Fast Coil

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Making a Fast Pulse Induction Mono Coil  A Practical How-to Guide and Tutorial   By: Joseph J. Rogowski  Also known as: bbsailor 

V er si sio on 1.0, D ece cem mber 200 2006 6 Introduction This article explains and shows you how to make make a fast fast mono mono coil coil fo forr a pu puls lsee induction induct ion (PI) metal detector. detector. The word “fast” refers to the ability of the coil to ope pera rate te at lo low w sa samp mpllin ing g de dela lay ys. Unfortunately, there is not just one thing you can do to make a fast coil, but there are a few few th thin ing gs you can do tha hatt colle collect ctive ively ly contr contrib ibute ute to the co coil’ il’ss  performance on your PI machine.

A little theory is needed to appreciate the design process necessary to make a fast coil. The value of the damping resistor  reve reveal alss the the tota totall coil coil an and d TX ci circ rcui uitt ca capa paci cita tanc ncee that that must must be cr crit itic ical ally ly damped. dam ped. Higher values values of the dam damping ping resistor cause higher current dissipation. This als also o cau causes ses faster faster cur curren rentt dec decay ayss due to les lesss capac capacita itance nce.. The dampin damping g resisto resistorr value value alon along g with the front-en front-end d am ampl plif ifie ierr inpu inputt resi resist stor or (R (R12 12 on the the Hamme Hammerh rhea ead) d) pl plus us the the two clamp clampin ing g di diod odes es al also so help helpss to co cont ntro roll co coil il’s ’s  bandwidth and also controls the t he peak p eak flyf ly back voltage generated by the coil. This arti articl clee desc descri ribe bess pr prac acti tica call wa way ys to minimize coil and TX circuit capacitance necessary to make a fast, wide  bandwidth coil. The mono coil coil fun funct ction ionss as bo both th th thee transmit (TX) and receive (RX) coil and is the easiest coil to make. The practical aspects of coil making will be emphas emp hasized ized,, lea leaving ving the mor moree detaile detailed d theo theory ry to be fo foun und d in the the re refe fere renc nces es sect se ction ion.. As the coil coil de desc scrib ribed ed in this this arti articcle is be bein ing g ma made de,, crit critic icaal coil meas measur ureement mentss are are do docu cume ment nted ed an and d shared sha red with the read readers ers to dem demons onstrat tratee the conc concep epts ts be being ing expla explain ined ed.. To my knowledge, knowle dge, this detaile detailed d appro approach ach to coil making has not been documente documented d before. Those Tho se seekin seeking g to und unders erstand tand the more

subt su btle le as aspe pect ctss of PI co coil il makin making g will will appreciate this approach. PI mono coils can be almost any shape and diamete diameterr but some some prac practica ticall des design ign considerations are:   Desired delay    Coil resistance    Inductance Inductance necessary necessary to match match  t he PI circuit design requirements   Size, composit composition ion and depth of   objects sought 

  

  Diame Diamete terr of the select selected ed co coil il housing   Weight of tthe he ffinished inished coil   Shielding technique used   Coaxial cable length.

Th Thee fo focu cuss of this this ar arti ticl clee wi will ll be on making a round coil but the techniques described can be used to make any shape an and d size size coi oil. l. If you ar aree us usin ing g a coil coil housing housin g that t hat is non-c non-circular, ircular, convert convert its shap sh apee to an eq equiv uival alen entt circ circle le and and reshape the finished round coil to fit the coil housing. This how-to guide will demonstrate the m king rhead oad f aPu fast Ind co iltion hel Hamme Haammerhe Pulse lse Induc uctio nfor meta mettal detector that can be used to locate coinsize size ob obje ject cts, s, go gold ld jewe jewelry lry and and gold gold nu nugg ggets ets.. Coil Coil data data will will be pre prese sente nted d alo along ng with with a co compa mpari rison son to alter alterna nate te construction methods. This method will for form m the basis basis of a tu tutor toria iall ab abou outt the techniques used to make this coil as well help you understand the consequences of  using coil parts you might have on hand. A fa fast st co coil il wi will ll oper operaate at low low pu puls lsee delay settings between 8 to 10 micro mic rose seco cond ndss (us (us). ). Thes Thesee low pu pulse lse delays dela ys are typica typically lly used used for detec detecting ting

small gold objects as their signal decays quickly quic kly.. For thos thosee who prim primarily arily seek  co coins ins or ot othe herr larg larger er ob objec jects ts that that use longer delays in the 30us to 50us range, these these co coilil-ma makin king g tip tipss will will he help lp also. also. Coils Coils for lon longe gerr delay delayss do no nott ne need ed a shie shield ld an and d typ typica ically lly op oper erate ate at low lower  er   pulse frequencies (100 to 400 pulses per  second) with higher peak current pulses due to the use of thicker coil wire which has a lower resistance. This thicker coil wire wire,, AW AWG G 22 to 26, sh sho oul uld d be the the stranded type to minimize eddy currents  being generated in the coil itself. At longer delays, an y ty typ pe of wire insulation insulat ion can be used. Unshie Unshielded lded coils still need the spiral wrap (3/16 to ¼” ID) or other method, such as electrical tape or lacing to secure the coil windings. If  you are using a longer delay coil in an area with high electrical noise, it is better  to sh shie ield ld the the co coil il to he help lp make make it ru run n quieter. At these longer delays, the coil shie shield ld will will have have a min minima imall imp impac actt on coi coill sensiti sensitivity vity even though though the shield shield might be detected at lower delays on a different PI machine operating at a lower  delay. All

coils

have

three

primary

ch char araacte cteris ristics tics:: indu inducctanc tancee (u (uH H), capacitance (pF) and resistance (Ohms). These The se cha charac racteri teristic sticss can can be adju adjuste sted d individ indi vidual ually ly but inte interac ractt to impa impact ct the finall coil fina coil perf performa ormance nce.. The para paramete meters rs tha that will ill affe affect ct your our coil’ coil’ss ul ulti tima mate te  performance are:   Coil housing diameter     Wire size (AWG or SWG)    Wire insulation thickness    Wire insulation material    Shield spacer material    Shield spacer thickness    Coax cable type and length 

 

Each Each of the these se char charac acter terist istic icss will will be examined in detail and measured (where  possible) as it relates to t o buil building ding a coil.

leng length th 6. 6.28 28 time timess long longer er than than the the combined comb ined wall wall thic thickne kness ss of the shield shield and spacer spacer mate materia rial. l. Add Adding ing this little little extraa leng extr length th to the coil coil circ circumfe umferenc rencee ensures a good fit inside the lid groove. Thiss ex Thi extra tra len lengt gth h ac acco comm mmod odat ates es the increase in coil ID from adding the coil spac sp acer er an and d sh shie ield. ld. There There is no nott muc much h room for error but with careful planning and measuring, measuring, it can be done.

