mh9162013: said:
But narrower coils have more trouble properly IDing deep targets, right? While I haven't had a metal detecting engineer confirm it for me, by looking at coil shapes and sizes, it's my understanding that wider coils (all else being equal) will go deeper...or at least be more accurate at depths that a narrower coil might incorrectly ID.
First, let me clear a few things up for readers who might not understand what is being stated with regard to
'width' and 'length' of search coils.
• Most search coils, regarding their physical shape, used to be round. Th internal coil winding or 'type' of coil configuration was something most typical Hobbyists never understood .... and quite frankly, I don't think most do today, either.
• Because search coils were, or could be, a round shape, then it is eqsy to describe them by their diameter, such as a search coils that is 5", 6", 7", 8", 9" or perhaps 11"
in diameter which can also be called
'wide' or the
'width'. Because the search coils are, or were, round-shaped, it didn't matter where you started your measurement of the radius, it was always the same and described the coils
'diameter' or
'width'. Naturally, with a round-shaped coil it also described the
'length' or
'front-to-rear' measurement which was the same.
• When we are working a search coil to hunt a site, were are sweeping the coil left-to-right and right-to-left hoping to find a desired target. We know that if a site is more littered, we want to be able to isolate a potentially 'good' target from an undesired 'bad' target. That is called 'separation' and to achieve the best 'separation' of close-proximity targets, we want to use a 'smaller-size' search coil.
A
smaller-size coil should have a smaller and tighter EMF
(Electro-Magnetic Field) due to its
'width' or left-to-right measurement. If we are hunting a site with minimal trash AND if we would like to increase the depth-of-detection, we would change to a 'medium' or 'larger-size' search coil. And typically that just meant increasing the
'width' or
'diameter' of the round-shaped search coil.
• Thus we have a
'wider' coil diameter to increase target depth capability, or a
'smaller' or
'narrower' 'diameter' for a smaller and tighter EMF to work in and around problem trash.
NEXT ISSUE to CONSIDER: That darn physical coil shape. We had round coils, and while some were Concentric and others Double-D internally, they were generally a round-shaped physical size. We sweep left-to-right and right-to-left so we ought to be most concerned about that coil measurement. The coil's 'width' or how 'wide' the coil is because that helps classify it as a small, medium or larger-size coil.
But way back when, somebody thought it was a nifty idea, especially with the DD coils that have overlapping Tx and Rx windings, to squeeze them into an elliptically-shaped coil rather than round. Do I have a problem with that? No, the concept is OK, but it is how people, both consumers and manufacturers, refer to these elliptically-shaped coils.
I spend the bulk of my time hunting places with a lot of trash, especially ferrous debris, and on other occasions I might hunt a modestly-littered site. Thus, I use a
'smaller-size' coil for a good 80% to 90% of my hunting, which is a 'narrower-width', and opt for a
'mid-size' search coil when it is more open with fewer targets. That might be just a little
'wider' in diameter, OR possibly helped by an elliptical-shape simply to enhance
'coverage' in a more open, sparse-target environment which would have less masking.
Some examples of
'smaller' or
'narrowed' coils would be a round 4½", 5", 6",or perhaps 6½" diameter, or an elliptical coil that could be 3½X6 or 4X7. This is how I interpret search coils, and a 'mid-size' coil might be 7", 8" or 9", if round-shaped, or an elliptical-shape such as a 5X8 or 6½X9 or 5X9½. Any of these could be a Concentric or a Double-D.
I do not sweep a search coil forward and backward. I sweep side-to-side so it is the
coil's 'width' or 'diameter' left-to-right that governs what 'size' it is and the category it fits in.
'Smaller' or 'narrower' is better for tight conditions or dense trash, and
'wider' to work more open areas with room-to-spare between targets.
Now, let's consider the longer front-to-rear elliptical measurement. It is that way because they squished in the
'width' of a round-shaped coil. An 11" round coil, of DD design, can be 'narrowed' to help, somewhat, with target 'separation', and if it is also shortened, front-to-rear, that will help make it fit better and also tighten-up the EMF. So let's take the new Garrett 'Viper' coil which is a 6X11 DD
shape.
The 6" 'width' is going to provide us with the primary search and target-response performance, and measuring 6" puts it in the 'smaller-size' category, but the lengthy 11" kind of bumps it into a mid-size group. The 6"
'width' is going to provide in-the-field performance more closely associated with a round-shaped 6" diameter DD coil. As for the 11" 'length', it is a narrow-shaped 11" and will not produce the 'depth' of a full-size or round-shaped 11" coil .... but it just might help
a little on increasing the typical 6" performance. It will add
a little better 'coverage, within a shallower-depth range, but what you really have is a 6" DD coil with
'slightly' better coverage and
'slightly' better depth .... but it is-what-it-is, a 6" coil.
And I appreciate White's labeling
(when they were alive) and Garrett's to properly describe their elliptical coils first by
'width' and then by
'length'.
mh9162013: said:
Think about it, why are so many "deep" coils wide? If you could get the same depth saving weight and going narrow, why wouldn't you? And people don't get wider coils to improve target separation. So what other reason does an engineer have to add weight and size to a coil by increasing its width?
They don't. The
'width' should simply be the assigned
'size' of the search coil for general application.
mh9162013: said:
It's not to improve ground coverage either, as then the engineer would make the coil taller instead of wider. The only logical explanation is that it adds depth and/or VDI accuracy at depth (these are both the same thing in practice for many situations).
A
slightly bigger-length elliptical coil might help with
slightly more depth and
slightly more coverage, and that little extra 'depth' is achieved by having
slightly more signal strength for deeper targets. With that, there is the possibility that you can achieve a
slightly better visual Target ID or VDI read-out.
mh9162013: said:
So if you want to increase your chances of your AT machine calling a silver target silver (instead of iron) when you're hunting deep, the last thing you generally want to do is get a narrower coil.
Correct. Get a wider-size search coil .... at least u to the point where it might help with smaller-size targets such as coins and rings, etc.
mh9162013: said:
My understanding is that getting one of Garrett's big concentric coils would be ideal for proper target ID. But they struggle (compared to an "equivalent" DD coil) in high trash areas and highly mineralized areas.
Close to correct. And here is where search coil TYPE is an important consideration because there has long been a noteworthy difference between Concentric and Double-D. Sadly, too many newcomers to this great sport came into the game with most manufacturers suppling DD coils, only, so they haven't experienced a good detector with a good Concentric coil.
We learned way back in the latter '70s and through the '80s how the Concentric coil, of comparable size to a DD coil, will provide slightly better depth, notably better Target ID, have better Discrimination, and still with the right size they can handle a lot of trashier places just fine, and a well-designed detector with Concentric coil can also work higher mineralized environments quite handily.
Trashfinder: said:
Some truth to that MH,, but in my experience it is mostly machine dependent.
And this is also so very true. You can have a good size coil like 5", and it might be a functional TYPE, be it Concentric or Double-D. But if you stick it on the end of a detector that works well for urban Coin Hunting and try to make it work well in a really challenging, Iron Nail contaminated ghost town, you might be surprised how that
detector DOESN'T work well.
That's why I am very selective about both the detector model I rely on, and the search coil(s) I trust them to perform well with.
Monte