It seems to be easy to go accessory crazy with detecting.  I think many times people who are not able to spend a lot of time detecting, instead start buying detecting accessories.  Maybe these detectorists are not having much luck and think a new machine or a new coil is just the thing they need to make better finds.

When it comes to coils, this is a tough one to answer.  Certainly for different types of detecting, having a smaller or larger coil can be an advantage.  But 90% of the time the stock coil will give you good results for coinshooting. 

There are two main types of coils available, concentric and double-d (DD).  Knowing which type your detector uses helps you understand how to get the most out of it.  A concentric coil puts out a cone-shaped field (shaped like a O), the DD field is more like the "center slice" of the concentric (shaped like an I).
Picture
The illustration above shows in grey, the fields of the concentric and DD coils.  The concentric is on the left, and the DD on the right.  The black bar at the top represents the search coil, the grey shows the penetration of the field into the ground.  You can see the strength of the DD coil here in that it can "get between" the bottle cap and the nail, isolating the target.  While the concentric coil will not give a good signal because there are three targets in the field at one time.
Picture
This diagram shows the DD field at a 90 degree angle on the far right.  As is obvious, the DD coil has much more detection area at the "coin" depth than the concentric coil.  But because it is also narrow, it has the ability to "see through" the trash.  But, to be able to "see through" the trash, you need a slow sweep speed.  If you race the coil across the bottle cap and nail, you're never going to be able to hear the "peep" of the good coin signal inbetween.

With the concentric coil, the deepest part of the field is very small, so if you are searching for a deep dime-sized signal, you have to really overlap your sweeps to maximize the depth.  The problem with this is that the interference from shallower objects and trash will keep the detector from even "seeing" these deeper items.  This is called target masking.

The DD coil is far superior in my opinion, as the narrow width of the field cuts out a good percentage of the trash and signals near the surface and the narrow field can really punch down around the trash signals.  Also by scanning an area at 90 degrees and 45 degrees to the original pattern, you will be able to find almost all the targets in an area, and target masking is minimized.

A small coil is said to be good for trashy areas.  With a concentric coil, this has some truth.  The small coil will help you cherry pick a few good items from the trash.  I think for the most part, smaller DD coils are no more effective than the stock coils.  Most often though, I see people reporting making more finds (or the "look what I missed!") with a smaller coil, but it is not the coil that made the difference!  What happened was they tightened up their sweep pattern and had to slow down with the smaller coil.  Had they used the stock coil, overlapped their sweeps and slowed down they would get even better results.
Picture
Look what happens when we put the small coil on.  The targets are in the same locations as the above pictures.  The small coil has reduced our depth, the coin is now not detectable at all and we are still getting signals on the trash targets.  We might have better results making sensitivity adjustments rather than using the small coil.  Looks like going slowly with the larger stock coil might work better than using a small coil based on this situation!

Large coils are touted for having great depth, the trade off is they have a larger field so they are much more prone to target masking.  Not only do these larger coils tend to be heavy, but they are really only effective in super clean areas due to masking.  People buy these coils because they think they need more depth, and while this may be the case at certain sites that have been filled, reworked, or if your area has a high "sink" rate for items, a bigger coil is not going to help you at most sites. In most areas almost all coins are within reach of your stock coil!  Any depth advantage is negated by the effects of target masking.
.
Picture
With the larger coil, we have many more objects detected simultaneously.  With the concentric coil on the right, I now have six targets I am hearing at one time!  The DD coil in the center and right diagrams do a little better, but we still have too many targets under the coil at one time to get a good signal, or a clear ID that says "dig".

Where a large coil has advantages is in covering a large relatively clean area.  A field, a beach... finding traces of an old homesite in the middle of woods.  These areas all have low trash concentrations, so there is little target masking and you can cover more ground faster with the larger coil.

Depth is, for the most part, a bogus issue.  If you are coinshooting, most coins in "natural" ground (grassy areas) are less than 12" deep.  Most newer name-brand detectors made in the past 15-20 years get adequate depth with their stock coils.  The reason we do not find all the coins is not because of lack of depth, it's rather from poor overlapping of sweeps, target masking, and inferior target ID meters on the machines.

Before you go out and buy a new coil, especially for a machine that is new to you, learn how to use the coil that came with the machine.  Understand what coil you have and how the field of that coil works.  Slow down, overlap your sweeps.  Remember that the coil that comes with your machine is selected by the engineers to give the best overall performance and also provide good weight and balance for your machine.  All the features of the detector are tuned to work optimally with the stock coil, not that 5" hockey puck or the 24" trash can lid! 

Master the stock coil and your detector, first.  Understand your site and how the coils work.  If then you decide you have a site that would benefit from use of a different coil, weigh the cost of the coil to the potential finds and see if that accessory coil is really a good investment.  I think most often you will find that improving your technique with the existing equipment, and digging more junk out of a trashy area will have a greater impact on your finds than a new coil.
 


Comments

nopurplecar

Wed, 02 Sep 2009 19:46:37

Great article! I learned something today.

