing, this balancing or positioning of the two windings TX and RX, its touchy as freaking hell. Just a tiny nudge and whammo the coil is out of balance. The factory has to get the windings balanced, then try to hold them in balance while the epoxy cures. Just the epoxy curing can pull the damn thing out of balance so yes its entirely possible for the factory to build a bad coil.

The factory probably has a range from perfectly balance to slightly out of balance that they consider good enough to ship but ideally you want a coil that's balanced perfectly for best depth on the deepest targets. This is why owning an oscilloscope comes in handy, its pretty easy to test coils. More on testing coils below.

The other thing that can screw up a coil, and is more frequently the problem imo is shielding. The coil needs to be wrapped in a blanket of conductive shielding, completely encased with no gaps or pin holes. Typically they use carbon black shielding paint. This paint is electrically conductive, but not so conductive that it can be detected by the detector. Warning: There are shielding paints on the market that are too conductive for building a coil, stick to carbon black. This shielding routes static (and some other complicated capacitive stuff I won't get into) off to ground so its not detected on RX and creating false signals.

This shielding layer then itself must be blanketed in a non-conductive layer, the coils plastic outer shell. If there is a gap in the shielding paint, even a pin hole the coil may false, if there is a gap in the outer plastic shell, even a pin hole again the coil may false. Especially in wet grass or on a wet beach. Water makes its way through the gap or pin hole creating an electrical path from grass, wet seaweed, etc. and will cause the coil to false like crazy. Original Explorer coils were susceptible to this problem when you got them wet. If you are building a coil and want to test your shielding, grab a tuft of about 20 blades of grass, gently rub them all over the coil, if there's a gap in your shielding or its too thin in some area you will get a false signal on the detector.

Finally there needs to be a minimum gap or distance between the shielding paint and the windings or again the coil can false. I think this was the problem with the early Explorer SE slimline coils which were noisy and falsed, some worse than others. You can't for example just spray the shielding paint on the copper windings, its not a huge gap but there needs to be some gap. What happens when during coil construction a loop of copper wire comes up to the surface of the epoxy very close to the shielding paint, bingo a coil that falses and that's what I found on those early slimline coils. This type of false signal is small, such that most users might assume its just the site conditions, bad soil, mineralization, and turn down their sensitivity when in reality the coil is the problem.

Suffices to say that while the components of an Explorer coil are simple, two windings of copper wire and shielding paint, construction of the coil is quite difficult, a sandwich of materials you have to get just right or the performance of the coil will be off.

TESTING A COIL - Its fairly simple, you will need a 2 channel oscilloscope, you will need to construct a test fixture e.g. a short bridge between the coil connector on the detector and the coil's connector so you have some bare wire to connect the scope leads. I purchased a male and female coil connector for my test fixture, soldered them together with about 3/4 inch of bare wire. Do NOT cross wire your test fixture, going from female to male to female to male its easy for your brain to screw up, draw it out on paper to make sure each of the 4 coil wires are passing straight through and not crossing over due to your brain getting flipped flopped. I'm speaking from my own DOH here.

Take care not to accidently short something during testing, I find it best to tape everything down. You are going to have coil cable and scope test leads laying around, tape them down so that if you bump something or get tangled up in a wire you don't yank on this tangle and short something.

Next you need to setup your test space so that there's no metal near the coil. Figure 2 feet in all directions, no metal above, below, or to the sides. Note large metal like a metal pipe or sheet metal from an appliance or workbench can be detected from several feet away. I would secure my detector to a bench with the coil and lower rod sticking out in mid air and that work well. You can test indoors but you need to turn your sensitivity way down due to AC electrical interference from your house.

With everything connected go ahead and turn on the detector, you should see a flat line on the RX receive winding if your coil is in good shape. What if its not? What if its out of balance slightly can you fix that? Some people have had luck rebalancing a coil by using a small piece of ferrite, you can move this ferrite around the coil watching the balance, and when the coil is balanced to a flat line again, glue the ferrite in place. In some factory coil construction one technique they use is, they leave a small secondary winding of RX free, just a few loops maybe 1 inch in diameter, they let the first pour of epoxy cure, then recheck the balance, they can move around this small second winding to rebalance the coil if the epoxy curing has knocked it out of balance.