Designing A Mono Coil Let’s start the mono coil design process. The The fir irst st con onssid ideerati ration on is the the coi oill housing size that will be used. This coil hou housi sing ng si size ze wil illl defi define ne your co coil il dia diame meter ter.. Co Comm mmerc ercial ial-lo -look oking ing co coil il hou housi sing ngss can can be obta obtain ineed th thro rou ugh www.hayselectronics.com.. HomeHome-ma made de www.hayselectronics.com housings can be made by the creative use of comm common on it item emss such such as a Fr Fris isbe bee, e,  plastic plate, plastic can lid or anything round roun d that that is not metall metallic. ic. The outside outside

I am going to use AWG 30 single strand Teflon wire that is 0.024”OD (+/- .002”). I ob obtai taine ned d my Teflo Teflon n wire wire fro from m eBay eBay (search on the words “Teflon wire”). If  you are usin using g di diff ffeere ren nt wire wire,, just just substit sub stitute ute yo your ur measure measured d wire OD and follow foll ow alon along g with the design design proces process. s. You need to accurately measure the OD of your wire with a gauge. For a 300 to 350uH coil with 18 to 20 turns, multiply the wire OD by 5 to closely approximate the OD of the wire wire bu bund ndle. le. This wire

diameter (OD) of the wire used for the coil will determine the thickness of the coil bundle and affect the calculated coil inductance.

 bundle will be 0.12” or about 1/8”. When Whe n ca calc lcula ulatin ting g yo your ur co coil il size size an and d inducta indu ctance nce,, go to the followi following ng on-line on-line c oi l inductance calculator. http://my.athenet.net/~multiplx/cgi bin/airind.main.cgi..  bin/airind.main.cgi





  MOSFET COSS output capacitance   Coil inductance inductance (d (diameter iameter and and number of coil turns) to match t he PI circuit design requirements.

If you choose to make a coil for delays  below about 30us, a shield must be used to prev preven entt th thee gro roun und d from from bein being g detecte dete cted d whe when n the coil is low lowere ered. d. The shield spacer and shield thickness add to the wire bundle cross-section diameter. I mustt repea mus repeat: t: the tota totall cross cross-sec -section tion of  the wire bundle bundle,, incl includi uding ng all spa spacer cers, s, mustt be sub mus subtrac tracted ted from the max maximum imum coil coil di diame amete terr that that fits fits yo your ur ho hous usin ing; g; otherwis othe rwisee yo you u will be wind winding ing anothe another  r  slightly slig htly smaller smaller coil. coil. Bef Before ore you make yo your ur coil coil for form, m, you you ne need ed to kn know ow the accurate inside diameter (ID) of the coil winding. This measurement measurement will define the co coil il for form m dia diame mete terr and and ult ultima imatel tely y determine the final fit of your coil inside the coil housing. If you want to be a little creative, you can use som someth ething ing like a plastic plastic paint can or bucket lid as a coil housing. You mustt do accu mus accura rate te me meas asure uremen ments ts an and d ca calc lcu ula lati tion onss to mak make sure sure that that the the finishe fini shed d coi coill fits inside the lid gro groove ove.. Wind one-turn inside the lid groove and secure the ends together so you can place the same one turn loop snugly on the coil form (expla (explaine ined d late later). r). Mak Makee this wire

complete comple te chart for accur accurately ately calculating coil bundle sizes from 1 to 61 turns: http://www.raychem.com/fetch.asp?filei d=980&docId=846 Accu Accura rate tely ly mak making ing the the fo foll llo owing wing measurements measure ments and calcula calculations tions ensures ensures that that yo your ur finish finished ed co coil il fit fitss yo your ur co coil il housing. For a shield spacer, I will use 1/16” 1/1 6”ID ID/1 /1/8” /8”OD OD po poly lyeth ethyl ylene ene spira spirall wrap obtained from www.usplastic.com from  www.usplastic.com.. When the spira When spirall wra wrap p is ad adde ded d to th thee wire bun bundle, dle, the bun bundle dle diamete diameterr will increase about 1/16” more. If you want to make make a co coil il with with mini minimal mal co coilil-totoshie shield ld capa capaci citan tance ce,, us usee two layers layers of  spiral wrap, one 1/8” OD diameter and the other 0.250” 0.250” OD. Subtrac Subtractt 1.5 times the total wire bundle diameter including all all lay layers ers of sp spir iral al wr wrap ap pl plus us sh shie ield ld added to the coil from the maximum coil housi hou sing ng diame diameter ter to ob obta tain in the actua actuall win windi ding ng ID of you ourr coil coil fo form rm.. The The reason I use 1.5 and not 2 is that there are two thicknesses of the spiral wrap on each sid idee, mak makin ing g up a to tota tall of 4 thicknesses. The two layers on the inside of the coil and do not increase the OD fit of the coil. The final coil bundle crosssect section ion diame diameter ter with with the Scotc Scotch h 24 shield and final tape wrap is .315” and  just fits the 11” Hayes Electronics coil housing with little room to spare.

Fi Fig. g. 1 Me Mea asuri suring ng OD of AWG AWG 30 Teflon wire.

Use the ID of the coil as th thee coil diameter input and the wire bundle size as the coi coill leng length th in inpu putt an and d en ente terr the number numb er of turns turns to calcula calculate te the coil’s coil’s inducta indu ctance nce.. Use the 1% answe answerr on the ab abov ovee on on-l -lin inee ca calc lcul ulat ator or an and d your  our  calculated inductance will be very close to the actual coil inductance. If you are ma mak king ing a co coil il with ith 25 to 27 turn turnss, multiply the wire diameter by 6 and if it is 34 to 37 turns, use 7 as the multiplier  to approximate approximate the wire bundle diameter. Go to the following web link to see the

Fig. 2 Measuring Measuring the diameter diameter of final shielded coil bundle. Making A Coil Form He Here re is a quic quick k an and d accu accura rate te wa way y to make make a coil coil fo form rm that that allo allows ws you to make any size coil. Obtain 17 “C” type scre screww-in in hook hookss (als (also o kn know own n as cup cup ho hoo oks ks). ). Pla Place two ho hook okss on a fla flat surface surf ace with the ope openin ning g of the hooks facing facing lef leftt an and d right right an and d th thee thre thread aded ed

 

sh shaf afts ts pa para ralle llel. l. Accura Accurate tely ly sp spac acee an and d measure the inside distance between the

Fig.. 4 Cup hook Fig hookss mo moun unte ted d to make make coil form.

