Question about an 18 inch coil for a TDI

[Ism]

I finished a dual 18 inch spider coil for my TDI. The coil works fine down to 10us with GEB OFF. However it sees almost every object as a high conductor when GEB is ON with conductivity in ALL. For instance, a US nickle will be detected at 12-14 inches with GEB OFF at 10us. When GEB is switched ON, the nickle is seen as a HIGH conductor at 10us but its detection range drops to about 6 -8 inches, very bad. If I change the delay to 17us then it switches to LOW conductor but still sees the nickle at a reduced depth verses when GEB is switched OFF. Adjusting GB does alter the outcome some but for the most part low conductor objects are seen as high conductor.

Is this characteristic to be expected for an 18in coil?

Coil parameters are as follows;
#22 teflon jacketed/stranded silver plated wire.
330uH at .63 ohms. Q 3.3

Thanks for any insight.

[Reg]

What do you mean your dual 18" spider coil? Is this one coil or two? PLease explain your coil design more thoroughly. Is this a simple mono in a dual spider housing or is it dual coils? Are the coil, concentric and in series? Do you have a damping resistor across one or both?

In other words, please try to explain the design as thoroughly as possible.

Your parameters don't work for the 18" coil either. If this is a basic mono and has 22 turns and is an 18" diameter coil, the inductance isn't going to be 330uh.

Reg

[Ism]

I don't like to use the term dual-field since its Whites name for their coils. Maybe not necessary but I would rather not irritate them.

The coil is constructed in a Hayes spider coil with #22 teflon jacketed stranded silver plated wire, not 22 turns,

It is 2 coils in series, inner coil is 100uH outer is 160uH (individual inductances) with a 1k ohm damping resistor across both coils. Shielding is #24 scotch shielding tape with a 3/4" gap where the wires enter the coil. The inner coil has 2 layers of spiral wrap between wire and shielding, the outer coil has 3 layers.

I didn't do a resonance test because I wasn't concerned that it work at 10us. However it worked fine in all metal mode at all delays. When placed in GEB mode things change drastically for the worse. Detection range is reduced. The TDI is very stable until I place an object in front of the coil and remove it. Then it starts oscillating its audio between high conductor and low (GEB-ON, Conductivity-ALL, Sensitivity-3)

Very strange behavior so I think my construction is flawed but can't seem to isolate the problem as everything is the same as my other coils that work fine. My testing area has metal and AC wiring within 4 ft of the sides of the coil but front and back is at least 10ft. I shut off most nearby electronics and appliances to see if that was causing interference but no help.

I thought my damp resistor failed so I checked damping on the scope and it looks fine. My resistance is pretty low but inductance is right.

When i first posed the question, I thought maybe a large coil behaved differently to low conductor objects but decided later that I have a problem with the coil. I probably should have made a mono but I like the way a dual performs.

[Reg]

First I apologize for not being able to read.

Now, I suspect your coil has too little inductance. I strongly recommend keeping the inductance at 300 or maybe just a little more. So, for starters, I recommend you increase the inductance of the outer winding.

You mentioned you check the coils for oscillation. How do you do this. Do you look at the signal inside the housing or just at the coil. If you are just looking at the coil, then you can't really see what is going on that well. So, a coil could be oscillating and not show up because the oscillations are so minute. They will show up when looking at the output of the preamp, but I don't recommend anyone checking there.

What a person could do is build a preamp circuit to check the coils so all checks are independent of the TDI. Just make the damping resistor a 1K. If you use two preamps, then you can set the gain of the pair at 1000 so you can check the coil more accurately.

So, what could be happening is oscillation is occurring which can cause strange things. Oscillations are not that uncommon on coils of insufficient inductance.

To be honest, it has been a while since I build a dual coil mono. So, I can't tell you what the total inductance should read once the two coils are wired in series. What I do remember is I always made sure there was sufficient inductance.

Reg

[Ism]

First I apologize for not being able to read.

lol, I'm the one that usually doesn't read thoroughly.

Now, I suspect your coil has too little inductance. I strongly recommend keeping the inductance at 300 or maybe just a little more. So, for starters, I recommend you increase the inductance of the outer winding.

The two coils seperatly come to around 260uH but when I add them together in the housing, the total in series reads just under 330uH.

You mentioned you check the coils for oscillation. How do you do this. Do you look at the signal inside the housing or just at the coil. If you are just looking at the coil, then you can't really see what is going on that well. So, a coil could be oscillating and not show up because the oscillations are so minute. They will show up when looking at the output of the preamp, but I don't recommend anyone checking there.

