Coils for TDI

[docguin]

Hello All,

I am a new member here but have enjoyed reading this forum for a while now. It is a fantastic site, with a wealth of metal detecting information. I am not an electronics wizard, but wish to learn to build my own coils for my Whites TDI.

One subject I see little about is the shape of the coil and it's effect on the performance of the coil. Is there an optimum shape for different kinds of treasure hunting?

There seems to be many ways to wind a coil. None seems to be generally accepted as better or best. How do these different styles of winding affect performance of the coil?

Can a coil be optimized for the size, depth, and composition of the the desired target by it's shape or method of construction? Is there a design reference or guide that may be used?

[Carl-NC]

Generally, round coils work best. TDI can go down to 10us delay, so you want to make decently fast coils if you're looking for small gold... read BBSailor's article on building fast PI coils if you haven't already.

If you are looking for deep caches, you can be a bit sloppier. Below is my 30-minute quick-build disposable 24" coil for cache hunting, made specifically to find a 3-pound cache of silver coins buried 2-feet deep. Unfortunately I cut one too many corners and did not bother to shield it, and it was just too noisy.

There is no formal design guide for coils, just a bunch of rules-of-thumb and trial-and-error.

- Carl

[Qiaozhi]

Generally, round coils work best. TDI can go down to 10us delay, so you want to make decently fast coils if you're looking for small gold... read BBSailor's article on building fast PI coils if you haven't already.

If you are looking for deep caches, you can be a bit sloppier. Below is my 30-minute quick-build disposable 24" coil for cache hunting, made specifically to find a 3-pound cache of silver coins buried 2-feet deep. Unfortunately I cut one too many corners and did not bother to shield it, and it was just too noisy.

There is no formal design guide for coils, just a bunch of rules-of-thumb and trial-and-error.

- Carl

Out of interest - did the TDI manage to detect the cache?

[Carl-NC]

In the air, it would detect a same cache at 27-28". But it was just too noisy on the ground to distinguish such a deep target. Interestingly, TDI found 3 really deep nails that I had missed when I cleaned out the cache site.

Once I got 8" of soil off, both TDI and V3 detected the cache.

- Carl

[docguin]

Carl,

Thanks for the input. That is a great looking coil! Light and functional. I may make something similar with a bit more finish. A little shielding, a little epoxy and that thing would be good to go.

I am trying to optimize a coil for hunting silver coins with the TDI. I think a mono would be a little less sensitive to small surface things than the stock coil, which is what I hope to do. A little less sensitivity to low conductors would be good too.

The TDI is a great PI, I am enjoying it very much and look forward to experimenting with various coils. I have read the article by BBSailor, I find it very helpful and the best I could find on the topic.

I am going to start with an 8" mono but feel a larger coil may offer less sensitivity to small and shallow and more sensitivity to coin sized objects a little deeper. I may have to make larger and larger ones till I find the right size for me. I wondered if the winding method (spiral, basket weave, etc), or the shielding method had an influence on how the coil performs at various depths.

I saw a post where aluminum foil tape was used as shielding. Not much follow up on it though. That would be a nice inexpensive way to go if it works.

[Reg]

Hi doc,

I see you made it here ok.

Now, to fill in a couple of blanks, aluminum tape really isn't the right choice for the coil shielding. You can use the scotch 24 for shielding or you can use a form of other tape that is no where as thick as the aluminum tape.

I like using the copper or silver plated tape, which is a strong plastic tape with a very thin plating of either copper or silver. This is thin enough that it isn't detected as a target and influence the decay signal. The down side to this type of tape is it is hard to come by in small quantities.

Here is a link to the Scotch 24 that is on sale on ebay.

http://cgi.ebay.com/3M-Scotch-24-Electrical-Shielding-Tape-5-rolls_W0QQitemZ370200401826QQcmdZViewItemQQptZBI_C onnectors_Switches_Wire?hash=item5631a9d7a2&_trksi d=p3286.c0.m14&_trkparms=72%3A1205|66%3A4|65%3A12| 39%3A1|240%3A1318|301%3A1|293%3A1|294%3A200

I use a tape similar to this;

http://www.lessemf.com/fabric.html

drop down to the Ni/Cu/Co FABRIC TAPE

and look at it. What I use is similar. I use a 1/2 wide tape rather than the 1". Some people can wrap the 1" on nice and neat. I find the 1/2" is easier for me to work with. I have cut some 1" wide tape into two pieces 1/2" wide before also. Regardless it is another option rather than trying to use aluminum tape.

