northwest First coil build and avoiding rookie mistakes
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: First coil build and avoiding rookie mistakes 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: First coil build and avoiding rookie mistakes  (Read 766 times)
autitch
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« on: Tuesday August 21 2018 15:12:03 AEST PM »

I’ve been working on my first coil build for to suit a GPX5000 and  I’ve been at it for a while as the learning curve is quite steep having never done anything of the kind before.  My aim is to attempt to realize a coil that will perform as well if not better than what is commercially available. 

As a whole I’m pretty comfortable I have the majority of the coil parameters nailed down to realise it a low capacitance 14” mono coil however I am unsure of the coil Q I should aim for and the influence of shielding capacitance .   Measuring my existing coils there appears to be a range of Q figures that is employed on commercial coils, see below data and generally from what I’ve seen the commercial boys use graphite paint shielding.
ML 11” Commander Mono Q 5.51 @ 1kHz, 290uH, 0.34 Ohm,  SRF 634 kHz
NF 12” Evo Mono Q 5.08 @ 1 kHz, 309 uH, 0.39 Ohm,  SRF 622 kHz
NF 16” Adv Mono Q 5.19 @ 1 kHz, 291 uH, 0.37 Ohm,  SRF 634 kHz
CT 16” Goldstalker Mono Q 3.64 @ 1 kHz, 310 uH, 0.56 Ohm,  SRF 567 kHz

 I’ve settle on constructing a mono coil to fit into a  14” housing ( kindly donated by a Coiltek 14” DD coil) basket weave spider patter wound coil for low capacitance, integral home made coax using the coil Litz wire also for low capacitance and a high strand count Litz wire.  The specs and estimate of parameters so far are as follows:
Wire 160/AWG38 1.75 OD  Litz
25 turns 240 ID 328OD
Inductance at plug  309uH (coil  305uH coil and coax  4uH)
Capacitance at plug  120 pF (coil 60 pF and coax 60 pF)
Shield resistance ~ 70 Ohms/inch
Resistance DC at plug 0.388 ohms
SRF 826 kHz
Coil Q 5.00

There two things that I’m unclear about:

1. What Q figure should I be aiming for and what  is the influence of coil Q on coil performance. 

2. How to measure the coil to shield capacitance and the influence of this figure.  The basket weave spider patter winding has a low capacitance as well as the coax I intend to make.

I have read on this forum some words from Stefan "One further point , if you minimize the shield to coil capacitance without making an appropriate reduction in the Q then the coil will be noisy. ie to much coil gain." 
In my design I’ve attempted to make a “fast coil” with low capacitance and hence a high SRF but I’m a little unclear if I may be pushing the design too far and in the process realising a “noisy” coil. 

Guidance and advice gladly welcome.

AuTitch
 

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Alex
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« Reply #1 on: Tuesday August 21 2018 21:51:32 AEST PM »

Hi autitch. Im new to coil construction and interested in the same questions.I measure coil to shield capacidence between shield wire and coil wire. ive made a few coils and found the lower Q seams to make the coil dull .The last coil wound as the same as the evo and the same specs you stated peformed the best in the gold fields.I however used stefarns shielding method and try for aprox 40pf per circumference foot and a 1.2 mm pitch seamed to work every time.I also constructed 1.2 meter coax using litz wire ,plumbers Teflon tape ,spiral wrap ,Teflon tape loom tube shrink wrap . 55 pf but I felt it made the coil a bit noisy so I went back to the original coax used wb 1508 I don't know why I assume to much gain .so maybe the srf also plays a big part
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autitch
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« Reply #2 on: Wednesday August 22 2018 08:58:36 AEST AM »


Hi Alex, thanks for the reply.  Interesting to hear your observations regarding the low Q making the coil dull and too low a capacitance making the coil noisy. It appears that there is a sweet spot to be found.   I’m assuming that the shielding method that you refer to is as per the below photo and you use 0.2mm or smaller enamelled wire.  The “40pF per circumference foot” I’m assuming is the capacitance measured between the shielding wire and the coil wire divided by the total length of the shielding wire in feet.  The 1.2mm pitch appears to be a very exact measurement just wondering how you keep this measurement when applying the shielding wire.

