Finding the coils "Sweet Spot"

[goldenIrishman]

hey guys... I've looked high and low in this section and have not been able to find any info on how to determine where the "Sweet Spot" on a coil is. I'm using a ML X-Terra 705 with the elliptical gold coil @ 18.75 Khz.

Could someone explain the procedure for finding the sweetspot please?

 

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[jmoller99]

If you have a concentric coil, you can locate the center point by laying a target on top of the ground and check for its center point (keep in mind that you have a motion detector, so you can't stop the coil over the target - it will disappear if your coil is not in motion).

If you have a DD type coil, there is no 'hot' center point. DD coils are more like windshield wipers - you have a line down the length of the coil that is very sensitive, and your target can be anywhere on that line, I turn 90 degrees and find the center of the target so I can pin point knowing the original target area). You still have to always keep the search coil in motion (that's always a requirement of motion detectors).

I don't use a Minelab, I use a Whites GMT or VSat. The process is the same to determine where your target is for VLF Detectors.

 

[trinityau]

Hello all, while I agree with some of jmoller99 remarks I have to throw in a few of my own. I am no means a tech on this subject but have been swinging coils a bit over 35 years all in the search for gold. I have tried to nail this subject down myself. I have used many coils on many different units and have found some to have a sweet spot while others do not. You can get ten matching coils and you may have a small and different location on each that may be a sweet spot, however some may not have a sweet spot. I have found my strong areas to be anywhere on a coil. I have DD's where a three inch area at one o'clock is the spot. I have a few others where the spot is at six and two o'clock. I have mono coils that do the same. As mentioned I have some coils that appear to not have any sweet spot. I have some coils that have two or more sweet spots. To me a sweet spot is just that, an area on the coil where it seems to get a better signal, clearer and a bit deeper.

I have had occasions where the sweet spot has helped in the hearing and recovery of a nugget. I would say however that my experience has shown me that a small coil that has a sweet spot has performed far better than a large coil with one. I have several ten inch elliptical coils for the GB Pro and three of them right now do not appear to have a sweet spot. However, the fourth ten inch elliptical coil has a spot right at twelve o'clock. With this coil I can get signals on sub-grainers twice and sometimes three times as deep as the other three coils. Guess which one I use the most. This has proven the same with larger coils but is a bit tougher to really catch simply because I normally find more small stuff than large.

Why a sweet spot ? I have talked to techs over the years and the same items keep coming up from many of them. It could depend on how the coil was made. The wires on one coil may be closer together because of insulating barriers pushing against them. There could actually be a higher grade of wire at one location of the coil. I have been told many things.

I am hoping some techs that know what they are talking about will chime in here. Dave Johnson, Reg, maybe a few of our more knowledgeable hunters, Steve H, Chris R. I would like to gain a bit more information on this subject. In the mean time I detect as always with whatever coil I happen to have on and try and learn its capabilities fully, and if there is a spot that appears to be sweeter then I am alright with that. These are just my opinions, TRINITYAU/RAYMILLS

 

[G.A.P.metal]

If your detector has VCO... holding the PP button, and listen for the loudest/highest pitch sound and your right over the target Do It Slowly.
You can also detune the coil...find what you think is the middle of the target with PP button...then let off and move 1" to the side..then PP button agin...this will make the PP area smaller.
Gary

 

[goldenIrishman]

Thanks guys! That explains a lot. i'm new to this MD stuff and know I've got a lot to learn. The MineLab I'm using is a loaner from a friend and is not the machine I'd have chosen if I was buying one for myself. Still it's a machine to learn on and from what I've been told it's not a bad unit even if it's not designed specifically for gold.

Gary... The graphic you posted was a great help! Now I understand more on how the field is shaped on a DD coil Vs a concentric one. This MineLab has some things I've got to remember when I'm in PinPointing mode. Like turn off the tracking so the target doesn't get masked!

Ray: I'm something of an science geek and can understand how coils of the same make and model can react differently due to differences in how they were made, materials etc. Anything made by people can have flaws but it sounds like that one coil you talked about has a really good flaw in it!

Again thanks to all for the help

Jeff

 

[Steve Herschbach]

Never did like those marketing diagrams showing the mythical DD blade shooting straight down. Be a neat trick, getting an electromagnetic field to do straight edges. I get the feeling not many lay people have mapped out coil detection patterns. Jeff Foster did some really good work in the regard.

Ray, I also have been told there is endless variation in coils within allowable specs. I have also noted rather randomly occurring "sweet spots" or "hot spots". I can't say I spend much time on it though, especially when nugget detecting. Get the signal, excavate the target, isolate it, and move on. Usually it is just enough to know the target is under the coil. The only time I employ the hot spots is when getting my scoop over the coil, especially with a PI. With the really tiny stuff the scoop needs to be right on the most sensitive part of the coil.

 

[L.C. BAKER]

What would be the best MD and coil set up for finding lead at a depth of 1-2 feet deep?
Thanks, L.C.

 

[trinityau]

Hello Steve, I do agree with your statement. Turn the unit on, detect, locate signal, dig and move on. Too many new hunters are focusing on things that really do not matter, just my opinion. I am old Army, KISS, keep it simple stupid. That was a general statement to all... Just about all newer detectors that are on the market right now are set to go with very few problems that a new person should have to deal with. After you get some time detecting under your belt and you have some confidence then start getting into details that may or may not even matter. In keeping things short as Steve did, NEWB buy a detector, read manual, practice out in the field, then go on an actual hunt.

I have said before that one of the best ways to break into gold detecting is to find someone that has experience that is willing to take you out somewhere and rub your nose in it. People pay pro golfers to show them how to golf, a person wanting to learn how to fly fish for Steelhead gets a guide. There is nothing wrong with paying a fee for a gold detecting course or joining a club to gain more knowledge before setting out on your own. This brings me to the seven P's - Proper Prior Planning Prevents Piss Poor Performance.

To all others, please read between the lines on this subject as I think a few people are mixing up pinpointing with a sweet or hot spot on a coil. There is a big difference. Pictures are great for pinpointing but digging lots of targets and doing it yourself will really help. If you use DD coils or Mono coils you will learn the differences with use. I love my mono coils for gold, however DD coils have their place. Again, just my opinions, TRINITYAU/RAYMILLS

 

[goldenIrishman]

Steve, In no way did I think that that actual magnetic field from the coil actually matched up with what was shown in the diagram that Gary posted. I know it's only showing a idealized view of what is supposed to be going on. Still it explained to me how I should be overlapping my swings compared to with a concentric coil.

Ray, Like you I'm ex-Army and am a firm believer in the K.I.S.S. method of doing things. This MineLab is pretty much a turn it on and go machine and I've already found several targets on my own, both in coin and prospecting modes. I do like the overall design and it's easy to switch its "size" for when my (much) better half is using it. We do belong to a prospecting club with several people that detect in it but as yet haven't had the time to get together with them to get some instruction. Seems that everyone (including ourselves) keeps having things come up that get in the way.