What else can I do with a scope? Well you can gather a bunch of targets and watch how RX reacts to different metals, depths, target orientation, swing speed very important in learning how swing speed effects depth. With the Explorer its possible to swing too slow and too fast. You can play around with co-located targets, a rusty bottle cap trying to hide a silver coin. Move a large rusty nail or some other iron near the coil, note how distorted the signal is vs copper or silver coins or gold, iron is butt ugly on a scope. Move some metal near the coil and hold it steady, not moving. While the Explorer is a motion machine you will see that metal even if the coil is not moving, is distorting and swamping the signal. If its pouring rain outside or its winter and the ground is frozen solid spend some in your lab time testing with your detector and oscilloscope.

BUILDING COILS - I have just barely touched on the subject of building coils above. Building your own coils can be a fun 2nd hobby, and it can be maddening its one of those things where the devil is in the details. But if people are interested maybe we'll start a discussion thread on this subject.

Re: Bad coil, testing a coil, building a coil
Posted by: BigTony
Date: February 19, 2017 02:15PM
Charles, thanks that was an interesting read for sure.
Why do after market coils hit harder on quarters and stuff then factory coils? Is it in the windings or better balance?
I use an Exp II with after market coils, one large and one small and love them both.
Thanks in advance.


Edited 1 time(s). Last edit at 02/19/2017 02:16PM by BigTony.

Re: Bad coil, testing a coil, building a coil
Posted by: Charles (Upstate NY)
Date: February 19, 2017 02:52PM
Which aftermarket coils are you referring to Tony? Coil geometry is important, and you have to think in 3D think of the bucket of space under your coil and the shape of the magnetic field being generated by your coil. The 15" WOT coil for example, well its much larger than the stock SE Pro 11 inch coil. Its going to create a larger/deeper magnetic field in the ground. I found with extensive use of the WOT coil that not only does it go deeper on say silver dimes, because of the geometry of the magnetic field it creates, it has a particular talent for nailing silver dimes on edge where with the stock coil most deep dimes and cents are laying flat.

You need to think in 3D in terms of how the magnetic field lines will strike the target. Silver dimes on edge at say 6 inches may get more field line strikes with the stock 11 inch coil vs the WOT coil. But silver dimes on edge at 10 inches, the geometry of the field lines from the stock 11 inch coil are flat, the bottom of the field and hitting the dime at an on edge angle hence hardly any field lines are striking the target. The WOT coil with its deeper magnetic field its field lines are still more straight up and down to a 45 degree angle vs the target hence more field lines strike the target and you get a signal.

Look at this image below, take note of how some of the magnetic field lines are straight up and down, vs some curved at a 45 degree angle vs some flat horizontal with the ground. Now imaging a target with that field, a target laying flat, a target laying on edge at a 45 degree angle, a target even straight up and down on edge. The number of magnetic field lines striking the target is what induces a signal into that target which the receive winding picks up.

Stand about 4 feet away from a car door, yep you can detect a car door from 4 feet. Even though the magnetic field at that distance is very thin and spread out, not concentrated like it is say 6 inches from the coil, enough magnetic field lines are striking the car door to get a hit.

So magnetic field density is also important. Which coil can detect the smallest target, a 15 inch WOT coil, the stock 11 inch coil, or a small DD 8 inch coil? The answer is the 8 inch coil, it produces a much denser field of magnetic field lines vs the larger coils. It won't go as deep because the geometry of an 8 inch coil produces a magnetic field that doesn't go as deep as an 11 or 15 inch coil, but within its depth range say 6 inches it produces a much denser field, more field lines striking the small target, the stronger the signal on that target.

