Large PI Coils ????

[CFO]

Does anyone know anything about making large (1meter+) coils for a PI detector? For example how would you calculate how many "turns" to use. Any info would be appreciated

Thanks,

Carl

[Geo]

Hi.
For a coil 1mx1m and impendance between 330-500uh you must try to wind a coil with 10-16 turns and wire diameter at least 0.5 mm.
Geo

[Woody.au]

I have found that when experimenting with big coils the capacitance, Both interwinding and faraday sheild become critical. I use a Minelab 3000 and build some very large coils in a cross field arrangement to lessen the ground mineralization influence on receive. I have found that increasing the distance between the coil and sheild have a positive effect on performance. I have also soldered chip resistors between windings to speed up the turn off delay with great success. Keep on experimenting with big coils as the big things are down deep.

[bbsailor]

Large Coils: one more thing

In addition to what Woody.au said, one more thing to consider is the dielectric constant of the coil-to-shield spacer and wire insulation. Spiralwrap is a convenient spacer but it comes in several types of material that have different dielectric constants. Choose one with the lowest dielectric constant.

Dry Air: 1
Polyvinalchloride (PVC): 3.8 to 8
ABS Plastic: 2,4 to 3.8
Polyethylene (PE): 2.2 to 2.4 (most practical)
Teflon (PTFE): 2.0 (most expensive)

On one of my more recent 300 uH coils wound with 30 AWG Kynar wire for a 10.5 inch diameter coil, I was able to obtain a 130pf shield-to coil capacitance.

Go to the following site to use an automated coaxial cable calaculator that can be used to estimate the coil-to-shield capacitance using different spacer materials and thicknesses. You will be suprised by the differences. http://www.mogami.com/e/cad/electrical.html

PE: Inner Conductor Diameter - 3mm Outer Diameter 6mm - 184pF/M
Outer Diameter 8mm - 130pF/M

PVC: Inner Conductor Diameter -3mm Outer Diameter 6mm - 257pF/M
Outer Diameter 8mm - 181pF/M

I did some curde measurements and concluded that for each approximate 100 pF reduction in total capacitance, as seen by the RX circuit, the coil speed can be reduced by about 1 uS. Once you use the lowest capacitance MOSFET, the shortest cable with 16 to 20 pF per foot capacitance, you now need to turn to the coil design. The shield adds a pretty large capacitance value and seems to be the place where some effort will produce results. See my small chart above.

I would appreciate if someone would validate my findings (100 pF per uS) and post the results to benefit the PI community.

I have been using the L/C Meter IIB from Almost All Digital Electronics to make the coil-to-shield measurements. I cut one foot test samples of a few known coax cable pf/foot values and the meter is right on the published specifications. I know I can trust the results. See my review of this meter on the electronics section of this forum.

I just received some 30AWG teflon insulated wire from ebay. It makes a 10.65" inside diameter coil, 300uH with 18 turns that self resonates at 1.115 MHz. The same coil with 30 AWG Kynar is 348uH and self resonates at 672 KHz. The different coil results are due to the dielectric constant of the insulation and slight difference in insulation thickness. My shielded 30 AWG teflon wire coil, with a lead tape shield over two layers of spiral wrap self resonates at 898KHz (no coax).

Teflon insulated wire makes some of the best PI coils when combined with a low dielectric shield spacer, low capacitance MOSFETs, low capacitance coax, and a DD coil design. DD coils allow a little sooner sampling.


bbsailor

[Unregistered Passerby]

BBSailor,

I've read here and there your posts regarding capacitance reduction, among other things, and especially appreciate the quantitative data you often provide.

Regarding the Teflon-insulated vs Kynar-insulated coil discussion you've presented, I would like very much to know the measured capacitance of the two coils in the unshielded state.

More generally, I'd like to learn from you and others measured capacitance values for a variety of coils (unshielded state), together with coil specification, so that a real-world, rule-of-thumb, order-of-magnitude conception of the numbers involved might be formed.