One of the shielding methods I do need to try is the aquadag. I haven't tried it yet. I did try a graphite paint but didn't get it on thick enough so the shielding was insufficient. When that happens, the coil is sensitive to bumping or touching certain plants, either of which can result in a false signal.

Don't forget the spiral wrap. I buy mine from US plastics. Their spiral wrap is cheap and works well. I usually use the UV black, 1/4 " size (41164) but also do use the natural in the same size (41156).

Here is a link to that;

http://www.usplastic.com/catalog/pro...oduct_id=12508

This should give you some options now to try.

BTW, one of the coils I use on a regular basis on my TDI is a home made 8" diameter coil. If I get a chance I will take a pic of it. My coil is a little bumpy because I fill the housing with two part foam once the coil is basically assembled. This makes the coil very solid and light weight as well as waterproof. On the down side, too much foam will cause some distortion to the housing. On the plus side, the housing is sealed enough that no additional sealing of the edges is really necessary. In a nutshell, the coils are not the prettiest but work quite well.

Reg

[docguin]

Hi Reg,

Yes, I have been here for a while reading the fabulous posts. It is an interesting place to say the least. I do not have a very deep understanding of a lot of the technical things, but these guys are working on some really great ideas and they are great fun to read. I have a whole new appreciation for what is in the TDI and how it works.

Thanks for taking the time to write up that additional information. I know that is a fair amount of work for you, and is very valuable to me. For me, a lot of the information I have been chasing down is so basic to everyone here that it is assumed knowledge.

I agree on the 8" coil. For some reason it seems to be a fundamental size to a number of detectors I have used. It just seems to outperform a lot of other sizes, the signals sound cleaner to me. I also have a preference for round coils though I have no sound reason to base that bias on. I have used coils from 6-18", and my two favorites are the 8" mono and the 15" WOT coils.

It is my hope to build a really good 8" mono for the TDI which will enhance the performance capabilities unique to this machine. I am getting the best materials I can find and plan to take special care with the winding in an attempt to get the clean crisp signals I like.

I am also considering building my own coil housings. That Styrofoam platform Carl showed is really a good place to start I think. Very complex shapes can be cut into it and epoxy makes it durable without adding too much weight. They also make a number of foam in place products in cans and you can choose the expansion characteristics to suit the application. I think the idea of foaming the coil in place is a good one and have also been considering an open weave winding design that might be foamed to hold the wire firmly before the spacer and shielding go on it. Still working that out in my mind.

One additional question I have. As I understand it, the shielding must not be continuous around the coil. It must have a gap so the ends do not touch, is that right? I have your TDI plug wiring diagram, but the coil wiring to coax is less clear. As I see it, the coil shield and one end of the coil winding goes to the shielding on the coax and the other end of the coil winding goes to the center conductor on the coax. Is that right? Does it matter which end of the coil winding goes where?

[Reg]

Hi doc,

First, you are right, the shielding, if wrapped around the windings should have a gap of 1/4" to 1/2" or so. If the shielding is a complete loop it forms a shorted turn and will reduce the sensitivity quite a bit among other things.

For starters, I wouldn't worry about getting fancy on the winding technique. Trying something like a basket weave is nice but hard to shield. Also, I haven't found it to help that much in reducing the capacitance. If you use Teflon coated wire or even simple 300v stranded hookup wire for the small coil, you should be fine. What can degrade this winding would be a shielding that could be detected.

If you look at one of the other posts under the coils heading concerning the wire used "When same cable isn't same cable..." and look at the two pics good and bad. You will see the bad cable has the long time lag and this is what happens when the shielding is detected or a strong target signal is present. In the case of the wire in the article, I suspect they changed shielding and that is what is causing the problem. It is hard for me to believe the capacitance could have that effect if a short wire run was used.

I have tried building coil housings and finally gave up. It is easier and cheaper for me to build a coil or two using one of Hay's forms. Remember, it is a good idea to have a cable stress relief, which comes with Hay's housings. It is hard to find these items in small quantities and get the right size.