Fig. 3 Cup hooks used for my coil.

hooks hoo ks,, wh where ere the wir wiree wi will ll lay lay,, to be exactly the same as the inside diameter  (ID) of your new coil. Now, measure the distance between the points of the two screw tips of the “C” hooks. This will be the diameter of the circle you will make with a compass on a piece of plywood. Equally divide up the circle so you can  place 16 “C” hooks on this circle. Make sure the “C” hooks go in straight so the actual ID of your coil will be accurate. If  you use the tall “C” hooks, you can bend oppos opp osite ite “C” “C” ho hook okss sl slig ight htly ly to ma make ke sm smal alll adju adjust stme men nts to the the co coil il insi inside de diameter.. Place a mark around diameter around one hook  that will become the coil start and end  point as well as the coil turn counting  point of reference. Before winding Before winding your coil, coil, you should check the accurate ID of your “C” hook  coil form by wrapping one loop of wire around all the hooks. Pull the wire tight as it goes over one hook and then place a mark on the wires. Measure the distance  between the marks on this piece of wire and divide this leng length th by 3.14159 3.14159 and you will confirm the actual diameter of  your coil. Adjust if necessary.

Place another hook in the coil center to hold the start start and end winding windings. s. Plac Placee the start of your winding on the center  “C “C”” hook hook an and d wind wind the the calc calcu ulate lated d numb number er of turn turnss ar arou ound nd the the 16 “C “C”” hooks.   I mpor tant: tant: se secure cure the w wii nd ndii ng

on this center hook with a few turns of  ele lecctri cal tape to ens nsure ure tha hatt the wi ndi ndi ngs don’t com come loose loose so te tension nsion i s maintai ainta i ne ned dr .whe hen n late later ad add di ng the spi spi r al wrap space spa cer My coil uses uses 19 turns of of AWG 30. Place the end winding on the center “C” hook, secure with tape and cut the wire. Inspec Inspectt the the wind inding ing to en ensu sure re tha that th thee coil winding wind ingss are snug and not snagge snagged d on anything. Applying The Shield Spacer With the coil wire still on the “C” hooks, you can apply apply the spiral wrap with without out the the ne need ed to lace lace it. Th This is is the the main main reason to use the “C” hook coil form. If  you measure the inductance of the coil on the “C” hooks, 19 turns of AWG 30

on a 10.5” ID gives you about 285uH. Once the spiral wrap is added, the wire  bundle is held tighter and the inductance is ab abou outt 317u 317uH. H. Sinc Sincee the the ID of the the spira sp irall wra wrap p is 1/ 1/16” 16” it will will ex expa pand nd to ac acco commo mmoda date te the 0. 0.12” 12” 19 19-tu -turn rn wire wire  bundle (.024” X 5= .12”). Cut the 1/8” OD sp spir iral al wr wrap ap 1. 1.65 65 time timess th thee coil coil circ circu umfe mfere renc ncee (n (no ot stre stretc tch hed ed)) to accommodate this expansion with a few inches to spare. Starting at the location of the two coil leads, begin applying the spira sp irall wra wrap. p. You You ca can n ac actua tually lly slip slip the spiral spir al wra wrap p under under the hooks where the wire crosses each hook.

Fig.. 5 Cl Fig Clos ose-u e-up p of sp spir iral al wrap wrap.. See See how spiral wrap can be easily slipped under each hook.

Continue wrapping around the total coil and only remove the coil from the “16 “C” hoo ook ks wh wheen you are clo lose se to finishing the spiral wrap. Turn a few “C” hooks 90 degrees and carefully slip off  the spiral wrapped coil bundle. It may be a little tight but the wire bundle will slip off. Adding the spacer will take about 15 to 20 minutes. Don’t rush! Compress the spiral wrap (sideways) after going under  each each hook hook ensu ensuri ring ng that that it is wo woun und d tightly tigh tly around around the coi coill bundle. bundle. If a few of the outer coil wraps are a little loose when finished, just pull the coil between you hands when it is off the coil form and and wo work rk you ourr wa way y arou around nd the the coil coil,,  pulling hard every few inches. This will seat seat the Tefl Teflon on wir wiree in insi side de the spira spirall wrap. You may need to go around the coil a few times, but the Teflon wire will snug snu g up insi inside de the spir spiral al wrap wrap.. If you wa want nt to ad add d a seco second nd lay layer of sp spir iral al wrap, you can add it with the coil off the “C” hook coil form. Fast Coil Factors “An 11” diam diamete eterr coil coil is optimum for  beach work and is the best compromise between small objec objectt sens sensitivity, itivity, depth,  pinpointing and coverage”   E ri c F ost ste er 

If you wa want nt to make make the the fast fastes estt co coil il  possible in the 10” to 11” diameter  range, rang e, use Tef Teflon lon insula insulated ted wire with with 300uH 300 uH to 32 320u 0uH H in induc ductan tance ce.. Teflo Teflon n in inssulat latio ion n makes a coil with with less less capa capaci cita tan nce tha than a coi oill with with PVC PVC insu insula lati tion on.. Te Tefl flon on has has a di diel elec ectr tric ic constant of about 2, compared to 4 to 6 for PVC. PVC. Te Teflo flon n spir spiral al wr wrap ap make makess a ve very ry go good od shiel shield d spac spacer er bu butt it is ve very ry expensive. As an alternative, you can use

 

 polyethylene (PE) spiral wrap that has a dielec dielectric tric constan constantt of about about 2.2 and is easily available and inexp inexpensiv ensive. e. When you combine the thinner wire bundle of  using AWG 30 wire with two layers of  PE spiral wrap you will make a coil that ha hass a low coilcoil-toto-sh shiel ield d capa capaci citan tance ce.. AWG 30 wir iree has about 0.1 Ohm resistance per foot and makes a 19-turn, 10.5” 10. 5” insi inside de di diam amete eterr (ID) (ID) co coil il with with about abo ut 5.4 Ohm Ohmss resista resistance nce.. When yo you u comb combine ine thi thiss re resi sista stanc ncee with with the onresista resistance nce of the MOSFET MOSFET coil coil driv driver, er, you you wi will ll ha have ve a coil coil that that has a pe peak  ak  curre current nt betwe between en 1 and and 2 amps amps (Ohms (Ohms Law). MOSFETS have an on-resistance that can range from 0.2 Ohms to about 5 Ohms ms.. The peak coi oill current is a fu func ncti tion on of the the co coil il resi resist stan ance ce pl plus us MOSFET MOS FET on-r on-resis esistan tance ce and any other  other  series ser ies resista resistance nce.. The time con constan stantt of  the the co coil il (L di div vid ideed by R) will will also lso govern the peak current in the coil for a gi give ven n pu pulse lse wid width. th. Long Longer er TX pu puls lsee widths will create a higher coil current an and d crea create te a hi high gher er fl flyy-ba back ck pu puls lse. e. Higher High er mon mono o co coil il curren currents ts tak takee longer  longer  for the fly-back pulse to settle down and results in longer delays. “On the Hammerhead it would help to reduce the coil current and the gain of    the preamp. Both will help in allowing   for a faster delay especially when trying  to get larger coils to work at less than 10uS.”  Reg Sniff  Insulation Thickness For comparison, to help demonstrate the insula insulation tion thic thickne kness ss point, point, I woun wound d an