[bbsailor]

Here are a few rules of thumb.

First. 18 Turns of AWG 30 teflon wire 10.6" diameter makes an almost 300uH coil. It measures about 250 uH when on the coil form. Then when I add the 1/16" ID spiralwrap, the wire bundle is held together more tightly and the inductance goes up to 295uH with no shield. Then when the shield is added the inductance reads 309uH. Thicker wire requires an extra turn or two to reach 300uH. Thicker insulation will spread the windings apart and tend to lower the inductance and require an extra turn or two. Above AWG 30 try to use stranded wire to minimize cross section eddy currents. This is cheaper than Litz wire and works just as good for treasure hunting purposes.

Second. The capacitance of unshielded coils are based on knowledge of two variables pluged into the automared LC calculator on the following web site. http://www3.telus.net/chemelec/Calcu...Calculator.htm

1. an accurate coil inductance.
2. an accurate seld resonant frequency.

Here is how to use the calculator. Just add various capacitance values in pFs into the formula along with the coil value in uH until you get your measured self resonant value. A good rule of thumb is that coils with higher self resonances have lower interwinding capacitance. The difference between the 1:10 scope probe loading and an FET scope probe (that has minimal to no loading) in this measurement range is about 30 Khz. So just add 30 Khz to your 1:10 scope probe resonant frequency reading to compensate for the scope probe capacitance.

Experiment. Go to Radio Shack and get a spool of AWG 30 Kynar wire wrap. Obtain some 1/16 ID spiral wrap. Obtain 16 tall "C" type screw hooks. Place the screw hooks on a circle 10.5" to 10.7" in diameter (based on your coil housing and how many space layers you plan to use). Measure the distance between 8 sets of opposite hooks to accurately set the coil diameter. Measure the inductance of the coil when on the hooks. Wind the 1/16" spiral wrap around the coil while still on the hooks. It can be easily done. Wrap and mark the 1/16 ID spiral wrap 1.75 coil diameters around the coil form around the coil as it will expand to fit the coil bundle. Cut the spiral wrap at the mark and you will have the right length to fit the coil.

When you wind a lot of coils on the same set of hooks, you will get a feel for the inductance values, tightness of coils and how the dielectric of various insulating materials affect the self resonant frequency.

Do a web search on the words "dielectric constant" as see which materials have the lowest values. Dry air is one of the best and it all goes up from there. Select the lowest value material you can find or afford. Check ebay for Teflon wire and spiral wrap from time to time.

A good rule of thumb is to try to achieve a shield-to-coil capacitance of 40 pf per foot or lower per coil circumference. 40pF/ft is pretty easy to achieve, 30pF/ft is a little more of a challenge.

I hope I answered your questions and set the bar for some coil challenges.

bbsailor

[Unregistered Passerby]

I'm familiar with estimating inductances and that w=1/2*pi*sqrt[LC], the basis of the Chemilec calculator you referenced. Actually, I was more interested in your *measured* capacitances of the Kynar and Teflon coils you described earlier.

By the way, I also own the L/C Meter IIB from Almost All Digital Electronics. I'm very pleased and impressed with it, would recommend it to anyone.

[bbsailor]

The short answer.

AWG 30 Kynar, 18 turns, 348uH, 672KHz, 161 pF
AWG 30 Teflon, 18 turns, 295uH, 1.1Mhz, 71pF

Measured with a 10:1 scope probe and a 1 megohm resistor between signal generator and coil. Add 30 KHz for unloaded measurement values.

bbsailor

[Unregistered]

Thanks,

Are these capacitances calculated from the resonant frequencies? Or measured directly? I apologize for belaboring the point but I'm curious to know how closely inferred capacitance based on measurement of the resonant frequency matches a capacitance measurement made directly with an LC meter.

[bbsailor]

You can't measure the capacitance of a coil with an L/C meter directly. You can only accurately measure the inductance, compensate for any scope probe loading, then infer the capacitance from the referenced web site LC calculator by accurately measuring the coil's self resonant frequency and plugging it into the formula.


