northwest f1a4 coil
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australian electronic gold prospecting forum.com  |  DIY forum  |  DIY forum  |  Topic: f1a4 coil 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: f1a4 coil  (Read 4830 times)
Alex
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« Reply #20 on: Saturday August 13 2016 21:16:50 AEST PM »

Thanks WM6 . i feel the rise in the inductance may be from incorrect measuring inside the house on a wood table with metal frame and tightening of the coil after wrapping the shield tape .The measuring devices i used was a Lc 100a meter ebay and     circuit connected to a Vichy vc97 frequency meter . I have since measured the coil in the paddock away from everything keeping the leads apart and got 686.1 khz 323.1uh .I havent measured the Q i dont have a meter. Perhaps its a good idea for the next coil .any suggestions .Im going away on tuesday for 3 weeks so ill start when i get back.
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Alex
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« Reply #21 on: Saturday August 13 2016 22:21:42 AEST PM »

Sorry link didn't work try       
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Alex
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« Reply #22 on: Wednesday November 9 2016 14:16:12 AEDT PM »

Hello again just used the coil recently constructed in the wa gold fields with some  success 9.5 gram nugget. I feel the coil is a bit noisy, ground balancing about every min and at times i had to wind the gain dial back from full to about half . I don't have a Q meter so i calculated the Q with 2 pi Fr I/ohms then compared it with other coils on geotech plus a coiltech coil and the original army coil. From this i think the coil Q could be about 4.5 The army coil could be about 4.3 I plan to use this method i want to construct an 18 inch to the same Q as the original army coil. Do members think this will work
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WM6
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« Reply #23 on: Wednesday November 9 2016 20:40:15 AEDT PM »

Hi Alex
It is hard to say how your new 18" coil will perform.
Coil Q is not only factor to imply, there is total resistance and parasitic coil capacitance too.
And coil should be well adapted to detector input circuit (properly damped).
You need to try and see outcome.
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Alex
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« Reply #24 on: Wednesday November 9 2016 21:16:28 AEDT PM »

Thanks WM6 .I thought if i mached the inductance resonate frequency capacitance and ohms as close as possible to the original 8 inch coil;then use those results to calculate the Q factor to try and get the same  I dont have a oscillow scope .Im using .2mm x36 or so strands to adjust the ohms.I relise the ohms increase as frequency goes up but i thought this way might get it close.any thoughts? thanks
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WM6
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« Reply #25 on: Wednesday November 9 2016 22:35:05 AEDT PM »

  

I relise the ohms increase as frequency goes up but i thought this way might get it close.any thoughts?


Generally this is due so called "skin effect".
Practically you need more wire surface to lower total resistance.
With more wire surface "parallel resistor" effect appear.
More wire surface you get with multi-strand wire (Litz wire).
This way you can somewhat control total resistance at given higher frequency (no effect at low frequency).
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Alex
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« Reply #26 on: Wednesday November 9 2016 23:06:05 AEDT PM »

Thanks Wm6  .So what your saying is because a f1a4 minelab detector operates at low frequencys .i have no need of a Q meter i could get away with calculating the Q with the known resonate frequency inductance and the ohms 2pi.f.l/ohms
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WM6
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« Reply #27 on: Thursday November 10 2016 00:12:24 AEDT AM »

Q is important to get tank (LC) circuit to oscillate.
To low Q will not allow tank circuit to oscillate (depend on frequency of course).

With (most) PI technology we use pure (square like) pulses to drive our sensor (coil).
If sensor would be pure R, then no Q factor implied in its functionality.

But this is not the case, cause coil has inductive as well capacitive components - it is LC(R) tank circuit.

When we drive LC tank circuit with pulse, those pulse "fall apart" or become composed out of sinusoidal signals.
Those by pulse created sinusoidal signals are composed out of main resonant signal with many added low and high harmonics signals.

Resonant LC tank (coil) sinusoidal signal depend mainly of coil LC value and not so much on (mostly low) pulse changing rate.
This way we are back to coil Q, which become somewhat considerable working factor in more complex PI technology.

Practically speaking: you can drive your coil by 100pps (100"Hz") pulses rate, but your coil has self resonant frequency at 500kHz and this
is important to establish coil parasitic capacitance at least and by this correct coil damping and adapting to detector RX circuit.

So, from your f1a4 point of view, to correct accept you new coil, such coil should be made as much like original by parameters: L, C (coil own parasitic capacitance), Z (impedance) and coil Q factor. In this case new coil will be seen by f1a4 as genuine coil and no inner f1a4 circuit adaptation will be needed. How to do this is another story, very hard without instrument to perform different measurements. Without instruments, the only way is "try and error" way - most usual in amateur world.
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« Reply #28 on: Thursday November 10 2016 16:05:28 AEDT PM »

  
Thanks WM6 .I thought if i mached the inductance resonate frequency capacitance and ohms as close as possible to the original 8 inch coil;then use those results to calculate the Q factor to try and get the same  I dont have a oscillow scope .Im using .2mm x36 or so strands to adjust the ohms.I relise the ohms increase as frequency goes up but i thought this way might get it close.any thoughts? thanks

Hi Alex,
You are going about it the right way and your 36 strands of 0.2 mm is what I use for an 18" coil the trick is a tight coil bundle I lace mine at 10 mm centers and have a 12 mm spacing minimum to the shielding to minimize coil to shield capacitance I have not tried one of my coils on the F1A4 yet but when I get time when I have got the splint of my left hand and can use it again (4 more weeks) I will try and let you know how I go.