Go to the following web site http://www3.telus.net/chemelec/Calculat ors/LC-Calculator.htm  and enter the coil inductance in micro Henries (uH). Then en enter ter a ca capa pacit citan ance ce es estim timat atee of abou aboutt 100pF. If your estimate shows a resonant fre frequ quen ency cy an answ swer er that that is high higher er than than yourr measure you measured d coil coil res resona onance nce,, ente enterr a highe hig herr ca capa paci citan tance ce nu numb mber er until until the an answ swer er ma matc tche hess yo your ur measur measured ed co coil il resonan reso nantt frequency frequency.. The magnet magnet wire 384uH 384u H coi coill with a 733 733KHz KHz resona resonance nce has has 123p 123pF F of ca capa paci cita tanc ncee whil whilee the the Te Teflo flon n wir wiree insula insulated ted co coil il of 31 317u 7uH H resonat reso nates es at 1.25MHz 1.25MHz and has 51pF of  capacitance. When Shielding Is Needed Shielding the coil is required for delays  below about 30us. The shield does two  primary things: 1 It eelimin liminate atess tthe he coil coil respo respondi nding ng to the ground at low delays when the

2

coil is lowered to the ground and It prot protec ects ts the the ccoil oil fro from m pi pick cking ing up electrical noise.

At lo low w de delay lays, s, findi finding ng a go good od shie shield ld material can be a challenge as the shield can become a target and can reduce the co coil il’s ’s se sens nsit itiv ivit ity y. Yo You u can can actu actuaally experiment with shield material using an existing exis ting coil. Jus Justt see if the candidate candidate shield material is minimally detected on your PI machine when held close to the coil while operating at the lowest delay settin setting g in the 8us to 15 15us us rang range. e. The The most most ac accu cura rate te test test occu occurs rs when when the the candidate shield material is moved near  the coil being held by a piece of wood or 

identic identical al 19-turn 19-turn coil, coil, 10.5 10.5”” diam diamete eter  r  us using ing ma magn gnet et wire wire with with thi thin n en enam amel el insula insulation tion.. Here are the coi coill statisti statistics. cs. Wire inductance on the hook coil form: 309uH, 1.25MHz self-resonance. With a PE spiral wrap added, the coil induc inducta tanc ncee we went nt up to 38 384u 4uH H with a 733 33KH KHzz resonance. This hig igh her  induc inducta tanc ncee oc occu curs rs beca because use the sp spira irall wrap compresse compressess the coi coill togethe togetherr but the resonant frequency is lower because the insu insula latio tion n is ve very ry thi thin n ma maki king ng the coil capacitance higher, 123pF.

 plastic rather tthan han your hand especially if  your coil is not shielded.

Here is how to calculate the capacitance of a coil after you measure your coil’s self resonance.

Fig. ig. 6 Pie iecce of Scotch 24 shield ield mate ma teria riall held held by a woo wood d stick stick being being tested.

Shield Materials There are a wide variety of materials that can can be us used ed effe effect ctiv ivel ely y as a PI co coil il shield. shie ld. One consid considera eration tion for a shie shield ld includes the ability to solder directly to the shield material. If you can’t solder to the the mate materi riaal, a gr grou ound nd lead lead must must be mechanically fastened to the shield using conductive glue/epoxy or electrical tape securing a bare wire to the shield. Wire carbon-based glue is available in a 0.3oz (9ml) jar from www.andesproducts.com from  www.andesproducts.com.. Another Anot her con conside siderat ration ion is the minimum minimum delay at which the shield material itself  is detected. Here is a list of some good and fair shield material material::   3M 1190 (good) (good) co coppe pperr fabric  tape, can be soldered   Scot Scotch ch 24 (g (goo ood) d) wi wire re mesh mesh  shielding tape, can be soldered, (used for the coil in this article) 





  C Con ondu ducctiv tive My Myla larr (g (goo ood) d),, mechanical shield fastening   Alumin Aluminum um fo foil, il, (fair (fair)) on only ly at long longer er de dela lay ys, mech mechan anic ical al shield fastening   Lead tap tape (g (goo ood d), can be soldered

Fig. 7 Sc Fig. Scot otch ch 24 me mesh sh sh shie ield ld bein being g secured to coil with tape over second layer of spiral wrap.

Th There ere are tw two o way wayss to ad add d the shie shield ld depending on the size and shape of the sh shie ield ld mate materi rial al used used.. If you use use two two lay layers ers of sp spir iral al wr wrap ap,, the the fina finall wi wire re  bundle diameter will be .25” with a circumfe circ umferenc rencee of 0.785”. 0.785”. This This allo allows ws you to use a .75” wide 3M 1190 shie shield ldin ing g tape tape app applie lied aro aroun und d the the circumfe circ umferenc rencee and ben bentt over over to alm almost ost tota totall lly y encl enclos osee the the tota totall coil coil wi with th a 0.035” gap.

 

The other method is to hold the tape roll at an angle and spiral wrap around the whole coil wir iree bun und dle with ithout overlapping the tape too much.   Be sure to leave a small shield gap where the two wire leads leads exit exit the the coil. coil.   Practice soldering on a piece of scrap tape first. Use a small piece of flat metal to act as a heat sink under the point where you are soldering to prevent burning a hole in the tape. Attach the wire so it is running in the same direction as the coil circumference. This can be a short piece of stranded wire or a piece of thin desoldering wick long enough to reach the co coax ax cabl cable. e. Apply pply a few few wra wraps of  ele electri ctrical cal tape over over the solde solderr join jointt to minimize strain on the solder connection. Thiss is one Thi one conn connec ecti tion on tha thatt you don’t don’t wa want nt to co come me undo undone ne afte afterr the the co coil il housing is sealed up.

Fig. 8 Scot Scotch ch 2 24 4 mesh mesh sh shield ield

Once the whole coil is wrapped (if you are are us using ing 3M 1190), 1190), cut the the tape tape just just  before the gap, slip the heat sink under  the tape and then solder the ground lead. I used used Scot Scotch ch 24 to shie shield ld my co coil il.. I wrapped the 1” wide shield mesh around the the oute outerr ci circ rcum umfe fere renc ncee of the the co coil il,, securin sec uring g it in eig eight ht places, places, then spiral spiral wrapped electrical tape around the coil to tightly secure the shield against the PE spac sp acer er.. The The me meas asure ured d coilcoil-toto-sh shie ield ld ca capa paci cita tanc ncee of Scot Scotch ch 24 wrap wrappe ped d around the 10.5” coil is 104.7p 104.7pF F Shielding Lowers Coil Self-Resonance It turns out that only about 20% of the 104. 104.7p 7pF F co coil il-t -too-sh shie ield ld ca capa paci cita tanc ncee reduces the with-shield coil resonance to 1.0445 1.04 45 MHz (with shiel shield d con connec nected ted to one coil wir iree). This is due to th thee distributed distribu ted capacitance of the shield over 

