Coil Wire Size for Fast PI Coil

[Bob_T]

bbsailor,

Thanks! I'm starting to make some headway in understanding what I'm trying to do.

mikebg, I've been detecting off and on for years but this is my first attempt at building a detector, and thanks to you guys here, I can build one. Suffering from information overload, and I've found I have to focus on bits and pieces at a time. One of the gaps in my understanding has been the effect of coil wire size. So, I started this thread and now I think I have most of the answers I was looking for. My apologies for my response to your comments above about the need for a coordinated fast network. I was just concerned that this thread would drift off in another direction before my questions were answered.

So, now my questions are answered and this discussion can go anywhere it wants to. I think the concept of a very fast PI detector is interesting. It reaches the point where the small gold particles are too small to be worthwhile digging and picking. However, a detector capable of detecting very small gold particles is very valuable as a prospecting tool. Once an area with very small gold particles is found with a detector, a drywasher or other more effective way of recovering the gold can be brought in.

Thanks,

Bob

[6666]

Gee I will take a bucket full of small gold particles any day,

I had a quick look through my notes for gold timings, not sure where these came from, but may be worth noting
PPS =1Khz
Tx pulse width 100uS
Duty factor 10%
1st delay 15uS
1st sample width 45uS
inter delay 200uS
2nd sample width 45uS

[dfbowers]

I think you'd be hard-pressed to see a difference between Litz and stranded in this case. You do want low R, so I'd use 18awg or such.

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Just for fun, I grabbed the highest "Q" coil I could find, around 270 uH 175/46 Litz, and connected it to one of my Baracuda circuits. I know.. the dampening resistor kills "Q" anyway but I wanted to see if I could minimize any eddy currents and eliminate all the parasitic capacitance that I could.. Not optimal at all for Litz but I thought that I would see some improvement over 22 gauge stranded silver with Teflon insulation.. No improvement at all!
I could however shave a good 5 or 6 us off of my decay curve by rewinding my 22 gauge coil to a basket weave.

[grungymike]

Went to the wetnet site and immediately noticed that the calculator is valid for a single layer of windings, which brings me to asking what the pro's and con's are between wound wire and printed circuit (ie. single plane) coils. I understand that the magnetic field shape generated by the two types is somewhat different, and apparently pcb type coils can suffer from high resistance, or potential for eddy currents, dependent on trace width, beyond that, I'm still reading and thinking. These questions are aimed at one of these days shooting the designer, and finally actually building a PI machine aimed at nugget hunting, based on the Hammerhead circuit, and (God help me) my own coils. What I have in mind is around 1k PPS, 50-70 us xmt, 10-15us 1st sample, sample width and 2nd sample location TBD. Any thoughts, input, directions, or guidance?

PS- thanks to everyone on this site. I'm in so far over my head, it ain't funny. If I hadn't stumbled on this place 3 years ago, I wouldn't even attempt this.

[bbsailor]

Went to the wetnet site and immediately noticed that the calculator is valid for a single layer of windings, which brings me to asking what the pro's and con's are between wound wire and printed circuit (ie. single plane) coils. I understand that the magnetic field shape generated by the two types is somewhat different, and apparently pcb type coils can suffer from high resistance, or potential for eddy currents, dependent on trace width, beyond that, I'm still reading and thinking. These questions are aimed at one of these days shooting the designer, and finally actually building a PI machine aimed at nugget hunting, based on the Hammerhead circuit, and (God help me) my own coils. What I have in mind is around 1k PPS, 50-70 us xmt, 10-15us 1st sample, sample width and 2nd sample location TBD. Any thoughts, input, directions, or guidance?

PS- thanks to everyone on this site. I'm in so far over my head, it ain't funny. If I hadn't stumbled on this place 3 years ago, I wouldn't even attempt this.

grungymike,
The webnet site calculator is very accurate and compares closely to making an actual meassurement with an LCR meter but use the 1 percent answer. Even though it states that is is valid for single layer coils ,the wire bundle will be 5 layers long and high for a 19 turn coil and 6 layers long and high for a 31 turn coil . Just use a proportional fraction such as 5.5 for 25 turns between 19 and 31 turns.

If you want to detect smaller nuggets, lower delays are necessary but the coil alone will not do it. The whole circuit must be capable of lower delays. See the hammerhead instructions http://www.geotech1.com/pages/metdet...ead/HHv1p5.pdf page 12 for the resistor modification for delays down to 8 microseconds. Also, keep the coax as short as possible. Many tips are in the fast coil article in the projects section of this forum.

bbsailor

[Bob_T]

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I could however shave a good 5 or 6 us off of my decay curve by rewinding my 22 gauge coil to a basket weave.

Now that sounds really impressive. Will a basket weave coil fit one of your standard coil covers, or do you need to use a full clamshell cover?

Bob

[Bob_T]

But then again, like everything else, the low capacitance from the basket weave coil is a compromise and comes at the cost of reduced magnetic field due to the spacing of the windings?

[dfbowers]

Now that sounds really impressive. Will a basket weave coil fit one of your standard coil covers, or do you need to use a full clamshell cover?

Bob

I have not seriously considered actually using just because I don't hunt for tiny gold. I use longer delays to eliminate salt water and help eliminate ground noise at the beach.

The diamond weave litz coils are held together with a slurry of melted packing peanuts and acetone. I'm not sure how I would keep a basket weave coil together.

Don