You can directly measure the capacitance of the shield to the coil. However the full coil to shield capacitance is not added to the coil interwinding capacitance. In my coils, about 1/3 of the coil-to-shield capacitance is actually imposed on the impact of lowering the coil's self-resonant frequency. I'm not sure if this is a constant but it is something I have noticed on many of my coils.

To find out the impact of the shield on the coil do the following.

1. Measure the coil self-resonant frequency with a tight spiralwrap but with no shield.

2. Add the shield and measure the new self-resonant frequency.

3. Measure the shield-to-coil capacitance.

4. See what percentage of the coil-to-shield capacitance is imposed on the new lower resonant frequency.

5. Post the results on this forum.

There is a lot to making fast coils that is not directly obvious. I have touched on may of the more subtle points.

bbsailor

[Stefan]

Building coil for ML SD2000, ended up too good

I'll relate the outcome of a coil building experience I had for a 12.5 inch mono designed specifically for the SD2000 I normally use. I have been building mono coils for many years and I have inproved them considerably. Some of the earlier coils were not the best.

Now back to the 12.5 inch mono and how I fabricate the coil. I use Litz wire but make the wire myself to my requirements. I use enamelled copper wire (0.2mm) and once the litz is complete, sheath the overall litz with a cotton layer. This is worth the effort as it reduces the intercapacitance effects within the coil. The coil is wound using a former. I start the first layer with 4 turns, then a thin cardboard strip is placed over this layer to form the continuation of the next round of 4 turns and so forth until the total turns have been wound. Each group of 4 has a thin cardboard spacer between the layers.

Once the coil is wound I then varnish the coil whilst in the former. The coil ends up as a rectangular flat coil with dimensions approx 15 mm x 9 mm and 12 inches in diameter.

Some specifications of the coil.

Self resonance is approx : 1 Mhz
Q of coil : 4.9
Inductance : 300 mico henries
DC resistance : approx 0.3 to 0.4 ohms


Once the varnish is dry I place a PE spiral wrap over the coil as a separation for the screen. The screen is then wound over the complete coil assembly.

Some more coil data.

Self resonance with screen is approx: 780 Mhz
Capacitance between coil + screen: 280 Pf

I then used a low capacitance lead 1.3 meters long of approx 67 Pf to complete the coil.

Almost final coil parameters

Self resonance of coil with screen + coil lead: 660 Pf
Q of coil: 4.5

After finishing the coil I gave it a field test at a small goldfield approx 200 Kms from home. I found a few nuggets and the coil was certainly sensitive to small nuggets but the ground balance drove me crazy. The ground where I live in W. Australia is highly mineralized. The continual ground balance signal changes made it difficult to detect and gave many false readings .

On return to home I had to desensitize the coil to make it user friendly. I did this by replacing the coax lead with one with a slightly higher capacitance and smaller diam inner core.

Final coil parameters

Self resonance of coil: 570 Mhz
Q: 4.3

Now a very good coil.


So in one sense I had made the coil too sensitive for the SD2000. I also tried the sensitive coil on my SD2200D which has automatic ground balancing again it was difficult to use.

Really if you are making coils for a specific metal detector keep within the broad specification required for that detector seeking of course the max sensitivity you can get away with. Matching coil to detector is a must.

Looking at the information from many other readers on the forum there are many ways to build successful coils. I hope this information will be of use to others

Stefan

[Woody.au]

I am currently evaluating a triple winding coplanar type coil for the GP3000 series. The outer TX winding is done as a vertical basket weave using my own homebrew Litz wire in an acrylic type varnish, this seems to give the greatest em field above and below the coil. I then make 2 coplanar reverse phase wound coils that concentrically sit 1/3 distance from each other and the TX winding. (Reduces TX RX coil coupling)

Experiments are going to be using this setup in the DD mode to test the configuration. I will be looking at using the 2 recieve coils in phase and out of phase with each other to test ground rejection and both small and large target capabillities. It will be interesting to test the diffence by using a Faraday sheild on all 3 coils or just the 2 receive coils.