the 19-turn coil separated by two layers of a PE spira iral wra rap p. Distr istrib ibu uted ted capacitance is a very complex lex calculation. It is best measured with an LC meter once the shield is added to the coil. Just clip one lead of the LC meter to the shield and the other lead to one coil wire ire and note your capacita tan nc e measure mea suremen ment. t. The coil coil self resona resonance nce with the shield added, represents 72pF of  capacitance compared to 54pF capacitance without it, only adding 18pF to lower the coil resonan resonance. ce. If you are doing these measurements to fol follo low w alo along ng wh while ile ma maki king ng your co coil, il, you wi will ll ge gett a sm smal alll vari varian ance ce in the the measur mea sured ed va value lues. s. When When yo you u ad add d the shield to the coil, the inductance of the co coil il will will ac actua tually lly me meas asure ure ab abou outt 7 to 15uH higher. When I added the Scotch 24 shield shielding ing tap tape, e, the inducta inductance nce went went up to 324uH (from 317 uH without the

the then the the coil coil sh shie ield ld lead lead to the the co coax ax  braid. Solder the other coil lead to the coax coa x center center con conduc ductor. tor. Use heat shrink  shrink  tubing over the connections to secure the wires. If you make the coil for use on the beach or for use in the water, the coil needs to  be sealed up to be waterproof. Once the co coil il is mad made, e, in insta stalle lled d in insid sidee the co coil il hous housin ing, g, test tested ed and and sea sealed led up, up, it is important that the coil be secure inside the coil housing. Any movement of the coil relative to the shield or movement of  the coax cable could cause a false signal. A sealed up coil is very difficult to open without damaging the housing so make it right before sealing it up.

shield). From this point forward, use this new coil inductance (with shield) in all yourr calc you calcula ulation tions. s. Anot Another her interes interesting ting co coil il ch chara aract cter erist istic ic is that that the co coilil-totoshield capacitance may be different by a few Picofarads when either the start or  the end windi winding ng is us used ed to make make this measur mea surem emen ent. t. Th This is is du duee to how the wire lays inside the coil bundle relative to the coil winding distance to the shield.

Fig. 9 Co Fig. Coax ax cable cable secur secured ed with with wire ti tie. e. Ep Epox oxy y ta tail il of wir wire tie to co coil il housing wall.

When ap When apply plying ing the sh shie ield ld to the co coil, il, ensure that there is a small gap where the two co coil il wires wires en ente terr an and d ex exit it the coil (repeated (repea ted for emphasis) emphasis).. If ther theree we were re full shield continuity around the coil, the shield would look like a shorted turn and

Shield Material Experiments Th There ere is a web web site site that that allow allowss yo you u to calcula calc ulate te the cap capac acitan itance ce of coa coax x cable cable with diff differe erent nt dielec dielectric trics. s. Thi Thiss will be useful for estimating your coil-to-shield capacitance capaci tance and see the impact of using a

the the se sens nsit itiv ivity ity of the the co coil il wo woul uld d be se seri riou ousl sly y degr degrad aded ed or may may no nott ev even en work.

va varie riety ty of spac spacer er mater materia iall an and d spac spacer  er  thick thickne ness sses es.. Go to the fo foll llow owing ing we web b site, http://www.mogami.com/e/cad/electrical .html   and and inse insert rt your our coil coil bu bund ndle le diameter in mm as the center conductor. Input the diameter of the shield spacer in mm. Finally select the dielectric and see the eq equiv uival alen entt capa capaci citan tance ce fo forr a one meter length of coax. Divide the answer   by the ratio of your coil circumference compared to one meter. The coil bundle de desc scri rib bed abo bov ve us usin ing g 19 turn turnss of  AWG30 Teflon insulated wire is 3mm, an and d two two laye layers rs of PE sp spira irall wra wrap p are are 6mm. The answer in th thee on-line calcu calcula lator tor is 18 184p 4pF/m F/met eter er.. Div Divide ide 39

Assembly Mount the coil inside the upper half of  the Hayes Hayes Electronics Electronics coil housin housing g coil shell. Install the waterproof strain relief  (s (sup upp plied lied by Ha Hay ys Elec Electr tron onic ics) s) and and expose exp ose just enough enough coax wire to all allow ow soldering and putting a thin nylon wire tie tie aro roun und d the the co coax ax insi inside de the the coil coil housing to prevent the coax from pulling ou out. t. Pu Pull ll it tig tight ht us using ing pliers. pliers. Yo You u can can epoxy the tail of the wire tie to the coil ho hous usin ing g wall wall fo forr ex extra tra co coax ax secu security rity.. Solder one coil lead to the coil shield and

 

inches (one meter) b y the coil circumference of 33 inches and you get .8462. Multiply 184 by .8462 to obtain the equ equiva ivalen lentt capaci capacitanc tancee of the coil coil circumf circ umfere erence nce which which is 155.7p 155.7pF F and is close close to the the actua actuall me meas asur ured ed co coilil-totoshie shield ld capa capaci cita tanc nce. e. Ente Enterr di diff ffer eren entt materia mate rials ls used used for a spi spiral ral wrap spa spacer  cer  and see that PE is very close to Teflon. Scotch 24 is a wire mesh shield and has less surf surface ace area area and les lesss capaci capacitanc tancee tha than n a so solid lid shie shield ld cove coveri ring ng th thee sa same me area. This is why the Scotch 24 measures 104.7pF, 51pF lower than the calculated capacita tan nce no note ted d above. Also lso, distributed capacitance of a coil is a little differe diff erent nt from capacit capacitanc ancee of a single single conductor coax. This exercise helps you do “w “wha hatt-if if”” des desig ign n trad tradeo eoff ffss and experiments using a variety of materials that you might have on-hand. Securing The Coil

Before Bef ore sealing sealing up the coil coil hou housin sing, g, try using the coil so that, if necessary, it can  be easily opened, inspected and modified. The easiest way to secure the coil inside the top of the coil shell is to cu cutt pi piec eces es of St Sty yro rofo foam am st stri rips ps to fit fit inside the top-half of the coil shell wings under coil (with coil upside down). The cleanest cuts are made with a fine-blade scroll saw. First I used two strips 0.5” wide, wid e, 0.2 0.25” 5” thick thick by 13” 13” long long.. The Then nI used two pieces 3/16 wide, ¼” high by 13” long to wedge around the inside of  the coil housing, forcing the coil bundle to the edge of the coil housing. I cut one  piece of Styrofoam 2.25” wide and 13” long to fit the center channel. I pressed the coil housing into the center strip to obtain an impression of the outer contour  of the housing edge. Using a fine tooth X-acto saw blade, I cut “L” shaped step on each end of this piece of Styrofoam to secure the edges of the coil firmly under  the coil as well as press the coil to the edge of the coil housing. This makes a secure coil that can be either sealed up fo forr wate aterp rpro roof of use use or temp tempor orar aril ily y assembled to test your new coil creation. Place the bottom piece of the shell on the top piece and seal with electrical tape for  temporar temp orary y use. use. This is not waterproof  waterproof   but allows you to try out the coil on land or the beach once you solder the coax to the the co coil il le lead adss and sh shie ield ld.. The coi oill

wi with thou outt the the co coax axia iall ca cabl ble, e, but but fu fully lly assembled with both halves and internal foam weighs weighs 8.3 ounces ounces (0. (0.236 236kg) kg) and with 33” of coax and connector weighs 10.6 ounces (0.300kg).

