Concentric coil for TGSL.

[scrungy_doolittle]

looks like 6.5 khz is all around better for coin shooting than the 14 khz. whites knew what they were doing.....

[Maurizio2]

I have redone the calculations and I believe there is a small mistake.
The turns for RX should be 183 and not 193.

After my success with a concentric for IDX, I though I would make a homebuilt CC for TGSL.
I have to wait for a few sheets of plastic to show up to vacuum form a pair of shells, but early test are promising!

Some advantages I am realizing by building CC over DD:

* Light! Maybe 1/2 the weight as very little epoxy is used.
* Cheap! maybe only $20.00
* Disc is excellent!!!!!!! A handful of steel tools over the coil is completely silent!!!!!
http://youtu.be/uNTnS6VoNL4
* Good detection range for the size. Coins at 25cm with a 20cm coil.
* Did I mention cheap?!

The down side - a little more challenging, but can be done.

For some details-

Tx coil - 95mm radius. It's build on a 100mm radius foam board with a groove cut in the edge with a hot iron. , 11mm thick. 105t using .25mm wire - 6.0 mH
Wire is wound Counter clockwise.

Rx coil - 43mm radius. It's built on a 43mm radius foam board, made into a spool (see pics). Foam board started at 11mm thickness but sanded a bit so all fits flush when finished. 193t using .25mm wire - 6.5mH. Wire is wound counter clockwise.

Bucking coil - 26 turns (about) on the outside of the Rx coil, again .25mm. Wire is wound clockwise.

Tx start is hooked to hot side of Tx circuit, end of bucking coil is hooked to ground.
Nulling is excellent.. although I have not built any shield yet.

Complete details to come when it's finished...

Don

[dbanner]

How did you arrive at 183 turns? Show us HOW calculation was made. By which method? Formula? Coil calculator software? How exactly? There is a coil calculator at the beginning which everyone uses. Did you use it?
It is not sufficient to say it is so, just by saying so. Hey, that's reminds me of a song from way back, "say it isn't so"

[Maurizio2]

correct
wrong

How did you arrive at 183 turns? Show us HOW calculation was made. By which method? Formula? Coil calculator software? How exactly? There is a coil calculator at the beginning which everyone uses. Did you use it?
It is not sufficient to say it is so, just by saying so. Hey, that's reminds me of a song from way back, "say it isn't so"

[Hyena]

How did you arrive at 183 turns? Show us HOW calculation was made. By which method? Formula? Coil calculator software? How exactly? There is a coil calculator at the beginning which everyone uses. Did you use it?
It is not sufficient to say it is so, just by saying so. Hey, that's reminds me of a song from way back, "say it isn't so"

Really? For example I totally do not care about number of turns and be carefull with "everyone uses". For rough calculation I use this formula: https://sub.allaboutcircuits.com/images/11098.png (the middle one). But I do not count turns. I only measure inductance. Thats the headline for coils

For example my small CC for IGSL-Musk has diameter of Rx80mm and its about (barely remember) 350 turns of 0,3mm wire. Try to make it by this instruction, depends on tightness of winding and cross shape... Better to be guided by 14,7mH.

[dbanner]

[ATTACH]46861[/ATTACH] correct
[ATTACH]46862[/ATTACH] wrong

Now clearly you have discovered a possible error in the document, which I can accept. That coil calculator is pretty accurate. Since the tx calculation in the document is bang on then it stands to reason that it could be an error but not necessarily. There could be some other reason. Only DB can say for sure.
Thanks for pointing that out.

[dbanner]

Really? For example I totally do not care about number of turns and be carefull with "everyone uses". For rough calculation I use this formula: https://sub.allaboutcircuits.com/images/11098.png (the middle one). But I do not count turns. I only measure inductance. Thats the headline for coils

For example my small CC for IGSL-Musk has diameter of Rx80mm and its about (barely remember) 350 turns of 0,3mm wire. Try to make it by this instruction, depends on tightness of winding and cross shape... Better to be guided by 14,7mH.

Whatever works for you.
The term" everyone uses" was meant to convey the meaning " widely used and by a lot of people "
In pointing out errors, it is always a good thing to explain how the calculations were derived.
No need to explain that.

[dfbowers]

Pretty tough for every builder to duplicate the design exactly for sure! Tuning is tricky but once you get it, it's worth it.

[Maurizio2]

Hi Don!
The top and the bottom shell are shielded in the same manner.
You masked off a small strip of tape that when removed, leaves a small gap in the conductive coating.
Therefore the top and the bottom shell must be coupled in the same point where this gap finish, to avoid a short circuit.
Is it right?