   

AuTitch


* Wire shielding method.jpg (38.45 KB, 415x382 - viewed 225 times.)
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IBGold
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« Reply #3 on: Wednesday August 22 2018 16:46:04 AEST PM »

Hi Autitch,

A Q of 5. is around the mark for SD coils it was around 3.8 it is all a ballancing act I will get more involved when I have finished my current project a total revamp of my old vacuum former for interest is a picture of my latest spider coil now ready for testing.

Regards, Ian. happy face


* 3 to 1 coils 001.JPG (493.78 KB, 1152x864 - viewed 208 times.)
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Regards, Ian.
Alex
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« Reply #4 on: Wednesday August 22 2018 19:43:15 AEST PM »

Very nice job Ian.Would like to see the specs.  autitch I got my info from Ian and Authere ,the shelding is .2mm wire spread onto 19mm tape using bolts with 1.2mm pich to space the wire then another lenth of tape to seal of on top ,for a 12 inch coil I make 2 length about 5m long ,wrap one leaving leaving aprox 15 mm space for the next tape .I use this methord because I can wrap measure unwrap adjusting shield spacing and wire wrap spacing until im happy .40 pf per circumfrecce foot is 12inchx 3.147 =37.76 divided by 12 =3.147x40=125pf I think just a guide I read on one of stefarns posts.The coax I constructed I used the same tape with aprox 50 strands wraped tightly.The coil you are making sounds like it would be fast .It would be great to see how it performs .
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Alex
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« Reply #5 on: Wednesday August 22 2018 20:42:02 AEST PM »

autitch. The last coil made and probably my best was wound as a double flat spiral like the evo coil .inside dia 278mm outside 310mm 318uh 606khz coil to shield 153pf ,coil spacing between shield from memory was only about 4 mm,iwas concerned with the extra width of the coil winding may increase gain and a noisy coil.also with extra width of shielding wrap increasing ground cap.but all seams good.. Q=5.1 .39 ohms at plug
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autitch
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« Reply #6 on: Thursday August 23 2018 13:59:30 AEST PM »

Nice work there Ian it looks like a purpose built unit it would be interesting to learn a little more about the project and certainly your input is most welcome.
Alex, thanks for the explanation of the shielding capacitance calculation.  From what you wrote the 40 pF per foot of circumference is a guide / starting point.  Using the thread of a bolt is a great idea to maintain an even spacing between stands, very clever.  Your last coil you realised from a numbers perspective appears to be very comparable to a NF 12” EVO and it must be very rewarding to have created a coil that is of that performance level.  Just wondering what you mean by “double flat wound” I just can’t picture how that would work.  Are there two layers of winding arranged as per the sketch below.   I’m pretty motivated to start winding the coil up.  I’ve experimented with the separate elements  of the coil now it’s time to put it all together. 

AuTitch


* double flat spiral.jpg (12.13 KB, 439x196 - viewed 188 times.)
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6666
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« Reply #7 on: Thursday August 23 2018 14:41:06 AEST PM »

I made my double flat spiral thus,
published way back in 2011 by TEC on GT forum.
Alex is this how you wound your coil ?


* TEC=coil-1.jpg (36.16 KB, 640x480 - viewed 189 times.)

* TEC-COIL-winding direction.JPG (53.49 KB, 789x418 - viewed 192 times.)

* TEC-coil-optional shielding.JPG (75.75 KB, 655x704 - viewed 196 times.)
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« Reply #8 on: Thursday August 23 2018 16:39:23 AEST PM »

  
Nice work there Ian it looks like a purpose built unit it would be interesting to learn a little more about the project and certainly your input is most welcome.
Alex, thanks for the explanation of the shielding capacitance calculation.  From what you wrote the 40 pF per foot of circumference is a guide / starting point.  Using the thread of a bolt is a great idea to maintain an even spacing between stands, very clever.  Your last coil you realised from a numbers perspective appears to be very comparable to a NF 12” EVO and it must be very rewarding to have created a coil that is of that performance level.  Just wondering what you mean by “double flat wound” I just can’t picture how that would work.  Are there two layers of winding arranged as per the sketch below.   I’m pretty motivated to start winding the coil up.  I’ve experimented with the separate elements  of the coil now it’s time to put it all together.  