Fig. 10 Styrofoam strips ready for coil insertion

Th Thee Ha Haye yess co coil il hous housin ings gs co come me wi with th some scrap pieces of plastic to be added to MEK solvent to make a glue paste to seal up the coil.   Caution: Use this in a well ventilated area.

coax cab coax cable le length length and MOSFET MOSFET being used. The damping resistance should be adjusted experimentally while observing th thee ou outp tpu ut of th thee firs irst amplifi lifieer  (Hammerhead IC6) with an oscilloscope. Place Pla ce a 12 1200 00 oh ohm m resi resisto storr in pa para ralle llell with a 5K ohm pot that has a 1200 ohm limit resistor in series with the pot. This allows you to adjust the damping resistor  value in the range of 600 to 1K ohms, (R11 in the Hammerh Hammerhead ead)) to opt optimiz imizee the the coil coil’’s perf perfo orman rmance ce on your our PI machine mac hine.. Once Once the critica criticall damp damping ing is set, measure the combined value of the 1200 1200 oh ohm m resi resist stor or and and pot pot (wit (with h the the series series resisto resistor), r), sub substi stitute tute with a fix fixed ed resistorr or resis resisto resistor tor paralle parallell comb combination ination to obtain the same value. If you want to ex expe perim rimen entt with with many many coils coils you can can inser insertt a 12 1200 00 oh ohm m resis resistor tor in R1 R11 1 an and d then then mo moun untt the 5k pot and 12 1200 00 oh ohm m limit resistor next to your coil connector  where the connection points to the coil at the connector are close.

The Final Steps The coaxial cable, connecting the coil to the PI cir circu cuit, it, ad adds ds ca capa paci cita tanc ncee to the co coil il an and d tends tends to slow slow do down wn the coil. coil. Coils attached with 7 ft of cable can add about abo ut 200 200pF pF of ca capac pacitan itance ce to the coil circ circui uit. t. Ty Typi pica cally lly,, 75 ohm ohm co coax ax ha hass about 16pF per foot while 93 ohm coax has about 13pF per foot.

By using a low output capacitance coil driver MOSFET (below 100pF), Teflon insulated insula ted coil wire, a polyethylene shield sp spac acer er,, pr prop oper erly ly ad adju just sted ed damp dampin ing g re resi sist stor or,, an and d a sh shor ortt co coax ax (abo (about ut 33 inches) you can squeeze the most speed out out of your our co coil. il. Th Thee va valu luee of you our  r  damping damp ing res resisto istorr is an indica indicator tor of the total total TX ci circ rcui uitt ca capa paci cita tanc ncee an and d co coil il ca capa paci cita tanc nce. e. Hi High gher er value aluess of the the dampin mping g resisto istorr in ind dica icates tes les less capacitance to damp while lower values of da damp mping ing re resis sisto torr va value luess in indic dicat atee a higher capacitance to damp. http://www.freepatentsonline.com/70753 04.pdf  Final Test The 300 to 35 350uH 0uH coil des describ cribed ed here will critically damp somewhere between 75 750 0 to 10 1000 00 Ohms, Ohms, depe depend ndin ing g on the

Fig. 11 Damping resistance set-up for adjustment.

“The switch off speed is governed by the damping damp ing self resona resonant nt freq frequen uency cy of the coi coill circui circuit. t. Cri Critica ticall dam dampin ping g gives gives the  fastest rate, but generally the coil is  slightly over-damped” E ri c F ost ste er  A Little Tutorial Coils Coils wil willl sudd sudden enly ly stop stop respo respond nding ing wh when en the PI circ circui uitt delay delay is ad adjus juste ted d lowerr than lowe than the coil’s coil’s nat natura urall min minimum imum delay. While waving a U.S. nickel, gold ring ring or nugg nugget et targ target et unde underr the the coil coil,, lis listen ten for a res respon ponse whil whilee slowl lowly y reducing the delay control. If you get to

a point where the PI machine suddenly

 

stops responding to the target, you have reached the natural delay for that coil at that that delay delay contro controll sett setting. ing. One way to speed up the coil is tto o reduce capacitance capacitance some somewh wher ere. e. If the the PI mach machin inee stop stopss respon responding ding very clo close se to the minimum minimum desired delay, you may be able to tweak  the dam dampin ping g resisto resistorr val value ue slig slightly htly to allow the PI machine to operate over its ful fulll de delay lay range range wit with h your our ne new w co coil. il. Somee of my own crude Som crude mea measure suremen ments ts indi indiccate that, for each 100pF 0pF of  capacitance I reduce in the coil or TX circuit above 10uS, I can speed up the coil by about 1uS. This is a good rule of  thum thumb b to ke keep ep in min ind d so you ca can n examine where you might want to look  to speed things up if possible. Other Insulation Materials Co Coil il se self lf re reso sona nanc nce, e, inc inclu ludin ding g sh shie ield ld capacitance and the coax cable, is a very compreh com prehens ensive ive measure measuremen mentt tha thatt tel tells ls

you much about about the poten potential tial speed of  your new coil. All coils have distributed capaci cap acitanc tancee between between the coil coil win winding dingss tha thatt is pr prima imaril rily y affec affecte ted d by the wire wire insu insula lati tion on thic thickn knes esss and and in insu sula lati tion on materia mate rial. l. Thicker Thicker insulat insulation ion hold holdss the coil turns farther apart which reduces the capacitance as reflected by a higher coil self-re self -resona sonant nt freq frequen uency cy.. Also Also,, thic thicker  ker  wire insula insulation tion tends tends to slig slightly htly reduce the coil inductance compared to thinner  wi wire re insu insula lati tion on.. Th Thee wi wire re or sp spac acer  er  insu insula lati tion on ma mate teri rial al has has a di diel elec ectr tric ic constant. consta nt. PVC has a dielectric dielectric constant range of 4 to 6 while Teflon insulation has a dielec dielectric tric constan constantt of about 2. The dielectric constant of any insulation materiall is an invisible characteris materia characteristic tic that has an imp impact on you ourr coil’ il’s se self  lf  reson re sonan ance ce.. If we comp compar aree two sp spira irall wrap spacers, one with a PVC dielectric co cons nsta tan nt of 4. 4.4 4 and anoth anotheer with with a Polyethylene (PE) dielectric constant of  2.2, we hav havee a differe difference nce between between the two materials of 2 (4.4 divided by 2.2). If the coi coil-to l-to-sh -shield ield capaci capacitan tance ce of the PVC is 14 140p 0pff the then n the coilcoil-toto-sh shie ield ld capacitance of the same coil using a PE spir spiral al wra wrap p is abo about ut half. half. Th This is same same thin thing g occurs wh wheen you win ind d tw two o identical coils using the same wire size and same same insulat insulation ion thickne thickness ss but each each having different different ty type pe of insulation insulation.. If we