The reason the larger coils have a less dense field is you have to stay within a very narrow range of electrical specs when building a coil. If you measure a bunch of different sized coils, regardless of size you will find they all measure very close to the same resistance, capacitance, and Q. A 15 inch coil has a much larger TX transmit winding, but can't have more resistance than an 8 inch coil. Lets say 40 feet of copper magnet wire of a given gauge gives you the correct resistance for the TX winding, on an 8 inch coil you are going to get a LOT more loops around that TX winding than on a 15 inch coil, more loops, the denser the magnetic field. If you are thinking well just increase the gauge of wire on the 15 inch coil TX winding to lower the resistance to get more loops and a denser field, nope that will throw off the capacitance and Q. Its a tug of war between these parameters. The RX receive winding is even more picky, for a given size coil you may find even a 1/2 gauge wire can be the difference between a coil that hits the specs and one that is out of spec.

And if you are not yet convinced how maddening building a coil can be, even the type of insulation on the magnet wire or the thickness of the magnet wire insulation can throw off your specs. How tightly or loose you wind the winding can throw it off. You will often see the RX winding wound quite loose indeed.

So coils of various shapes and sizes, there are trade offs. They will be good and some things, not so good at others, just depends on the type of detecting you are doing and your soil and trash conditions you are working with.

Re: Bad coil, testing a coil, building a coil
Posted by: Charles (Upstate NY)
Date: February 19, 2017 03:03PM
I once built a 9 inch concentric coplanar coil for my Explorer, yeah a round coil like on the Whites machines. There's no rule that an Explorer must use a DD shaped coil. The concentric coplanar coil has three windings, a large round TX winding on the outside, then a smaller round RX winding inside that, then a third even smaller round RX winding inside that but wound in the opposite direction of the outer RX winding. This inner winding is what balances the coil, rather than overlapping TX and RX to balance the coil as in a DD coil, on the concentric coplanar its the number of loops and position of the inner most opposite wound RX winding that balances the coil and produces a flat line e.g. no signal on the RX winding.

Why care? Because this coil produces a different shaped magnetic field. Again visualizing in 3D this coil produces a cone shaped magnetic field. Its like having a 9 inch pin pointer. It won't cover as much ground as a 9 inch DD coil, but at the center it will go deeper, and around the parameter nearby trash is less of a problem. I once dug an Indian head cent with this coil, the coin was about 6 inches deep and a strong clean signal. I dug a 6 inch plug and examined the plug of dirt, good lord there was a bunch of trash in that plug that coin was still there after years of detectors sweeping that site because it was hidden in that trash.

So after you exhaust a site with the stock coil, if there's a fair amount of trash and/or rusty nails you may pick off a few more targets by hitting the site with a different size or in this case style of coil. No one coils gets them all.

Re: Bad coil, testing a coil, building a coil
Posted by: MikeO
Date: February 19, 2017 06:21PM
Awesome post Charles. Very informative for sure.
I am moving this though to the classroom as it is not a find and falls into the Explorer Classroom kinda of area.

Detecting since 1979
Findmall member since 2000
Became a dedicated Minelab user in 2000 with a XS

Favorite find ever a 1834 Half Dime (15 years ago)
Oldest coin ever found a 1734 Real (11 years ago)

One of my favotites from 2012 season an 1857 Flying Eagle Cent.,1795264,1795264#msg-1795264

Favorite coin from 2013 season an 1835 Capped Bust Dime.,1962866,1962866

Favorite coin of 2014 so far an 1849 Large Cent.,2034861,2034861#msg-2034861

Favorite Silver of 2014 an 1854 Seated Half Dime.,2106714

A hunt that will likely never be repeated. Silver Three Center and Seated.,2115392,2115392#msg-2115392

Oldest and largest for 2015 so far 1807 KG,2167872,2167872#msg-2167872

Re: Bad coil, testing a coil, building a coil
Posted by: BigTony
Date: February 19, 2017 10:16PM
Charles, thanks for that interesting and informative post and also your second post below on concentric coils.
I use a 10 x 12 SEF, 15 inch WOT and the five inch Excellarator.
I agree with what you said about different coils and going back over an area with them.
Don't forget that FBS machines have two modes to use, conductive and ferrous. I use both of those and still turn up more stuff.


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