Detection Depth Food For Thought

[cougarmccxxi]

I have always been told a larger coil equals deeper detection depths of similar sized smaller objects, for example a dime. I have never read any information on the relationship to detection depth and coil size and why until today. When I think of how an electromagnetic field works the following excerpt from the explanation of coils and how they work does make sense.

"A metal detector creates a current in its search coil that results in an electromagnetic field. This field extends to a depth that is about equal to the coil's diameter."

Granted there are more variables in the depth of detection than only the search coil size but I found the information very useful and helpful in understanding the whys and wherefores as far as coil size and depth of detection are related.

The website for the excerpt is; http://www.metaldetectors.com/metal-...ilsarticle.cfm

If these guys are way off base I am all ears to the truth of the matter as the subject interests me.

HH.

[DIGGER27]

This is pretty right on for the most part.
In the real world things sometimes don't work exactly this way, but mostly they do.

What this article does not mention is that the larger the coil, the less discrimination ability you usually have.

Large concentrics go deeper than smaller ones, but they don't pick up really small objects as well.
Smaller coils we call snipers are more sensitive to smaller objects and can move around and through trashy areas because that scanning field is smaller...although usually not quite as deep.

DD coils are fantastic for hunting large areas because the scanning field is longer, and in mineralization they will give you greater depth than the round ones, but certain trash objects like round pop tops will sound just about like a quarter or won't disc out or break up like the same object might do with a concentric.

There is a reason that we sometimes have a collection of different coils for different situations, and this is also the reason we always strive to learn the language of any detector we swing with all the different coils we purchase for it.

Like they say on those public service TV commercials...

"The more you know..."

[beest]

Originally Posted by cougarmccxxi

"A metal detector creates a current in its search coil that results in an electromagnetic field. This field extends to a depth that is about equal to the coil's diameter."

I don't believe this part at all. With my 8" coil this would mean i wouldn't find anything deeper than 8" because the field doesn't go deeper?
That's why i regular have to dig up sodacan's and other big things on 18" or more

[pulltabMiner]

I think the missing part of information here is that the coil is calibrated for coin-size objects.

My personal standard for determining the maximum depth of a detector is a U.S. dime. Besides the size of the coil, soil composition will play a big part.

Last, this is a subject dear to my heart but it's not very popular among the 'gurus'. I think getting hung up on one particular aspect of the hobby is not the way to go BUT I also believe metrics should be an important part of our hobby as it will allow us to evaluate new technologies as they arrive.

[cougarmccxxi]

Originally Posted by beest

I don't believe this part at all. With my 8" coil this would mean i wouldn't find anything deeper than 8" because the field doesn't go deeper?
That's why i regular have to dig up sodacan's and other big things on 18" or more

The depth compared to the coil size was referring to small dime-sized objects. Large metal objects of course are a totally different story.

Example; my old Radio Shack detector did great at detecting crosscut saw blades buried two feet down with an eight inch coil but it couldn't see a dime at 8 inches.

HH.

[spacecoast]

Electromagnetic fields theoretically extend to infinity, it's just that they get too weak to detect or are overwhelmed by competing fields. Hence the difficulty in hunting under power lines.

[beest]

@cougarmccxxi
I just reacted at my quote made of your quote, which litterly says that the field extends to that depth same as the coils width.
We likely all agree it extends deeper, so that quote is nonsense like i said.

OT
My experience with bigger coils is that sensativity has to be turned back a little, by which depth will be a little less. Sensativity back more on one detector than another, so effectivly detectorrelated.
I once had a detector with huge coil (30cm. about 14"??) on which sensativity for not being restless had to be that far back that it's depth was about the same as with it's 8" coil. Only remaining covering bigger surface, less swinging (not so handy in small corners and edges).
Imo everything has a pro and neg to it.

[b-boop]

Good info, Digger 27. Thanks!