compare the self resonance of coils with PVC insula insulation tion and Teflon Teflon insulati insulation, on, the co coil il with with the Teflon Teflon insula insulated ted wire has a higher self resonance than the PVC insulated insula ted wire. Kynar insulati insulation, on, the type used used on wi wire re-w -wra rap, p, ha hass a diel dielec ectr tric ic co cons nsta tant nt of ab abou outt 6. A 320u 320uH H 10 10.5 .5”” dia iam meter coil made with Kynar  insul insulat ation ion (AWG (AWG 30 30,, 0. 0.01 019” 9”OD OD), ), 18 turn turnss ha hass a se self lf re reso sona nanc ncee of abou aboutt 861K 861KHz Hz wh whil ilee the the sa same me size size Te Tefl flon on insulated coil (AWG 30, 0.024”OD) has a 1.2 1.25MH 5MHzz se selflf-re reso sona nanc nce. e. Ad Addin ding g a second layer of ¼” OD PE spiral wrap to the Te Teflo flon n co coil il lower lowerss the co coil’ il’ss self  self  res resona onanc ncee by ab abou outt 11 11KHz KHz to ab abou outt 1.239MHz. This is due to the additional PE materia iall bein ing g in the coil’s electrostatic field. The insulation on the Ky Kyna narr insu insula late ted d wi wire re is also also a litt little le thin thinne nerr than than the the Tefl Teflon on insu insula lati tion on thickness, thickne ss, and this t his also contributes to the Kynar insulated wire coil having a lower 

see the TX MOS MOSFET FET capac capacitan itance ce and the additiona additionall loading loading of the RX inpu inputt resistor. The same 19-turn coil wrapped wi with th a PV PVC C sp spir iral al wr wrap ap spac spacer er ha hass a 1.167MHz self resonance while the PE spiral spir al wra wrap p spa space cerr has a 1.2 1.25MHz 5MHz self  resonance. This difference is due to the dielec dielectr tric ic co cons nsta tant nt of the spira spirall wra wrap p materia mate rial. l. As a rule of thum thumb, b, the fina finall self self resona resonance nce of a coil, coil, sh shield ield space spacer, r, shield and coax should be about half of  the coil-al coil-alone one self resona resonance nce.. To get a coil to operate at or under 10uS delay, shoot sho ot fo forr a self self reson resonan ance ce ab abou outt 40% lower than the coil-alone self resonance. Anything you can do to make the final coil (with shield and coax) self  resonance higher will make a coil with less less capa capaci citan tance ce an and d on onee tha thatt ha hass the  potential to operate at a lower l ower d delay. elay.

self resonant frequency.

things first. 1. See if you ca can n fin find a lo lowe wer  r  output outp ut cap capaci acitan tance ce MOS MOSFET FET coil driver. 2. Use Use a low lower er cap capac acita itanc ncee coa coax. x. This is useful if you must make a co coil il with with a lon longe gerr co coax ax that you want want to op oper eraate at the the lowest possible delays. 3. Add Add a re resi sisto storr of a ffew ew o ohm hmss in series with your coil. This will reduc reducee the fly-b fly-bac ack k vo volta ltage ge,, lower the coil current and allow you to sample a little faster.

A good good so sour urce ce fo forr PE sp spir iral al wrap wrap is www.usplastic.com. www.usplastic.com . I used the following two item number numberss to make the coi coill in this article:   4116 41164, 4, Poly Poly-E -E Sp Spir iral al Wrap Wrap  Black ¼” OD 100 ft and   4116 41163, 3, Poly Poly-E -E Sp Spir iral al Wrap Wrap  Black 1/8” OD. US Plast Plastic ic Cor Corp. p. ha hass a cata catalog log full of   parts that will help coil makers improvise coil parts. To see the impact of capacitance on your  co coil’ il’ss pe perf rform orman ance ce,, measu measure re the self  self  resonance with the: 1. Coi Coill al alone one,, 2. Coil Coil with with just just the sp spir iral al wra wrap p spacer, 3. Coil with the shield, shield, 4. Coil w with ith sshiel hield d an and d the coaxia coaxiall cable. What you will see is that the coil alone has the highest self resonant frequency. It will be slightly lower with one layer of  spiral spir al wrap space spacer, r, very slightly slightly lowe lower  r  with a second layer of spacer, lower with the shield added, lower when one lead of  the coil wire is connected to the shield and finally lower when the coax cable is attached. Then when the coil is connected to the PI TX/RX circuit it will

Befo Before re rede redesi sign gnin ing g you ourr coil coil in an attempt to make it faster, try these three

All design is a compromise. Try to understand the variables that you can control and the things that are relatively fixed. Cable Length Design Consideration Th Thee co coax ax cable cable is a large large capa capaci citan tance ce contrib con tributo utorr to lowe lowering ring the coil coil’s ’s self  self  resonance. If we use a RG8X coax with 27pF 27p F per foot capac capacitan itance, ce, then a coil with 2.75 feet adds 75pF (2.75 times 27) to lower the coils self resonance. Seven feet of coax will add 189pF (7 times 27). Th Thee value value of th thee damp dampin ing g resist resistor or is higher with the shorter coax as there is less capacitance. RG8X coax was chosen  because the center conductor is stranded and is easily available in marine supply stores. stor es. If yo you u are really determine determined d to

 

make a fast coil, use Phillips PXT1000 audio coax cable (solid center conductor) or equivalent at 17pF per foot. Extended flexi flexing ng of this this cable cable ma may y be beco come me an issu issuee if used used fo forr a long longer er hiphip-mo moun untt configuration. This is a good reason why some PI designs mount the control box on the shaft with a short coil coax, and remotely mount the heavier batteries and  possibly some final audio amplification in a body-mounted box. If you make only one coil for your PI machine, set the damping resistor value fo forr th thaat coi oill on th thee circ ircuit uit boar board. d. Howe Howeve ver, r, if yo you u plan plan to us usee multi multiple ple coils on your PI machine, place a little higher high er val value ue damp damping ing resisto resistorr of about about 1000 to 1200 Ohms (0.5 to 1 watt) on the ci circ rcui uitt boar board d an and d then then trim trim the the fina finall damp dampin ing g valu valuee by addi adding ng a ¼ wa watt tt resistor between 1000 and 5000 Ohms in the connec connector tor hou housin sing g in par paralle allell with the coil connections. This will allow you to ch chan ang ge co coil ilss an and d hav have op opti timu mum m da damp mping ing for each each coil. coil. Th Thes esee va value luess mustt be ob mus obta taine ined d expe experim rimen ental tally ly by mon mo nit itor orin ing g th thee outp output ut of the the firs irst amplifierr (IC6 on the Hammerh amplifie Hammerhead) ead) with an osc scil illo losc scop opee whil whilee adj djus usti ting ng a variab var iable le dampin damping g resisto resistorr tempora temporarily rily  placed across the coil to achieve critical damping (no ringing). ringing).

