Metal detectors coil detection patterns

[chuck a]

Anyone know of a book that would explain what the metal detection patterns look like for different size coils and different type detectors?

Thanks Chuck

 

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[stevemc]

Circular coils usually have a cone shaped pattern, with VLFs a fairly small cone, the more powerful such as a PI, will be out farther than the coil and much deeper, but still cone shaped. You can verify this by putting a coin down and pinpointing it, and going up until it cannot be found. Then with a large metal object find the outer limits of that cone. You should be able to see your coils detection pattern. DD coils or SEF cols have a centerline running through the 2 coils that runs deep, Instead of a pointed cone bottom it is like a big V from front to back, a much larger, deeper area to find things. Why many coil companies are making DD coils for units that can use them.. Also they have a wider detection area at the bottom. This can also be verified by the coin on the ground trick. Some weird shaped coils like a 24" wide by 6" deep, will of course have a wide shallow cone. Even large coils like a 1 meter by 1 meter PI coil will be a giant cone going down many feet on large objects, and to the side also. The double coil or box models have a seperate transmit and recieve coil seperated, which makes the field larger. But these are VLF types and still dont have a huge range. The pattern is basicly what is between the 2 coils. Maybe a little beyond. They are meant for detecting walls and shallow ground. They dont go as deep as even a small coil PI.

 

[Sandman]

Steve did a good job of describing them but maybe a picture will help more.

Last pic is hard to read unless you use ZOOM. It shows the importance of over lapping your sweeps.

 

[finderskeepers]

Anyone know of a book that would explain what the metal detection patterns look like for different size coils and different type detectors?

Thanks Chuck

Lots of info here

Truth About Search Coils

 

[chuck a]

Thank you finderkeepers that was just what I was looking for.

 

[finderskeepers]

You're welcome and welcome to the site and the hobby! That site has A LOT of info about A LOT of different aspects.

 

[Frankn]

Chuck a, Forget about the coil patterns. What you really want to know is where does it pick up the target. VLF detectors which is most of them use two basic types of coils,- concentric, that is a coil that transmits on the outside and a coil that receives on the inside of the loop or coil. This type of coil has the highest signal return dead center of the coil. - then there is the DD coil which has a transmitter coil on one side and a receiver coil on the other side of the loop or coil. This coil type picks up best on the edge of the coil. When the lead edge of this type of coil just comes over the target, that is the strongest signal. There is a Pulse Induction type of detector also. This detector coil also picks up best on the lead edge, that is the left side of the coil as it goes over the target. I don't know why the left side picks up better, but it does in actual tests. There is one more type known as the 2 Box detector. It has two coils, each mounted in its own box with the electronics. The transmitter is in the rear box and the receiver is in the front box. This detector throughs out a "ball" shaped pattern and picks up from all directions in the air. That is it will pick up an object in a wall beside you, in front of you, or behind you, but only in air, that is non ground areas. It will only pick up in the ground directly below the operator. I use an old Hays 2 Box and it's range is over 6' in any direction, but the target must be larger than a softball. Hope this helps. This info is based on actual operational experience, not text book theory. Frank

 

[chuck a]

Hi Frank,
thanks for all the information. I am still trying to figure out my new V3i. Sorry it has taken so long to answer your reply.

Chuck

 

[kycoinseeker]

Is there any difference in a sef butterfly coil pattern and a dd coil?

 

[Frankn]

Chuck a, The first Whites V3 models had DD coils and had many complaints about pinpoint problems. As a result, the next variant of V3 had a concentric coil and was back to the normal excellent White pinpointing. I think there are 3 versions of the V3 out now. Frank

 

[Frankn]

Kycoinseeker, I have never used a SEF coil, but my guess is that it is a variant of the DD concept. Frank

 

[woof!]

If the question is what the search response pattern of a searchcoil is, that's a question to which there is no straight answer. Gross generalizations of limited relevancy are as good as it gets. a lot of what's been posted in this thread shouldn't be taken too seriously, and some of it has been just plain wrong.

http://www.tekneticst2.com/tekfiles/searchcoils essay revised.htm

--Dave J.

 

[Frankn]

Woof, it is very easy to determine the search pattern of any coil. Just place a coin on a piece of barren ground and move it back and forth at different heights. You will soon know the effective pick up pattern of that coil. Forget the theory and go by actual use. Frank

 

[Jason in Enid]

Like Frankn said, there is a big difference. The size and shape of the actual EM field is one thing. Where an object needs to be in order to give a detectable response is completely different.