Fast coils: worth it?

[Monolith]

First of all this could be the “hunting” area: http://maps.google.it/maps?hl=it&ie=...,0.703125&z=11

For sure Roman settlements and beaches at the end of bathing season.
This means that my targets could be coins or jewelry and metals like gold or bronze…
Not at all gold nuggets, never heard!

On bbsailor article I read “…can be used to locate coinsize objects, gold jewelry and gold nuggets.” This made me believe that, without this kind of “fast coils”, it would have been very difficult find gold objects.

Now, I know that my English is absolutely horrible (on the other hand I'm Italian...) and thus can be hard to understand what I mean trough my tarzan style writing, but what I would like to read is somebody who can make statements like “yes, first I built a coil just with enameled wire and wasn’t able to find, later I built a fast coil and now I can detect gold rings and jewelry and nuggets…. I use it on my xxxx homemade detector”. And, if possible, some other comparative data.

That's all. Easy, huh?

Gold chains and thin rings, small gold coins etc, have a fast decay time. To be able to detect them, your detector needs to be able to sample at 10us, even less is better.

The HH detector can be used for that, using the highest pulse repetition rate and a fast coil.

Monolith

[mario]

I have built a few coils. One of them, a fast spiral with an ID less than 6" using litz.

I love this coil, it samples quickly. I can swing away and not miss anything. Unfortunately, all is not good. Even though it is the most sensitive coil I currently have, I dont use it much.

This coil is far too sensitive to hot rocks. Unless your detector has hot rock descrimination circuitry, it will have you digging a lot of hot rocks. I dont know if it samples faster because it covers less surface area. I dont know if it samples faster because it is smaller -It might be a combination of the two factors.

I am no expert on the issue. However, slower timing settings might help with the issue...??

[scuba840]

Coil+ Coil- and Shield

I just constructed a "fast coil" to the specs of Joe Rogowski's article on Fast Coils.

My 2 questions:
1) After shielding it with scotch no 24 (and leaving a small gap), does it matter which wire (coil+ or coil-) I attach the shield to? (my designation of coil+ and coil- are arbitrary)

2) Does it matter which direction the current flows when I hook up the wires? I vaguely remember a right hand rule with EM fields, although I don't think it matters here.

I've attached photos for eye candy.

[mario]

That's a thick coil!

And to answer your question, it depends on your detector. On mine, the shield must be connected to coil ground (which ironically is VDD).

Experiment if you're not sure. Mono coils are simple enough.

To which detector are you making this coil for?

Also, I noticed your shielding might be too thick....If your coil is not sensitive, try experimenting with different shielding material

[scuba840]

That's a thick coil!

And to answer your question, it depends on your detector. On mine, the shield must be connected to coil ground (which ironically is VDD).

Experiment if you're not sure. Mono coils are simple enough.

To which detector are you making this coil for?

Also, I noticed your shielding might be too thick....If your coil is not sensitive, try experimenting with different shielding material

I am doing a microprocessor based PI metal detector. I am looking at the Hammerhead and GoldPIC schematics, but I am heavily deriving my own low noise amplifier based upon the AD797 and a AD600 amplifier. zow they are expensive.

Thanks for the heads up for attaching the shield to the coil ground. In the hammerhead, I assume that this the "common" and not -Vbatt?

I was lucky, there was a guy on fleebay selling some teflon insulated 30 ga wire with a insulation OD of 0.031", which is great for reduced inter-winding capacitance, but makes for a thicker coil. The coil you see uses 2 wraps of PE spiral wrap used to maintain spacing between the coil and the shielding. The shielding is Scotch No 24. mesh shielding single thickness with almost zero overlap.

It fits like a glove into my Hays electronic 6" coil housing. Attached is photo....

[mario]

I've used scotch 24 mesh tape on my coils with very little success. I usually lose 2-3 inches easily. I currently use it as a junction between the unit's coil ground wire and the coil's shielding material. I wrap it tight against the coil's shielding and solder it to the coil's ground. With further experimenting, you will notice that shielding material doesn't have to be metalllic. As long as it is a poor conductor, any material may suffice. The less metallic the better, since it will less likely be picked up by the detector.

You might want to experiment before sealing everything. I'm not saying it won't work on your unit, since all units differ.

Also, teflon insulated wire is probably appropriate for the bigger coils. For the smaller coils, I would use magnet wire.

In any case, it's your first coil. See how it works. We all start some where.

Good luck