1 inc nch h of wire togeth theer to fo forrm a “g “gim immi mick ck”” ca capa paci cito torr that that us usees the the capacitance between the wires separated  by the wire insulation acting as the dielectric.

stream beds need smaller coils to fit into tight spots and need the extra sensitivity of a smaller coil with more coil turns to obta obtain in a resp respon onse se fr from om thos thosee smal smalll nuggets. The techniques described described in this article can can be us useed to mak make eac each co coil il of a doub double le D (DD) (DD) coil coil wh wher eree ther theree are are separate transmit and receive coils. The details for making DD coils require some additional design considerations that will  be the basis for a future DD coil design article.

Fig. 12 Coil self res resonanc onancee test set setup. up. Courtesy, http://www3.telus.net http://www3.telus.net/chemelec/  /chemelec/ 

Coil Size Considerations Using the right coil diameter for the size and depth of the targets sought will help to optimize your your searc searching. hing. If a target is  just detected at a depth equal to the coil’s radius, then the coil size is just right for  that particu particular lar target target size size and metall metallic ic composition. Going to a larger coil will reduce the detection depth on that target. If the detection depth is more than the coil radius, then you can increase the coil diame diamete terr an and d inc incre reas asee the de dete tect cting ing depth for that particular target.

Coil Self Resonance Measurements To do basic coil measurements described in this article, you will need a few pieces of test test equip equipme ment nt.. A signa signall ge gene nerat rator  or  with with a 2M 2MHz Hz max maxim imum um freq frequ uency ncy (pr (prefe efera rably bly wit with h a di digi gita tall fre frequ quen ency cy

PI mono mono co coil ilss of vary varyin ing g di diam amet eter erss ha have ve differ differen entt nu numbe mbers rs of coil coil tu turn rns. s. Smaller coils have more turns to achieve the the sa same me indu induct ctan ance ce as a larg larger er coil coil wo woul uld d have have.. Smal Smalle lerr co coil ilss are are more more sensi sen sitiv tivee to ne nearar-co coil il sm small aller er ob objec jects ts  because the magnetic field is stronger  due due to the the incr increa ease sed d numb number er of coil coil turns, turn s, but the far-fiel far-field d strengt strength h is less less..

display) is used to stimulate the coil. An oscilloscope is necessary to observe the waveform form across the coil as th thee frequen freq uency cy from the signal signal genera generator tor is swep wept to fin ind d th thee coil’ l’ss resonant frequency. A scope probe can load the coil circuit down enough to alter the self  reson re sonan antt mea measu sure reme ment ntss by 30 KHz to 100KHz (or more) lower than the actual resonance. One way to minimize this is to use use a 10 10X X pr prob obee wh whic ich h ha hass lowe lower  r  capa capaci citan tance ce tha than n a 1X pr prob obe. e. See See the followin foll owing g diagram diagram for a goo good d coil coil test setu se tup. p. No Note te tha thatt the there re is a sm small all value value capa capaci citor tor in se serie riess wi with th the sc scop opee to minimize mini mize loading loading on the coil. coil. A sma small ll capacitor can be made by twisting about

Larger diamete Larger diameterr coi coils ls have have fewer fewer turn turnss than a smaller coil but reach deeper for  larger targets. Since the identity of most targets targ ets is unknow unknown n until until retriev retrieved, ed, it is  better to us usee a coil size of 10 to 12 inches i nches in diamete diameterr that that acc accomm ommoda odates tes a wide range of small objects and only go to a specialized coil when your objective is to se seeek a sp spec ecif ific ic targ targeet size, ize, meta metall co comp mpos ositi ition on,, an and d de depth pth,, or when when the ground conditions dictate a specia specialized lized co coil il.. Be Beac ach h hunt hunter erss ten tend to want want to cover a lot of ground with each sweep so larger round coils or rectangular coils in the 6” by 18” size range work well in this this en envi viro ronm nmeent nt.. Nug Nugget get hunte unters rs seeking very small nuggets in streams or 

Acknowledgments I want to thank Eric Foster for sharing man y technical tips on his PI Technology Forum over the years. Carl More Mo rela land nd’s ’s Ge Geot otec ech h Foru Forums ms ha have ve evol evolve ved d into into a ve very ry co comp mpre rehe hens nsiv ivee technology-o techno logy-oriented riented detect detecting ing resource resource that I frequently use. Carl has provided

valuable suppor valuable supportt and encourag encouragement ement in finalizing finalizin g this article article.. Reg Sniff provid provided ed many technical tips about making fast PI coils. This article is the result of reading many years of Eric’s and Reg’s postings on Eric ric’s and Ca Carl rl’s ’s web web si site tess and setti setting ng up my own wo work rksho shop p to wind wind many many PI coils coils while while do docu cume ment nting ing th thee results.

Joseph Jose ph J. Rogowsk Rogowskii   is a retir retired ed U.S. U.S. Army strategic planner for an Army-led De Depa part rtme ment nt of De Defe fens nsee di dist stri ribu bute ted d co comp mpu utin ting sy syst stem em.. He is a fo form rmer  er  co coll lleg egee fac faculty lty memb member er teac teachi hing ng Audio/TV Audi o/TV pro produc duction tion and and managi managing ng a coll colleg egee medi mediaa de depa part rtme ment nt.. He is an award winning TV Producer for the U.S. Ar Army my,, and and late laterr beca became me a tech techni nica call writer to manage Technical Manuals for  Army Arm y co commu mmunic nicat ation ion an and d electr electron onic ic systems. He conceived and managed the

 

Ar Army my’s ’s fi firs rstt inte intera raccti tive ve vi vide deod odis iscc technicall manual relea technica released sed in 1989. He is a Wo Work rkfl flow ow Ma Mana nage geme ment nt Coal Coalit itio ion n (WfMC) Fellow. He sails Barnegat Bay,  N.J. (bbsailor) in the spring and summer. Winter and fall finds him on N.J. barrier  islands (Long Beach Island and Absecon Is Isla land nd)) meta metall dete deteccti ting ng.. Hi Hiss firs firstt ex expe peri rieenc ncee with with co coil ilss came in his yo youn unge gerr ye years ars fro from m de desi sign gning ing gu guita itar  r   pickups for N.J. custom guitar makers.

References http://geotech.thunting.com/pages /metdet/info/coils.pdf 

http://geotech.thunting.com/pages /metdet/info/induction.pdf  http://geotech.thunting.com/pages /metdet/info/lancaster/lancaster_1 50.pdf  http://www.docsdetecting.com/do csplace/jlange/confusion.html http://www.epanorama.net/docum ents/wiring/coaxcable.html http://groups.yahoo.com/group/Pr  ospectinginOz/files/ http://www.freepatentsonline.com /7075304.pdf 

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