DD-Coil design guide for a VLF detector?

[dingbatca]

How well would a Garrett AT Pro coil work?

https://www.ebay.com/itm/Garrett-AT-Pro-5x8-DD-Coil/254581706423

I realize that is a 15kHz machine. I am just fine with adjusting my machine to match said frequencies. I just need to find a coil that I can trust, so I can make sure the coils I build work as expected.

[Carl-NC]

Probably fine, but it has a proprietary connector you would have to replace.

[dbanner]

I'd be rather suspicious of those Garrett coils on eBay. You can buy so called Garrett coils on Alibexpress for cheap. They are not original.

Be sure it's a genuine Garrett coil.

[dingbatca]

Thanks for the warning. I doubt it will be in my price range by the time the auction is over. :-(

Turns out it is hard to come by a good search coil with a broken connector/mount... I will keep waiting.

[dbanner]

But that is not to say that those Chinese MD-6250 coils are not good. They sell two coils compatible with the Garrett Ace series. The price is very cheap. Those coils are good for teardown and inspection of construction and shielding. The Chinese make an exact clone of the ace250. Coil too.

[dbanner]

Here is a link with a whole assortment of coils, the prices are superb for what they are.

https://h5.aliexpress.com/item/32981142898.html

These are cheap enough to do invasive testing and teardown.

[dingbatca]

Besides the lower operating frequency (6.5kHz), are the Ace coils just simple TX/RX with no other circuits in the way? Any one have a good tear down/disassembly of said coils, including the pinout?

[Altra]

Besides the lower operating frequency (6.5kHz), are the Ace coils just simple TX/RX with no other circuits in the way? Any one have a good tear down/disassembly of said coils, including the pinout?

From some old notes. Did not open the coil it was potted in epoxy so can't say if there were any caps or resistors inside. I was able to make a 12in dd coil with 64 turns #24awg for the Tx and 48 turns #34awg on the Rx. Sorry I do not remember the coil former diameter (9-10in?). But you could probably use a coil calculator and work backwards with number of turns and inductance. It IDed targets correctly so it must have been close enough to the factory coil.

Ace 250 stock coil concentric
Tx coil = 2.48mH 4 ohm
Rx coil = 1.67mH 32 ohm

Uses custom coil cable. Dual coax pair similar to s-video cable

5 pin connector
Pin 1 Tx shield
Pin 2 RX shield
Pin 3 Rx
Pin 4 Tx
Pin 5 not used

[kt315]

you think to open a coil is simplest job?

[dingbatca]

Thanks for all the good info.

I NEVER expected it to be easy to open a coil. I don't mind trying to open one, but that is why I have a hard time buying a "new" coil. Most likely I will destroy it in my search for understanding exactly how it was built.

[gurdal]

Hı
it is not difficult actually,
There are very accurate coil information in the forum.
Choose a brand model, wrap the coil in accordance with the information and gain experience.
you will succeed at the end.
Best regards

[dbanner]

There are perhaps hundreds of photos of the internals construction of coils here on the forum. Along with the coil specs.

Induction balance coils fall into three main types, concentric, DD and Omega.

Each type has variations of shape and size, but they are all the same principle of operation.

If you want to replicate a coil, all you need to know are the geometry (shape and dimensions), the inductances and series resistance, the number of turns is good to know as well but not absolutely necessary.

So long as you know inductances and series resistance of a coil, you can know what size wire and number of turns are required for a given size of coil.

Having an actual coil opened up gives you the opportunity to see how they shielded the coils and other internal circuitry if any.

The mainstream commercial coils have all been analyzed and tested and photos and information have been uploaded. So no need to reinvent the wheel.

Finally, having a good theoretical knowledge of how coils work and good grasp of the calculations required and what to do in terms of balancing and phase response, transmitter efficiency ( tx topology ) and feedback coil principles is all you required to make a good working search coil.

Start by observation of the different types of of tx oscillators, and the coils they drive.
You will begin to see why they chose a particular inductance and wire size.

Look at the RX preamp and you will see why they chose certain RX coil parameters. Compare different schematics and the coils they use and you begin to understand some of the reasons for choosing such coils and why some coils will not work in some circuits properly regardless if they are tuned to the correct operating frequency.

[dingbatca]

Coiltek Wot Coil Minelab Sovereign?

https://www.ebay.com/itm/124184036793

Looks like its a simple pin out:

http://www.treasurenet.com/forums/minelab/523005-sov-coil-connectors-pin-outs.html

[Skippy]

I'm pretty sure all Sovereign coils have the internal pre-amp circuit board.

[dingbatca]

Confirmed the internal pre-amp

http://www.treasurenet.com/forums/tech-talk/529564-minelab-rcb-rx-coil-amplifier-board.html

If it can get it for cheap, I don't mind ripping out the pre-amp. Lets see how much the auction goes for.