Just before I go off and re-invent the wheel has anybody else tried this configuration? The coil I am building is 22" in diameter and the homebrew Litz equivelent is made from 140 strands of .012mm enamel Copper (OverKill)

I am also making the RX coils out of fairly low ohm Litz under 2 ohms as my own tests on commercial coils in a DD arrangement show that the receive winding R of up to 18 ohms in some cases kills the faint signals from deep objects. Previous experiments with DD type coils using .4 ohm on the RX as well the TX gave large sensitivity improvements. I myself thought that it would not make much difference but the increase in sensitivity was in the order of twice the signal amplitude.

Comments and pointers most welcome.

Peter

[Reg]

Hi Stefan,

Your building of a coil and what you had to do to reduce the sensitivity is interesting. I stumbled into the same thing some time back while building coils for my Eric Foster designed PI's.

What I found was this, if a coil was right on the edge such the sampling occurred on the decay curve, the coil became much more sensitive. I called it "hypersensitivity". Such a coil is much more sensitive to small gold, but also more sensitive to everything else including the ground signals. Just shortening the decay time of the coil, or increasing the time before sampling just a little would settle the coil down dramatically. Fortunately, the ability to adjust the sample time on Eric's machines is what finally led me to figure out what was happening.

Now, what you found was you could add a little capacitance via the cable and that would make the coil more stable. Yep, that works on certain coils. It certainly does sound strange, but on a slightly overdamped coil, adding a little capacitance can speed things up a little so the sampling is done closer to 0V. On a critical coil, just shortening or lengthening the coax could make a big difference on how the coil worked at the same sample delay setting.

I know I scratched my head a few times before I finally figured out what was happening.

This coil building stuff is much more complex than I ever thought it would be.

Reg

[Reg]

Hi Peter,

First, how in the heck are you? Hope all is well.

Now, when I first read your post, I thought you had 3 receive coils. I built something like that some time back but didn't have much success. My design was one with 3 receive coils, two for noise and ground balancing and one for actual receive. I found it didn't work well, but that was a long time ago and I need to try it again.

Now, I am still not sure about your design when you say 1/3 distance from the transmit coil. Are you saying they are on a different plane?

As for a receive setup having two coils, I have built them in a "salt coil" configuration, where the two coils are out of phase. This works well on a long coil using a rectangular housing. The reason being there is a sensitivity loss in the area where the two coils approach each other, so longer receive coils allow for greater sensitivity. Now, I haven't separated the coils as you have indicated, so I would be interested in the final results, especially the sensitivity.

As to your discussion about the resistance of the receive windings having a distinct effect on the sensitivity, I haven't seen that, but haven't been looking for it either. Are you sure something like my discussion about hypersensitivity isn't the real cause of the sensitivity change? I am just wondering if the coil with the higher resistance wasn't allowing the sampling to occur on the decay curve. I know this can make a real big difference.

Anyway, it is something more for me to work on. Thanks for the info. Also, let us know how the coil works out. One thing I have not done is to try a coil where the transmit isn't shielded. Let us know how that works also. I have been meaning to try that but just haven't gotten around to it. I am thinking that there should be some interaction not seen on a shielded coil. What I don't know is, can that be used to an advantage.

Reg

[Woody.au]

Hi Reg, I am well.

I have been locked in my Lab doing a lot of theoretical R&D on Gold detecting and also wide band antennas for Spread Spectrum radio modems. The Gold bug is really starting to bite again and now the detector/coil side of things is starting to take over again...lol I was up to 4:20 AM building coil jigs and trying different winding configurations. The coils are all in the same plane, just think of it as a 3 ring Bullseye. Thoughts go to using the inner and outer on TX, The outer on TX with the 2 smaller in phase, then out of phase.