Coils for TDI

[wyndham]

Reg and others, I have always had problems with meters that I wrapped a wire around like the example of the LCD meter where the coil wires were wrapped around the probe tip. Does this give any false signal when measuring ?
I have another lcr that only has blade slots and what can i use for a probe in that case, thanks Wyndham

[Roughwater]

Wyndham,

I'm not as knowledgable as Reg when it comes to electronics but I doubt that wrapping the ends of a coil wire around a probe a time or two will make any significant differance in your coil inductance readings. As to your other meter having "blade slots" I can't help you there. Maybe if you send a pic in your meters connections I might be of more help.

Best Regards,
Terry

[Reg]

Hi wyndham,

I have three different LCR meters and all have wire leads as a means of connecting to the coil. However, one of the meters does have slots also. Normally I only measure capacitors with the slots but I guess one could use a stiff wire inserted in the slot and then connect the winding leads to the stiff wire.

As for any inaccuracies because of the meter leads and the possibility of any winding of the lead around the coil, please keep in mind that most of these meters only have a 2% to 3% accuracy to begin with. Any error as the result of LCR probe interaction is going to be far less than the general numbers of the coil, so I don't worry about it.

Just use common sense when measuring the final result. By this I mean, don't set the coil windings on a metal surface, or have large metal objects under them. Keep everything relatively neat, so there is no wrapping of the coil leads around the meter leads.

Finally, when in doubt simply check. If you wonder if something might alter the final results, simply try it to see. That is the best test and then you will know for sure. Make sure to test several times to be sure of the consistency of the testing until you are assured of the answer. A test of one seldom provides a truly accurate answer.

Reg

[wyndham]

Thanks Reg. Since I am a beginner those question about "What am I Doing" always crop up and with the slots I was using a stiff wire but wanted to double check the method was ok.
Is the teflon coated wire the better one for coils on the TDI. I have a roll of Lietz that I have run several bench test to learn the meter and the calculation for mh
Wyndham

[Reg]

Hi wyndham,

Litz wire may work just fine. The problem with Litz wire is there are different types, and number of possible wires that make up a Litz type. As an example, they make served Litz and non served. The type I have seen is the served has an extra wrapping of string type material around the strands.

Now, does this served non served issue make a difference? Yep, the extra string type material adds a little space between windings and this alters just how the coil will work.

Since Litz is made up of individual strands, this further complicates things because of the wide variety of individual strand sizes plus number of total strands. Again, all of this can make a difference.

Now, with the exception of the ML's, if you are making a coil for a PI such as the TDI or the Hammerhead, or even some of the other kits, the best way to know for sure if a coil wil work is to simply try it. As long as the inductance isn't dramatically off, or there are no shorts in the soldering or wiring, then nothing bad should happen.

With proper wiring techniques, the worst that can happen is the coil may oscillate a little, or the decay time may take too long to work at the minimum delay. In either case, the coil should work at a little later delay.

Half the battle in learning to build coils is to see just what happens and then experiment to find the best solution. It is also a great way to learn.

So, rather than worry, simply build a coil or two and see what happens. Don't forget to keep in mind that if the coil doesn't work at the shortest delay it will most likely work fine at a little later delay.

Also, keep in mind the best size of coil to start out with is a smaller one. This helps reduce the capacitance so it is more likely to work at the shorter delays. It is far easier to make an 8" coil work at 10 usec than it is to make a 16" coil that will work at the shorter delay.

Reg

[wyndham]

Rail gun question

I maybe should have started another thread but as Ideas pop up, I try to follow it for a bit.
I was watching the science channel and the subject was a rail gun, where opposing currents on parallel rails launched an aluminum block at high velocity to a target.
I believe I understand that in a PI coil, we have alternating induction pulses to create the TX field.
If there were 2 coils parallel and transmitting a pulse at the same time but in same fashion as the alternating pules of a tx coil so that the two coils produce the field could this benefit in strength or size (depth) or speed of the TX?
I can see an oblong coil of 8 " and 2" wide set vertically with a space between the rail coils of some small distance.
the Rail gun seems to me to be a version of a tx part of a coil but then this maybe far from what really is. In this case the two alternating currents pulsed at the same time might null each other, would it?
I may have had too much coffee this morning, Wyndham

[Roughwater]

Finally almost finished my 18X14 TDI coil

After making 3 18X14 coils I finally got the 3rd one large enough to fit the housing I purchased. The first two are both less than an inch short but no cigar. I'll use those later on with some home made housing maybe make one round and one a 22X6 or something like that.
The positive side of having made several is that I can now put a good working coil together very quickly. I started this evening about 4PM and by 8 I had put it together almost from scratch and briefly tested it. The only thing left to do is to epoxy or foam foam the housing and seal. It works great though at least with air testing especially in "ALL" mode. I didn't know I had so much metal junk in my yard! Seemed to work fine even with the pulse delay at 10.