AuTitch

Hi AuTitch,

I have just done a prototype coil that has similar winding as you have shown but the winding starts at 1 then 2 is where 12 is and 3 is where 2 is and 4 is where 11 is and so on  and that seems to be excellent from thee reports I have had  any way attached some pictures of the Vacuum former  to give you an idea.

Regards, Ian. happy face


* Vacuumn Former sheet hold down mechanism 001.JPG (499.69 KB, 1152x864 - viewed 184 times.)

* Vacuumn Former sheet hold down mechanism 001.JPG (499.69 KB, 1152x864 - viewed 182 times.)

* Vacuum Former 006.JPG (451.07 KB, 1152x864 - viewed 189 times.)
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Regards, Ian.
Alex
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« Reply #9 on: Thursday August 23 2018 19:31:26 AEST PM »

rds Hi guys ,and yes it is wound the same as 6666 has shown.But I used a mosaic mesh to wind one layer then wound the top layer on another contact mesh and placed it on top of the other one . I posted the mesh and my plan on this forum under   Tools or methods related to coil building .The mosaic mesh has a strong contact adhesive once the paper is pelled off , makes winding spiral easier .I made litz wire the same way explained in Geotech where strands are wound in anticlockwise and clock wise directions.
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Alex
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« Reply #10 on: Thursday August 23 2018 20:22:41 AEST PM »

Hi Ian interesting winding pattern.and what do you heat the plastic with,it looks like battery terninals in your pic
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« Reply #11 on: Saturday August 25 2018 16:27:20 AEST PM »

Hi Alex,

 In the bottom picture you will see a stainless steel box that houses two 3 bar Quartz 2400 watt heaters that is the heat source picture attached of the underside these came from Bunning's on special as suggested by Muntari earlier what you see in the top pictures is the 12V leads and switch mechanism that pulls the ABS sheet holder onto the bed when the ABS is at the correct temperature or that is the plan the vacuum bed is now fixed to the vacuum box and my great thanks go to Muntati for CNC drilling the bed for me and me being able to pick it up from him yesterday again THANKS.

Regards, Ian. happy face


* Vacuum former quartz heater 002.JPG (529.16 KB, 1152x864 - viewed 155 times.)
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Regards, Ian.
Muntari
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« Reply #12 on: Wednesday September 19 2018 13:49:54 AEST PM »

Hi Ian,

How did it go with your Vac former revamp, all working now?

Work has slowed down for me now so I'll be back in the shop getting onto mine but just wondering how you went.
be good to see some action shots lol

Any surprises happy face?

I'll post a few photos of my progress soon

Cheers

Muntari
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« Reply #13 on: Wednesday September 19 2018 16:17:42 AEST PM »

Hi Muntari,

The solenoids did not have enough pull so I have changed it to a manual lever pull down system and that works a treat the only thing to do is work out the correct heater height above the ABS I have not tried it out yet as I only have one panel left so waiting for the next project the wire for which is coming then I will try it out playing with pin pointers at the moment I will add a picture tomorrow of the latest upgrade I have been meaning to contact you.

Regards, Ian.
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Regards, Ian.
Muntari
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« Reply #14 on: Thursday September 20 2018 13:45:49 AEST PM »

Hi Ian,

That's a pity on solenoids, maybe with a leverage arm it will work ok.

I've been looking for a decent Litz wire supplier here in OZ but haven't found one yet so turning my attention to oversees suppliers.
The problem there seems to be shipping costs more than anything else.
I don't really want to make my own but if I have to, I will.

Cheers

muntari
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« Reply #15 on: Thursday September 20 2018 15:30:52 AEST PM »

Hi Muntari,

Yes it was a pity but the manual system now in use is simple works is foot operated and once down on the bed it stays there held by the vacuum until you turn the vacuum off so as far as I am concerned job done.

Regards, Ian. happy face
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Regards, Ian.
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australian electronic gold prospecting forum.com  |  Detector Technology and Electronics and new detectors  |  Detector Coils  |  Topic: First coil build and avoiding rookie mistakes « previous next »
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