[dfbowers]

[ATTACH]46872[/ATTACH]

Hi Don!
The top and the bottom shell are shielded in the same manner.
You masked off a small strip of tape that when removed, leaves a small gap in the conductive coating.
Therefore the top and the bottom shell must be coupled in the same point where this gap finish, to avoid a short circuit.
Is it right?

Well, not really because the bottom shell overlaps the outside of my top shell so there is a gap already. Unless you are using a highly conductive spray like nickel I don't think it matters. Graphite is a few kohms per square so it might not be so critical.
Anyway, that's my thoughts.

[dbanner]

In the construction of concentric coils, I highly recommend the Cscope method, which is to wrap the bucking coil on seperate winder with the same diameter as the RX coil, instead of wrapping the buck coil on top of the RX coil.
The RX coil contains a lot of windings such that the bucking coil, if wrapped on top of RX coil, is now slightly larger in diameter which makes balancing a little more tricky.
The calculations for number of turns for the bucking coil is made on the basis of the same diameter as the RX coil, which is now thrown slightly off(diameter of the buck coil) when wrapping the bucking coil on top of the finished RX coil.
Also, wrapping them on separate winders(spools) makes it possible to remove some turns on RX coil as well if that is deemed necessary.
Have a look at some photos of a Cscope concentric elsewhere on forum for some pics.
I think this approach makes concentric coil building much easier in terms of getting it to balance within required specs.
With this proven method, one can make any size search coil for any detector. Just something to consider.

[MartyJ1963]

Thanks Dbanner,,,I've been following all your posts on 'Coil tuning' info etc,,,it's been really helpful to me. Since joining the forum in 2016 I have learnt a great deal from everyone on here,,I can now manage to build Mono (PI) Cols fairly fast, accurate and neatly now but I've been trying to master 'DD's and Concentrics for over a year now so I can eventually, successfully make coils for my TGSL/IDX/ and IGSL. I've managed now to make 'Neater' and more accurate DD's and can tune them a little better (thanks to the advice you gave me in the past),but have still not successfully made a single working coil yet,,,hopefully soon though!! I've struggled more with the concentrics,,and it's like you say, it's when I get to removing or adding to the bucking coil, or adding a turn to the TX etc,,,I've gotten a little further though following your past advice so I'm going to try what you sad and make a separate Bucking coil,,hopefully it will be successful and save me wasting the last 3 Concentrics I made,,,Fingers crossed!,,Thanks again,,Regards,,Marty.

[waltr]

I have made a Concentric for my HH2 PI detector. What I did to 'tune' the buck coil is leave some extra wire going to the buck coil. This is then moved around slightly to tune the null. I know this is not for a VLF (TGSL) detector but the methodology is the same.
Check my thread for details and pictures here:
https://www.geotech1.com/forums/show...ake-on-the-HH2

[dbanner]

I have made a Concentric for my HH2 PI detector. What I did to 'tune' the buck coil is leave some extra wire going to the buck coil. This is then moved around slightly to tune the null. I know this is not for a VLF (TGSL) detector but the methodology is the same.
Check my thread for details and pictures here:
https://www.geotech1.com/forums/show...ake-on-the-HH2

yes Waltr, the little extra wire is moved around, however that assumes that you are pretty close to the null, the little wire is then used to fine tune the final setting.
However, just one turn too much or to little on the bucking coil(vlf concentric design) is enough to throw the null out by a wide margin. Just imagine your calculation for the buck requires 20 turns on 10cm diameter, then if you wound the bucking coil on top of the RX coil, the starting diameter is now 10cm plus say 4mm. That extra 4mm is enough to render your calculation of 20 turns too much by one or two turns believe it or not! Now you are left with trying to remove a turn or two, endless tinkering etc.(and so by removing turns on buck coil) it changes the inductance of the TX coil and messes with the TX oscillator frequency! Many headaches!!
So by winding the buck and RX separately, one can avoid these issues to some extent. You have a much easier task of getting it close enough to null based on initial calculations so that just the little extra "loop" moving around is sufficient to bring it to a final null.
Here is a photo of a Cscope coil. Notice the double spool for RX and buck. Just print a few such forms and perhaps an outer form for the TX(although not necessary), and now life in the world of concentric coil construction is much easier.

[waltr]

Was not trying to say the double inner coil form was a bad idea, It is actually a good idea.
Just adding some info to assist tuning null.

What I did when making my concentric is wind the buck wires with the RX wires. This averaged the diameter of both and null was then easily achieved with the little extra buck coil wire.

The calculation of the buck coil turns come from the article on building concentric coil on the Geotech projects.
https://www.geotech1.com/cgi-bin/pag...e=projects.dat