Jump to content

Metal Detector Coils?


Recommended Posts


In a nutshell:

Big coil = more depth on big targets, less depth on small targets, worse separation

Small coil = more sensitivity to small targets, less sensitivity on big targets, better separation

Concentric coil = Slightly better depth, easier pinpointing, worse ground handling

DD coil = Slightly worse depth, harder pinpointing, better ground handling

There is way way more to it than this, I would need to write 3 chapters to do it justice.

  • Like 6
Link to comment
Share on other sites

Here's a short description by (another) one of the metal detector design+engineer gurus:

http://www.fisherlab.com/hobby/davejohnson/searchcoilsessayrevised.htm

And a little closer to home from a detectorist guru 😁:

 

 

Link to comment
Share on other sites

I have always wanted to have every factory coil made for a given detector because they all have slightly different characteristics/applications. Each sees the ground/target differently.

  • Like 1
Link to comment
Share on other sites

On 9/20/2022 at 7:26 PM, Geotech said:

In a nutshell:

Big coil = more depth on big targets, less depth on small targets, worse separation

Small coil = more sensitivity to small targets, less sensitivity on big targets, better separation

Concentric coil = Slightly better depth, easier pinpointing, worse ground handling

DD coil = Slightly worse depth, harder pinpointing, better ground handling

There is way way more to it than this, I would need to write 3 chapters to do it justice.

Hello Carl, could you please explain this to me?

Link to comment
Share on other sites

Quote:"could you please explain this? : "small coil has .. less sensitivity on big targets""

On a VLF machine, the precise inductance and resistance of both coils ( transmit and receive ) is important. Hence all coils, small or large, for a given machine need to have exactly the same inductance & resistance. This then determines how many turns of wire go on the TX & RX coils. This combination of different size loops, different number of turns, affects the strength of the transmitted magnetic field, and the sensitivity of the receive coil. A smaller coil simply gets a smaller signal from a target, so the target needs to be closer to be detectable. Vice-versa for the larger coil.
On the plus side, a smaller coil also picks up a smaller ground signal, which helps the detector pick out the 'target signal' from the 'ground signal'.
Other relevant factors include: Does the detector know it has a small / large coil fitted ?
If it does, then small changes to the way it operates, such as gain/sensitivity changes, ground filtering characteristic changes, can be made, to mitigate the effects of the new coil. Machines that will almost certainly do this include XP Deus/Deus2, ML Equinox, XTerra's. But the majority don't, and it's down to the operator adjusting sensitivity up/down etc to make the best of a different coil size.
  • Like 2
Link to comment
Share on other sites

@PimentoUK  - So in other words you say a small coil will have trouble to detect bigger items? Wow.

All the written from Carl makes sense to me, except this phrase. If the detector/coil combi will detect a cent in 5 inches it will also detect a bin lid in 5 inches, but I think the bin lid will be detected deeper, too.

And what my experience in the field is, the shape of big metal-parts can also be better determined with a smaller than with a large coil.

And thanks for trying to explain Induction Balance, but that wasn't the question at all. I also know that the detectors frequency will take effect to the targets detectable size so most gold detectors have higher frequencies like 50kHz and more, VLF wise.

Link to comment
Share on other sites

Quote:"So ... you say a small coil will have trouble to detect bigger items?"

Yes, a small coil will give a smaller signal than a 'standard' coil, on a large target. And a large coil will give a larger signal than the standard coil on that target.

Roughly: Going from a 10" coil to a 5" coil will result in 1/6th the target signal. But ground signal strength will be down by a half, so there's the possibility of increasing gain and/or sensitivity, to leave you with a 1/3rd drop in signal. This would equate to 80% of the standard coil depth.

Quote:"Thanks for trying to explain Induction Balance"

I didn't explain it. I did point out there are engineering design constraints for small ( and large and standard ) coils .... the designer can't simply wind three times the number of turns on a small coils' loops to make up for the loss of signal, because the result would be incompatible with the detector.
Link to comment
Share on other sites

There are some air test data in the following report on the Gold Bug 2 with three different coils.  I plotted those data (except the 1 ozt row) and did some curve fitting:

GoldBug2_air-test.thumb.png.5c355fa1b1fe0fc1bba91449c4c3a98d.png

You can see in the 12-14 grain range that the three coils give about the same 'depth' (in air, not ground) and that for lighter nuggets, smallest coil is best and above that range, the largest coil is best.  (Note:  about 15 grains equals 1 gram).

  • Like 1
Link to comment
Share on other sites

On 9/20/2022 at 1:26 PM, Geotech said:

 

There is way way more to it than this, I would need to write 3 chapters to do it justice.

Geo has it right, So many factors involved with each machine and coil. Ground conditions play a major roll as will machine programs and adjustments. These are the basics but it takes a bunch of experimental runs to find out what works best, where and with what settings. This will span every detector you own. It can take months to find a combo for each area you hunt, with each individual machine. That's the fun of this hobby. Once you dial in, It make machine and coil choice easy. Keep a log of settings, machine, coil and area. It takes the guess work out.

 

 

 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...