Here's a real quickie explanation of how I made this one:
I already had a set of hooks screwed into wood to wind the coil on, I just added an additional loop to lengthen the coil. I tightly wound my 24ga teflon coated stranded wire around the hooks till I throught I might be close then I checked the inductance with my meter. It came up 180 uh so I wound about six more windings around the loop and it came up about 330 which is about what I was looking for. Then I cut the wire from the reel giving me an extra 8" or so and wrapped the end around a screw 2 or 3 times to keep the winding tight. I then made a series of half hitches all the way around the coil. Then I took my cauking gun and ran a bead of silicon all the way around the top of the coil winding. Then I smeared the silicone all over the surface of the coil. Without letting the silicone dry, I wrapped a single layer of scotch 24 shielding tape around the entire winding except for about the last 1/2" as you don't want the shield to form a complete loop. I have found that winding the shield over the wet silicone really helps to hold it in place once it's dry. I then put just one half hitch or 2 at the beginning and one at the end of the shield and leave about 3 inches of shield at the end that I twist up till it looks wire like to give me a lead I can solder to. At this point I usually check the inductance of the coil again to confirm all is well. I have found that wrapping the shielding directly to the coil affects my inductance reading a bit, usually less than 40uh so I like to see my pre shielding inductance between 330-350. Some recommend using a wrap material between the shield and the coil to help reduce any interaction but I unknowningly wound my first coil without the wrap and it worked great as does this coil. I probalby would have used the wrap with this coil as I have some but the wire I'm using is 24ga and it makes a pretty thick loop and I wanted to avoid making it any thicker. Had I used 26 ga wire or thiner I might be tempted to use the foam wrap but I got a great deal on 2Kft of this wire so I'm gonna use it. Moving on, I then run my single conductor mike cable into the coil. I try to keep it as short as possible inside the coil as it contains metal of course. I solder one side of the coil wire to the mike center conductor and the other coil wire goes to the coild shield and to the mike shield. Of course I use plenty of shrink tubing on all wiring. Then I solder the other end of the mike cable to a 5 pin mike connector. The ground side goes to pin 1&5 and the single conductor to pins 2,3&4. I split the mike shielding in 2 parts and solder to pins 1 and 5 for extra strength. Slide one large piece of shrink wrap over the 5 soldered connections and shrink. I once again check my inductance before testing the coil by connecting one lead to either pins 2,3 or 4 and the other lead to pins 1 or 5. Your inductance should be very close to your last check as adding the coax should cause little change. Hope this helps someone. A month ago I had no idea where to start when it came to building a coil but thanks to Reg and his posts and his generous sharing of information both here and at the TDI forum I have learned so much but for sure still have so much more to learn. Thanks again Reg.

Terry

[Reg]

Hi Terry,

It looks like you are doing just fine in the coil making arena. Now, my question to you is, does this large mono coil work at the minimum delay setting of 10 usec? If it does, you are really doing exceptionally well, since that is very hard to do. I would think it may need to have the delay adjusted up just a little.

Now, as for your coils that are too small, you might look at another housing reasonably close and you might want to try something different with them. You can take them apart of course, or you can form them into two coils and use it in the 14" by 10" housing. In this case you will form a figure 8 with one part of the eight smaller. The larger one will form a large mono loop around the outer part of the housing and the smaller coil will fold back into the large coil and fit in the narrow center zone of the housing. This will give you sort of a dual field design and still be a very large elliptical coil. I am not sure this makes sense to you so if it doesn't I will try to draw something to help.

Anyway, it does provide a way of using your other coils you wound.

Now, getting back to your large coil you just made, do you have a particular use for it in mind?

Reg

[Roughwater]

Extra coils

Reg,

Thanks again for the advice. The large coil does seem to work at 10 but my initial feeling is that it may work better at the medium setting. I'll test it better later but I'm off to the beach this AM to visit my family.

I wondered if I might make a DD out of one of the extra coils but you confirmed it. I really don't want to take either back apart so I think I might try and make a DD.

I figured I would use the 18X14 in areas where there is little trash and targets are spread out a bit. When I was at the beach recently I found places where targets tended to be far apart. If I every make out to gold country again such as at Rye patch I'll definitely use this coil some and it might find use looking for metorites as well.

