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 2784 times)
Alex
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« on: Sunday June 5 2016 12:31:22 AEST PM »

Hi all have wound my first coil for  a minelab f1a4  wound similar to IB gold and Stephans method. same self wound litz and used polyporylene pallet straping for a spacer between 5 turns  11 inch with i think 23 turns  results are  305uh 1.139mhz .4 ohms. I have enough cable to continue straight to the control box . Is this a good idea should  the lead be sheilded or would i be better of with a coax cable
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IBGold
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« Reply #1 on: Sunday June 5 2016 17:27:31 AEST PM »

Hi I have been using the coil bundle start wire as the centre conductor and making my own coax using a combination of heat shrink and spiral wrap as the coax insulator then using the coil end wire as the shield spiral winding the separated strands it as the shield wire with a couple of extra strands to connect to the coil shield then cover the lot with heat shrink the more gap you can get between the centre conductor and shielding the lower the capacitance will be and the faster the coil will end up this also applies to the coil shielding.

Regards, Ian. happy face
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Regards, Ian.
Alex
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« Reply #2 on: Sunday June 5 2016 18:18:00 AEST PM »

Thanks Ian dos it make much difference whether the start coil wire or the end one is used in the centre conductor
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WM6
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« Reply #3 on: Sunday June 5 2016 23:12:22 AEST PM »

  

 1.139mhz


Probably you mean Mega-Hertz (MHz)  and not mili-Hertz (mHz)?

Is this coil self resonant frequency?
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Alex
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« Reply #4 on: Monday June 6 2016 15:57:59 AEST PM »

Sorry my mistake and yes self resonant
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IBGold
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« Reply #5 on: Monday June 6 2016 16:15:13 AEST PM »

  
Thanks Ian dos it make much difference whether the start coil wire or the end one is used in the centre conductor
I really don't know I have always used the start of winding to coax center and always wind in a clockwise direction whether it makes any difference I don't know it is just what I have always done I have asked the Guru's and never got an answer maybe Techno Bob could make a comment.

Regards, Ian happy face
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Alex
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« Reply #6 on: Monday June 6 2016 19:29:44 AEST PM »

thanks Ian .I thought i would ask i have one lead to short
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Alex
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« Reply #7 on: Sunday June 12 2016 19:19:27 AEST PM »

Hello again .Ill take off my hat to all the sucssesfull minelab coil builders. My first attempt i wasnt happy with . second one no good third one was a real dogs breakfast .My fourth attempt reads .4 ohms 1441 Mhz but i only have 275 uh i could tighten it up a bit but do you think it would work . the coil size is 9 inch   thanks.
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IBGold
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« Reply #8 on: Monday June 13 2016 16:14:31 AEST PM »

Hi that should work fine and should be very sensitive on small nuggets some of the sensitive commercial coils have inductances down to 260 uh so you are in the ball park.

Regards, Ian  happy face
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Regards, Ian.
Alex
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« Reply #9 on: Monday June 13 2016 19:57:09 AEST PM »

Thanks again Ian ill try to construct the shield described by Stefan if successful ill attempt a larger one . like the chalenge
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« Reply #10 on: Tuesday June 14 2016 17:12:50 AEST PM »

My condolences You are hooked that is how I started out. good luck

Regards, Ian. happy face
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Regards, Ian.
gef12
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« Reply #11 on: Tuesday June 14 2016 17:28:11 AEST PM »

LIKE???? ????
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Alex
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« Reply #12 on: Monday July 4 2016 17:07:46 AEST PM »

Hi all I need some advise. After reading a post by stefarn about a coil to sensitive for WA goldfeilds i felt the coil i just wound may be the same so i wound another 1.380Mhz 286uh . after winding the sheild i thought that would drop the resonate frequency to about 800Khz .however i came to 1.333Mhz with 111pf coil to sheild. please correct me if i an wrong but from this point my only option would be to fit about 2 mt of coax 16 /per foot pf to bring the resonate frequency to aprox 600 to700 Khz for WA gold fields .the military coil is 364uh 581Khz .the detector ran fine with a coiltech 323uh 693Khz thanks.
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Alex
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« Reply #13 on: Sunday July 10 2016 21:50:17 AEST PM »

Hi all Have had success with the coil .works well thanks to past experiments by members . Could someone explain why the inductance rose from 286uh to 324 after fitting the coax .is that normal.
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WM6
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« Reply #14 on: Monday July 11 2016 04:38:24 AEST AM »

  
Could someone explain why the inductance rose from 286uh to 324 after fitting the coax .is that normal.

It is normal.
Cable is part of coil windings, it prolong coil wire and by this coil inductance go up somewhat.
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Alex
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« Reply #15 on: Monday July 11 2016 18:39:41 AEST PM »

Thanks WM6 that also explains why the military coils inductance is so high
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Doug
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« Reply #16 on: Tuesday July 12 2016 12:00:13 AEST PM »

Coax Inductance
Calculate the Inductance of a Coax Cable
  
  
doug smile
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Stefan
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« Reply #17 on: Friday August 12 2016 12:03:54 AEST PM »

Hi Alex,

Can’t explain why the resonant frequency  went up after the coax was connected.  Normally the resonant frequency will go down as you have added the capacitance of the coax to the coil.  I’ll get to that shortly.  Suggest you backtrack to what you did with the coil when you connected the coax to the coil leads. If you tightened up the coil bundle then there will be some increase in the inductance but not to the extent you noted.  Also make sure that when you measure the capacitance of the coil you keep the coil well clear of any metallic material especially steel.

Now back to the coax:
The capacitance of the coax will be much greater than the inductance, so capacitance effects on the coil with the introduction of the coax will have the greater effect and will lower the resonant frequency of a   coil. 
This is why us coil builders like to keep the inter-winding capacitance of the coil, the shield capacitance and coax capacitance to a minimum.  This is inherent to the coil construction practice used by the coil builder.

Next parameter you should consider is the Q of the coil. This is the quality or the gain of a coil at a measured frequency. Traditionally the Q of a coil was measured at a frequency of 1000 hz but many capacitance meters will measure at 100khz or higher.  The Q of a coil is defined as the reactive resistance of the coil divided by the dc ohms resistance of the coil.

The measured  Q helps you construct coils with the same gain irrespective of the size of the coil.  The total coil resistance R is equal to the actual resistance of the coil plus the resistance introduced by the  coax.  Increasing the total resistance decreases  the  Q of the coil.
The original Minelab mono coils had a Q of 4.5 measured at 1Khz. The current coils probably have a   Q much less than this.

I hope this info helps you in your coil endeavours.

Stefan
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Alex
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« Reply #18 on: Friday August 12 2016 22:15:58 AEST PM »

Thanks Stefarn .Ill give that some thought .I recently tried the coil in WA goldfields and it seemed to work well even though no luck .In a few weeks ill start attempting an 18 inch . I did notice the coil preformed much better on the small settings than on the large setting .This f1a4 is a mod 2 Jack Lange
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WM6
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« Reply #19 on: Friday August 12 2016 23:50:55 AEST PM »

Hi Alex

How did you measure coil resonant frequency? By galvanic coupling testing probe to coil or loosely by inductive coupling?
By galvanic coupling you can add test probe leads capacitance to your measurement and get improper results.
You need to take in consideration that in some cases cable capacitance (depend on cable used and sort of its connection to coil)
can be in serial to coil parasitic capacitance and not in parallel as well that with improper measurement we can easy measure
some of parasitic frequency instead of right resonant one.
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