Regards, Ian. happy face
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Alex
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« Reply #29 on: Thursday November 10 2016 20:39:00 AEDT PM »

Thanks Ian and Wm6 for your input. Ill look forward to hearing from you Ian. When i made the 11 inch i based the coil parameters on a coiltech 14 x9 from a gp3000 .i got all parameters very close . Maybe if i tried the coil tech in the goldfields it could be noisy with the f1a4.Ill copy the 8 inch army coil  for the 18 inch parameters.The army coil is 363 uh ,i thought that was high but every way i check it i get the same reading .Ian do you bulk wind your 18 inch or use a spacer as Stephan does with his 12 inch  thanks again.
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« Reply #30 on: Thursday November 10 2016 23:22:04 AEDT PM »

  

Hi Alex,
You are going about it the right way and your 36 strands of 0.2 mm is what I use for an 18" coil the trick is a tight coil bundle I lace mine at 10 mm centers and have a 12 mm spacing minimum to the shielding to minimize coil to shield capacitance I have not tried one of my coils on the F1A4 yet but when I get time when I have got the splint of my left hand and can use it again (4 more weeks) I will try and let you know how I go.

Regards, Ian. happy face

Hi Ian

Curious, what would be resonant frequency of such 18" coil using 36 strands wire?

Have you ever been determined (measured) it?

Regards and best wishes to your great work with coils.
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« Reply #31 on: Friday November 11 2016 17:14:27 AEDT PM »

Alex, I make my wire from the required amount of .2 mm enameled copper wire when the wire is made I put 1 or two layers of  clear heat shrink tubing I buy it in 15 meter lengths very time consuming but it seems to do the job once the coil bundle is wound and tightly laced I put it in a Styrofoam former which I make in two halves and bond together I then apply the shielding over the former then put that in the housing which I either vacuum form or fabricate from ABS I hope that answers your question.

WM6, Yes I measure the resonant frequency of all my coils depending on the type of wind the resonant frequency of an 18" coil can be from 550 KHz to around 785 KHz as I said depending on construction method and type of winding.

Regards, Ian. happy face
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Alex
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« Reply #32 on: Friday November 11 2016 18:55:52 AEDT PM »

thanks Ian
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WM6
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« Reply #33 on: Saturday November 12 2016 21:11:04 AEDT PM »

  

WM6, Yes I measure the resonant frequency of all my coils depending on the type of wind the resonant frequency of an 18" coil can be from 550 KHz to around 785 KHz as I said depending on construction method and type of winding.

Regards, Ian. happy face

Ian, thanks for info.
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« Reply #34 on: Wednesday December 14 2016 20:41:28 AEDT PM »

HI guys I have a question perhaps for Stepharn . For say a 16 inch coil using a shielding tape with .2 mm wire spread evenly across 19 mm tape .Would it be better to wind one 18 meter length or say 6 x3 meter lengths
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authere
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« Reply #35 on: Thursday December 15 2016 14:32:44 AEDT PM »

Hi Alex,

I have made Stephan's shielding and it was quite easy to make it one length to do the job

I used my verandah posts which were 3m apart and to each post connected a bolt with the correct spacings in the thread for the spacings in the shielding

I then ran the wires along the verandah posts thru the threads

I then made a former which I travelled with along the tape to keep the spacings as I set it to the tape

Time consuming but you get there eventually,but to make it in three pieces it would have to have the ends connected and soldered joints build up eddy currents which would negate a lot of hard work

I think it would be a lot easier to do what ML do and coat a sheet of water proof paper and apply one to each side of the coil

Ron(but I am no expert)
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« Reply #36 on: Thursday December 15 2016 17:24:50 AEDT PM »

Hi Alex I use both Stefan's method and commercial Electrodag with Stefan's method I use 24 or 25 mm tape and use a 6.0 meter length of 50 x 25 RHS with the required pitch bolts set  on each end the RHS is held in two vices with one layer of tape held sticky side up on the top of the 25 mm side the wire is then strung and another layer of tape applied over the top time consuming but does the job and I have not needed any more than a six meter length I have even used masking tape with no problems at odd occasions.

Regards, Ian. happy face
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Alex
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« Reply #37 on: Thursday December 15 2016 17:35:47 AEDT PM »

 Thanks Authere, I used Stepharns sheild on a 11 inch and i found trying to get even spaced wires on a 5 meter length difficult .Your idea using bolts sounds good .Thanks again ill go with the full length . one more thing ,roughly how wide would you space the shield wires on a 16 inch bundle wound about 10 mm with about 15 mm polystyrene spacer
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Alex
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« Reply #38 on: Thursday December 15 2016 17:41:20 AEDT PM »

Thanks Ian . Which is better aquadag or stepharns
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Alex
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« Reply #39 on: Thursday December 15 2016 17:49:33 AEDT PM »

opps  i meant electrodag
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