I'll soon make a test garden so I can better compare my homemade coils with each other and with the stock coil. However, during my quick backyard test of the 18X14 I definitely had to lift the coil much higher off the ground to loose the signal compared with the stock coil.

Terry

[Roughwater]

Further checked out home made coil in yard this evening

Reg or whoever,

I checked out the 18X14 coil a little more this evening and it looks like it will go very deep (12"+) on a dime in all mode judging from air tests. However It's not as impressive in the High conductivity mode. The 18X14 works well at a delay of 10 so long as you keep the coil close to the ground. At a delay of 10, if I raise the coil several inches off the ground and wave the coil back and forth it will want to "sing" like it's picking up a weak signal. If I turn the delay a 1/4 turn it no longer falses in the air.

I also checked it over a nugget. It was kind of a chunky nugget though probably weighing close to a pennyweight or so but the only one I have left. It air tested Ok in the "all" and "Low" conductivity settings at a delay of 10 and higher.

I used the big coil to try to find a small patch out of my 2 acress to put in a target test garden that was not littered with metal. Not an easy chore but I finally found a semi quiet area. It's very rocky ground so I don't look forward digging holes in it but I figure it's the best way to really evaluate the strengths and weakness of any detector or coil.

Terry

The attached pic has nothing to do with building a coil, Just a pretty view of upper Mills creek AK with an old rusting trommel in the background.

[Reg]

Hi Terry,

The best way to tell how a coil is working is to turn the GB off. This shuts of the GB channel and only uses the main channel. Now, when in the off mode, check to see if a target signal is weak. Slowly turn up the delay while passing a target over the coil and see if the signal suddenly gets better.

To eliminate the earth field effect, two samples are taken for each channel. There is the normal sample and a much later sample. This later sample basically eliminates the signal from the earth's magnetic field if the two signals are equal.

When you swing the coil in the air, you are basically testing the subtract process to eliminate the earth field effect. If you hear a moan or a tone when you swing the coil in the air, it says the two channels are not balanced or equal This occurs when the main channel isn't leveled off at the time of sampling, so the sample is only a partial sample.

By turning the delay up, you sample a little later. So, you can also wave the coil in the air like you did and very slowly turn up the delay to the point where the moan or as you say, singing just stops. That would be the proper or minimum setting that will work right. Just before that point, you may find the coil to be somewhat hypersensitive.

If the delay is too short for the coil to work properly but you have the GB on and trying to detect a target, the target signal will probably be low or very weak. In the worst case, the target signal you hear may be inverted or opposite of a normal target signal (when the GB is set properly). This means if it is a coin and the GB is at 9, the silver or clad coin such as a dime may generate a high tone instead of the normal low tone. This can happen if the coil sample in the main channel is not proper and the ground sample is actually stronger than the main sample. This can only happen if the main sample is taken way too soon because the coil signal recovery time is too long.

To be honest, what you are experiencing is what I expected to happen on such a large coil. It is almost impossible to get that size coil to work at 10 usec.

Reg

[Roughwater]

Coil Inductance

Reg,

I know that the Ideal inductance for a TDI coil is 300uh and better on the high side than low. Why can't we just wrap one or two heavy windings and make up the differance with an inductor?

Terry

[kt315]

Carl, I try to describe your test in russian forum but people does not believe in the zero result. I am in some prostration... can you do some short video of your cashe test with White's 6000/di Pro with 24" Magnum "cache" coil and TDI?
TIA
all the best

[Roughwater]

Answering myself

Reg,

I know that the Ideal inductance for a TDI coil is 300uh and better on the high side than low. Why can't we just wrap one or two heavy windings and make up the differance with an inductor?

Terry

Hey Reg or whoever,
I am glad no one answered the question I posted yesterday which is in quotes above as I believe I know the answer and I realize now it was a dumb question to start with. The inductor in this circuit is the detector coil and if we place another inductor in the circuit to try to make up the coils lack of inductance, though the inductor may be very small, it will most likely also react like the primary coil reducing signal to the main coil and probably cause noise or impart a constant tone to the detector.

Terry


Terry

[Roughwater]

Start build of 1 Meter coil

To Doc, Reg and all,

Just got a start on building a 1 Meter coil. For the moment it's actually 28X44 due to the width of my backing.

Building it simple and sticking to what has worked for me so far.
For wire I'm using 24ga stranded single condutor teflon coated wire. 10 turns of wire seems to give me a suitable inductance reading of about 370uh. 9 turns only came up to 290uh so I went with 10 turns. Attached are a few pics. Will add more info and pics as I go.

Terry