Its probably far from the best way but I read of its use somewhere and its one that seemed to work well for the smaller coils. I induce a pulse into the coil from either the TDI or another detector (Gary's pulse). I put a 5k pot parellel across the leads adjust till the damping removes all ringing seen through the scope. The resistances read afterward on the pot coincide with average damping values mentioned in the forums (usually 800-900 Ohms). I tried direct readings from the PI output but couldn't accuratly define a value for damping.

What a person could do is build a preamp circuit to check the coils so all checks are independent of the TDI. Just make the damping resistor a 1K. If you use two preamps, then you can set the gain of the pair at 1000 so you can check the coil more accurately.

So, what could be happening is oscillation is occurring which can cause strange things. Oscillations are not that uncommon on coils of insufficient inductance.


To be honest, it has been a while since I build a dual coil mono. So, I can't tell you what the total inductance should read once the two coils are wired in series. What I do remember is I always made sure there was sufficient inductance.

Reg

I do see what looks like oscillation embedded in the damped waveform that my other coils don't exhibit so I suspect that is the problem. I have built around 8 coils, a couple mono's, a couple dual coil mono's, and a DD, all with very nice results (the second generation coils showed improvments)

This 18" baffled me because I built it using the same techniques I used for the other coils but got careless and potted it before thoroughly testing. A costly mistake in time and money. I will be more careful next time and watch for the pitfalls you mentioned. As far as the technique I use for damping, I would appreciate your critique....good, bad, or ugly

Thanks for your thoughtful reply.

Randy

[Reg]

There are a couple of other possibilities and one is there is too much damping. If this is the case then what you are experiencing could be very true. If the coil signal is very slow to rise, and the signal is taken on the curve and not at flat line, there could easily be a reduction in gain on the main signal. If that happens, then the GB signal will be dominate and cause all signals or most to act like high conductors.

If the coil works right at 17 usec then it sort of makes what I say more likely. In other words, the coil is actually working right at 17 usec and this would be the minimum delay that can be used.

The other possibility is the shield is seen as a target. So, I need to ask just how you shield the windings. If too much shielding is used and it is detected then there will be a droop in the rise of the signal and that will give you the results you are getting also.

I am not sure the extra spiral wrap is the best idea, but more importantly, I am concerned about the actual shielding procedure. So, we need to discuss that a little more also.

So, what could be happening is simply the shielding is too thick and/or too far from the coil windings themselves, thus it is being detected as a target which will cause the decay curve to be distorted. When the shielding is very close to the windings themselves, it is less likely to be "seen".

So, the shielding of any coil should be just enough to do the job and no more. The trick is to find that minimal acceptable level.

Reg

[Ism]

Hi Reg,

My shielding is the Scotch #24 split down the middle with scissors. I then open it up to create a 1" wide single layer of shielding tape. I wrap this around the coil spirally with a slight overlap. (see pic) This is the way I have done it with all my coils.

I am beginning to think you have discovered my construction error in the number of layers of spiral wrap. I got this great idea from reading the forum that the further away the shielding from the coil, the lower the capacitance. I didn't think that an extra layer of spiral wrap would be a problem.

You see, last year I built an 18" dual coil mono in a donut housing. A bad idea because the weight isn't properly distributed at the rod connector. It would constantly need adjustment as well as tiring me out after 15 minutes of testing it outdoors.

The 18" donut coils had 2 layers spiral wrap. Although it wasn't fast enough to work at 10uS, it worked nicely at 12.5 and above.

I couldn't convert the coils from the donut housing to the spyder housing because of the size difference so I disassembled them and made new coils for the spyder.

but the spiral wrap is the only construction difference with the spyder coil from the original 18" donut coil. It makes sense to me that it could be the problem lies.

You revealed a couple things that hadn't occured to me, hopefully this will make me a better coil builder. I guess the next step is to build a new 18" coil and see if thats where I went wrong. I will carefully watch damping also.

Thanks again.

Randy

[Ism]

Reg and anybody else interrested.

I came to the conclusion that it was the different shield distance between the two coil windings. The one coil having 2 layers of spi-wrap between coil and shield. The other coil having 3 layers of spy-wrap. I made another smaller coil in a similar fashion and found the same effects as the 18" coil but not as pronounced. Hope this saves another builder of dual coils a bit of grief.

Happy trails

[IBGold]

I have a similar problem to you with both my latest 18" dual-Field and Tri-Field coils but the shielding spacer on both coils is rigid Polystyrene foam with coil to shield spacing of around 7mm with low coil to shield capacitance the coils are Litz wire the shielding material is SS wire mesh on one and spiral wrapped .2mm enameled wire-tape with wires spaced at 1.2mm spacings on the other they are designed for use with Minelab detectors but I have also tested them on a TDI Pro but are unsatisfactory on both but with my coils the coil to shield spacing is the same on both coils but I have noticed the coils wont Ground balance with the SD or GP detectors and are sensitive to capacitance from the hand when brought near the coil.
Regards, Ian.