 

terry everly

Wed, 16 Sep 2009 08:30:26

good article nick as far as it goes..Have you personally ever tried hunting with a smaller or larger coil??
Try hunting right up along a fence or the edge of a concrete wall with an 11" stock coil and it wont work.
From experience and trial and error,even with my SE,i can tell you that hunting a very nail filled area such as an old one room school or church that was burnt or just knocked over it does make a difference.

 

Nick A

Wed, 16 Sep 2009 08:47:59

Great comments Terry. I have used larger and smaller coils on several machines I have owned. There's no doubt specialty coils do have their place. I see lots of people not finding "goodies" rushing to buy coils they don't need thinking that it will solve their "problem".

I feel it is important to weigh the cost of the coil vs. the benefit. If you are hunting construction/fire sites all the time, then that small coil may pay off. But a $200 coil to get a few more coins is a very poor return on investment. Whereas just expertise with one coil may yield even more than those few coins for no additional investment.

When hunting next to concrete with rebar, turning down the sensitivity and using the shape of the field of the DD coil will help you get quite close to the edges.

 

Dan Hudnell

Thu, 22 Oct 2009 14:33:23

I've got a buddy who loves to fish. He spends a lot of time at Lake Erie. Every year he goes to Canada for a week-long fishing trip. He owns several rods and reels for trolling and has tackle boxes filled with lures. This summer he spent just over $12,000 on a lake boat. He's spent a LOT of cash on his favorite activity.

In light of that, no one has ever asked him, "Have you caught enough fish to pay for that boat yet?". No one has ever told him the thousands of dollars he spent is a poor investment.

In comparison, I have spent less than $2,000 dollars on my metal detecting equipment - to include what I spent on the 15x12 s.e.f. butterfly coil. Yet many people have asked me, "Have you paid for that thing yet?". lol That's funny.

I enjoy hunting with a different coil. It's fun. I took it to a section of a park I hunted hard with the Pro coil and had more fun digging up coins. I ended up finding 15 more Wheat cents, an 1892 Indian Head cent, a 1919 Mercury dime, an 1896 Barber dime and a 1905 Barber dime. The collective value of those coins may be a poor return on my investment of $189, but I sure had a great time. I got a great return on fun and look forward to more good times. :o)

Dan "Gibsondan" Hudnell




 

Dan Hudnell

Tue, 03 Nov 2009 03:15:28

Let me add that I agree with you, Nick. You certainly don't need an additional search coil. I am very impressed with the stock Pro coil. It gets amazing depth and separates very well. I bought another coil in an effort to wrench a few more good finds out of the heavily hunted local sites I frequent in my small town. I would probably be more productive by simply traveling to another city to hunt. But sometimes I just want to get out for a little while and hit the old familiar sites. It's a fun challenge to get them to produce good finds again.

 

Bob (Mirage)

Mon, 25 Jan 2010 12:02:48

Very good article.
I typically hunt with the Explorer II stock 10" coil. I am very hesitant (know I am loosing depth) to put on my 7.5" DD coil. However, some situations "force" me to use it. Some wooded and brushy areas the larger coil is hard to maneuver between saplings and trees. Also I find I can hunt a lot longer with the smaller coil before my hand starts going numb (carpel tunnel) and my back starts aching.

 

Neil

Sun, 18 Apr 2010 19:27:21

the profile of the concentric coil may sometimes be true but certainly not always. several concentrics put a straight field down, not a cone shape as you have pictured. this is misleading.

 

Nick A

Mon, 19 Apr 2010 07:47:05

Just a note here that this article is meant to be a brief and simple explanation of how coils work, and not a completely detailed discussion.

I asked Neil to explain a bit more about his comment and I wanted to share that here for those who are interested as it does go into a little more detail.

"I didn’t mean to imply you were purposely misleading, just the info is misleading because it’s not complete. Actually a lot of the concentrics do as you have pictured but not nearly as pronounced as the cone you’ve got pictured.

One example would be the Troy Shadow series of detectors, they detect almost straight down off the coil except at their greatest detecting depth where they only hit dead center. dead center is where they all hit strongest including the widescan coils and a note on the widescan coils, a lot of them hit on the outside of the coil as well, not just the center line. Those also will taper near the bottom, kind of a belly pattern.

Here’s a good example you can try for yourself to see what does what: I took a gold ring that comes in a little below a nickel, buried it at about 8" with a rusty nail buried about 3"from it, same depth. I took a Tesoro Tejon with stock coil and tried to detect the ring, barely got it, broken signal. put on a widescan coil(10X12) and hit it repeatedly from all directions. dug up ring and nail, removed the nail, recovered the ring, and with the stock coil the Tejon hit it very well just like with the widescan. so the Tejon was spreading its signal farther out than the cone shape most think of implies.

Over the years of hunting with CZs and many other detectors I’ve seen this over and over. try it with any concentric you have, you might see you can detect a target at depth across more of the coil than you would have thought. Now, I mentioned the CZs, unfortunately they do follow the pattern you have posted with their 5" and 8" coil but not the 10.5"."

 



Leave a Reply