If I were to build a wide range of coils for the TDI for coin hunting, I would build a 6", 8" 10" and 14" round coil. This would probably cover about all the sizes one might need for coin hunting. I might consider an 18" coil for cache hunting or large meteorite hunting.

Reg

[docguin]

Thanks Reg,

Your help has been invaluable. Following your advice I think I will just wind one up with Teflon insulated in the simple fashion you recommend. I think you are probably right about the coil housings too.

Did I have the right idea on how the coil is wired into the coil wire?

[Reg]

Hi doc,

Yep, one lead of the coil and the shield connect to the coax shield. I have not found that it matters as to which wire on the winding is connected to the shield, but I haven't done much experimenting on this either. You might try both before sealing the coil.

If the windings were such that one end was on the inside and all the outer windings were closer to the other end, I would connect the outer one to the shield. However, that isn't easily done when winding a simple PI coil.

Now, if I were winding a concentric coil to be used on a VLF where I had a second transmit winding wound on the receive windings, I would be more careful. It could make a difference as to which receive end is connected to ground or the shield.

Now, one more idea for the shield is to use one of the different types of conductive cloth listed on the LESSEMF website. Some of those types of cloth could be used just fine and either wrapped around the windings or glued to the coil housing. However, some may be too conductive. I would try Veilshield first if I were to try one of the fabrics.

Reg

[docguin]

Coils for TDI

Thanks Reg,

That makes it a lot more clear. I have most of the materials on the way now and hope to make the first one soon. Most of the expense seems to be in first purchasing the materials, after that each coil doesn't really take that much so I should be able to experiment with a variety of coils to try and optimize for how I hunt.

[Roughwater]

conductive carbon pain as shielding

I have a question to Doc, Reg or whoever might know the answer.
As some of you know from the TDI forum I have built a 7" TDI coil with much help and encouragement (on line) from Reg. I am in the process of building a second TDI coil. I used the scotch 24 tape as shielding on the first coil and was planning on using some Conductive carbon paint I purchased as shielding on the second coil (18"X14"). However I read a statement by Reg that the conductive paint should be low resistance.
The carbon paint I bought has a resistance of 1200 ohms according to the manufacture which seems typical of conductive carbon paint.

Ok, finally to my question: Resistance wise, should this paint suffice as shielding for the second coil? Also, if so, can I paint it directly on my coil which I have half hitched and siliconed or should I first wrap a layer of the spiral wrap on the coil?
I did paint a 2" section of the coil just to see how it applies and acts upon drying.

Thanks,
Terry

[Reg]

Hi Terry,

The shielding paint normally goes on the coil housing on a PI. I know a guy who did apply the shielding to the coil but added a layer of tape first and painted the tap. This was for a coil for a VLF type detector, though. PI's require more shielding to be effective.

Now, I suspect the conductive paint you have will not be low enough resistance to work properly on a PI. I have a piece of the shielding paper from a Coiltek coil somewhere. If I can find it, I will measure the resistance of it. For some reason I think the resistance was somewhere around 40 ohms an inch or so, but I will check if I can find it.

Reg

[Roughwater]

conductive paint

Thanks much Reg for the quick reply. I have found some 18% silver paint online that is said to be .4 ohm/in. I guess that would should work but it's much more expensive than the carbon paint. The scotch 24 seemed to work fine but takes a little time to cut and wrap and add another series of half hitchs to hold it firmly in place on the coil. Just looking for a way to reduce my time and labor without sacrificing quality.

Terry

[Reg]

Hi Terry,

I found the old Coiltek coil and measured the carbon paper. It measured close to 75 ohms per inch.

Now, I don't recommend the copper or silver paint. The resistance will be too low. This can interfere with the depth and may be detected.

In other words, you can't have the resistance too low or too high when using a shielding paint. This is not the same as wrapping the windings in terms of how it works. You can have a highly conductive tape you use for wrapping the windings and that will not have a negative impact as long as it isn't detected.

I have some graphite based paint I need to try again. I suspect it will require several coats to get the right resistance. Also, I may try some y-shield before it is all said and done.

As for the use of the scotch 24, you can simply tape one end, spiral wrap it on the winding and tape the other end to keep it snug. Then simply add a layer of electricians tape over the shielding to keep it snug. I don't add any half hitches on the shielding.

Reg