Explorer II coils ?

[Tamis]

Hi guys,

Anyone knows how to make explorer II coils ?
Any tutorial around ??

[Charles (Upstate NY)]

Building Explorer Coils

I build my own Explorer II coils and some others are starting to build them as well. Here is an overview of what you will need equipment wise and how to put one together.

Equipment:

1. Oscilloscope - you can buy a new one now for as little as $207 http://www.testequipmentdepot.com/instek/oscilloscopes/gos-310.htm though a 2 channel scope is best. Or take a chance on a used scope purchased off ebay. I bought two used scopes off ebay, a BK Precision for $75 which came with 2 probes and worked well. I also bought a Tek 453 for about twice that, its a little flaky though so last week I purchased this one http://www.testequipmentdepot.com/instek/oscilloscopes/gos-622.htm because the Tek scope trace was too fuzzy and got to be annoying. You need a scope to balance the transmit and receive windings (TX and RX). You don't need anything special, a 5-10 mhz scope will get the job done just fine.

2. LCR Meter - and not one of the cheap $79 units, you should plan on spending $170-$230 for the next level up in meter quality, Extek or BK, it should also measure Q. On this subject, R = resistance, C = capacitance, L = inductance (why they didn't call it "I" I have no idea) and Q = quality factor. You will use the LCR meter to first measure your factory coils to establish a range for each measurement. For example RX L (inductance) on my factory coils range from 501uh to 518uh. I then wind my custom coils to within that range.

Thats pretty much it equipment wise, of course you need wire cutters, soldering iron, small hand tools etc.

PARTS...

Magnet Wire - You will find that LCR varies depending on the guage of wire you use, for example you might wind RX with 30 guage wire and while L is right on the money at say 505uh you find R (resistance) is too high by 1ohm at 7ohms and you need to lower it. To do so you need to use a different gauge wire. I'll save you a lot of time and money, TX you can use either 19 guage or 20 guage, 19 falls in the middle of the range I established for my factory coils, 20 guage is on the high side of that range but is desirable because its not as bulky. For RX there is only one choice, 29 guage. Bernie recently tried some 28.5 guage and it would not fall into spec and I have tried 30 guage and its way out. So 20 and 29 guage would be my recommendation.

Side note on Q, the Explorer likes low Q windings, if you for example wind RX then wrap it nice and tight with some tape it will boost the Q out of spec and also very likely increase L way out of spec. You don't want it birds nest sloppy but you don't want to wrap it up tight either. Bernie hereafter known as "Bernie the Vice" wrapped his first RX winding so tight it went from 500uh to like 650uh! I'd say...you want it secured snug in a few places around the winding, just enough to keep it together. The RX wire is so fine that it will tend to spring out of shape. TX on the other hand you can wrap tighter, its so thick you can't really squeeze too much L out of it.

Coil Cable - Perhaps you can scavenge a cable from a bad coil thats one option. There is no readily available commercial cable on the market that I am aware of except an S-video cable. The Explorer cable has 2 conductors, each individually shielded and isolated from one another. The s-video cable is the same configuration but the wire guage is tiny compared to the Explorer at least on TX. I had 1,000 feet of custom cable made, 150 with the curly cord section the rest is straight. I could part with some of it if you need a cable.

Coil Shells - Hays Electronics sells coil shells, I have begun offering them as well, those are the only two sources of coil shells that I know.

Flex Fittings - e.g. coil cable strain reliefs, I use Sealcon fittings with the o-ring and supply them with my shells.

5 Pin Mic Connectors - female inline, they are widely available and range in cost from 99 cents to $4 for pretty much the exact same part so shop around.

Shielding Paint - I'm currently using Electrodag 502 http://www.laddresearch.com/General_Catalog/Chapter_2/SEM_Conducting_Paints/sem_conducting_paints.html but we are experimenting with conductive fabric tape at the moment. It seems fine for the Sovereign, not yet sure if its going to work for the Explorer. I hope so because shielding the coil is pretty much the one difficult, and highly annoying part of building a coil. If the tape works, this will greatly simplify things. I already built a prototype, the tape as applied was too conductive and the Explorer would detect it if the coil was bumped, but overall it was quite useable. Further testing showed that the Explorer could not detect the tape at a given size, say 1 inch strip even when rubbed directly on the coil. We are thinking it may be possible to shield the coil with smaller sections of tape, each insulated from the other, but tied together with a fine drain wire creating what may be akin to a tiny gold chain that the detector can pick up.

Okay thats the equipment and parts you will need to build a coil, seems pretty simple right, some copper wire and a cable, not exactly rocket science. But lets talk about assembly, thats where things can become difficult.

I will assume that you have fashioned some jig for winding RX and TX to fit whatever size coil you are building and have wound both to with factory coil specs.

Its best to lay the windings out on a board with a paper template of your shell (if you trace your shell out on paper note this will be backwards, you will be working on the coil bottom side up), tape them down and balance them on this template first, this is to make sure the windings will still fit within the shell parameters when balanced. How much the windings overlap depend on the size of the windings, its possible to make the windings a bit too large or too small and not have enough room in the shell to balance them, in which case you will have to adjust your winding jig.

Balancing the coil - okay lets tape the windings down with removeable masking tape within the shell template. Note you are working with the coil facing bottom up, rear of the coil nearest you, front of the coil away from you. From that starting point TX should be on the right, RX on the left. RX should be taped down first, then TX over the top of RX. Out of the Explorer control box you have TX main and TX shield, RX main, and RX shield wires. Connect TX main in such a way that it flows in a clockwise direction, if you think in terms of a garden hose, water will flow out of TX main into the TX winding, and flow around in a clockwise direction, come out of the other end of the TX winding and into the TX shield wire. RX is wire just the opposite, RX main should connect to the RX winding and flow in a counter-clockwise direction, exit the RX winding and connect to RX sheild.

Its is HIGHLY advisable to build a test fixture for doing this work. You will need a female 5 pin connector which will plug into the Explorer control box, with bare wires out the other end for connecting test leads between the control box and your windings. Be sure you mark them clearly so you always know which is TX and which is RX. You can test a factory coil to see which pins are which, RX will measure about 6ohms resistance and about 500uh, TX will measure about 1 ohm resistance and about 580uh. WARNING: If you get the wires crossed you may smoke your control box. I have had a few senior moments when I did this and the Explorer didn't much like it. Mine at least can take a beating and keep on ticking but I would not bank on it being able to handle even a single short. Remember you are going to be moving these windings around balancing them, be sure to tape down the wires so they don't accidently touch and ruin your day.

Okay now connect your scope, I have a two channel scope and connect channel 1 to TX and channel 2 to RX. I set channel 1 to 5 volt division and channel 2 to 1 volt division (to start). Both probes are grounded to the scopes ground source. Check all your wires again and when you are sure they are correct, turn on the scope, then turn on the Explorer. You will see a TX square wave form on the scope. RX will likely be quite a ways out of balance and so you will see this large square wave induced into RX from TX on the scope. You want to adjust the overlap between TX and RX until this flatlines at 1volt div. Now adjust channel 2 to say 50mv division, again adjust the overlap betwen the two windings until it flat lines e.g. no signal is being induced from TX into RX e.g. thats balanced. You can continue increasing your volts division on channel 2 but I'd say anything past 10mv division is probably not required. Most of the factory coils we have measured are slightly over coupled e.g. slightly overlapped too far e.g. instead of the square wave being completely flat, there is a slight signal induced in RX, about the height of the trace on the scope. This varies from coil to coil but you have a range from slightly under coupled to slightly over coupled where your coil will ID the targets correctly. Its good to have a nickel, a clad dime, silver dime, silver quarter, and silver half handy to ensure that your coil is properly ID'ing those targets. If you are out of whack the clad dime might rise up to the silver dime range and likewise the silver half which generally ID's lower on the screen than a silver quarter, will rise up to the silver quarter range.

You will find that a VERY slight nudge is all that is needed to knock a DD coil out of balance by quite a distance. Thats one of the things that make building a DD coil difficult. Tip: You will pour your coil with epoxy most likely, all of them shrink to varying degrees. As they do they tend to pull your windings together e.g. under couple them. So if you balance your coil perfectly you may find that once the epoxy has cured its out of balanced towards the under coupled side. Therefore its not a bad idea to over couple the coil somewhat and let the epoxy pull it back to the sweet zone. How much really depends on the properties of your epoxy e.g. the shrinkage so select your epoxy carefully from this respect.

By now your coil is probably balanced and you have tested it with some coins and are all giddy about how cool it is to have built your own working coil on the bench! Congrats! Now comes the hard part, assembling this thing into the shell.

As I mentioned before, shielding is the hard part. If you are using the shielding paint you have to decide well do you paint the inside of the shell? You can do that but you need to consider, how will is the epoxy going to stick to the paint verses a bare shell? You also must take great care not to get shielding paint down near the bottom edge of your shell because if there is a path for water (even due from wet grass) to your shielding paint the coil is going to false. Ideally you want the shielding to be isolated from the outside world. You could build an inner shell like the stock explorer uses, shield that, then fix that in an outer shell. There are any number of approaches, all have issues. Thats why we are exploring the shielding tapes. Once you figure out how you want to shield the coil thats 90% of the battle (remember you will need to run a drain wire through the shielding paint and connect this to the RX shield wire) and you can begin assembly. Some use hot glue to tack down the windings in balance, some drizzle epoxy, I use both small pieces of masking tape and the drizzle method. I let that setup and if the balance still looks good I'll pour the coil with epxoy and I'm done.

I have skipped attaching the coil wire as that seems rather obvious.

WEIGHT!!!! Nothing I hate more than some pile of bricks heavy coil and epoxy is heavy. If you are making a small coil its not really an issue but if you are making a coil say the size we just made for our explorers, 11x14 you WILL want to reduce the weight. That coil shell holds 16 fluid ounces and can easily weigh 2 pounds. There are various types of fillers out there which you can mix with the epoxy to reduce the weight. Microspheres, polyfiber, etc. You could glue some foam blocks in the shell at certain locations to soak up some space. Making a coil thats both stable and lightweight is a bit of an art form and will require some experimenting.

Well thats its for now, I guarantee you that your first coil will not be cheap and you may scrap a few prototypes but thats part of the process but I also guarantee that there is nothing quite as cool as building your own coil then going out and finding some good targets with it. Hopefully the above information will help people who have an interest get started and avoid some of the pitfalls.

Charles

[concad]

Charles,

Man thats alot of great info.

Thanks
dave

[Golum]

Sovereign coil

Hi!

Very much interests inductance for TX and RX Sovereign XS coils

[diminute]

And finaly

Size and turns for this coils ????


Thanks,

diminute

[diminute]

Another

Explorer has the same circuit that is show in Has anyone built a Sovereign Coil? by Carl??????

Thanks,

diminute

[diminute]

HI

No body answer this ????

diminute