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Double D coil myth or truth

T

TxGold

New member
Both the manual and the book titled "Mastering the Minelab Explorer XS & S" state that the target will only respond when it is underneath the "hot strip" of the coil. Are both the manual and book incorrect?

During tests with coins, the target also responds when underneath the outer ring of the coil as well. What gives.
 
No, that is not correct. A target responds the strongest when under the hot strip. Even this depends on depth and size of target. The difficult with pinpointing with a Double D is in part due to the response other than in the hot strip. The actual electromagnetic footprint is considerable larger than the coil. I think it is more or less a generic statement that a target only responds when under the hot strip as the distance increases from the coil of a coin sized target. I would change the "only responds" to responds the strongest.
 
Thanks Cody. So how do you know whether the target is beneath the hot strip or the coil's edge?
 
[quote TxGold]Both the manual and the book titled "Mastering the Minelab Explorer XS & S" state that the target will only respond when it is underneath the "hot strip" of the coil. Are both the manual and book incorrect?

During tests with coins, the target also responds when underneath the outer ring of the coil as well. What gives.[/quote]

---------------------------------

"Hello Tx". "Hello Cody"

The Manual or book statement, is not absolutely correct.

The error lies not in what is stated, but rather in what is NOT stated.

That omission is:-

The case where the orientation of the target is other than normal.
(NORMAL = Where the plane of the target is parallel with that of the search-head).

What is abnormal?

The simplest example, is a coin on edge.

That will be displaced to either left or right of the apparent 'centre line', as deduced by pinpointing.

That simple 'in air' test should give you an idea of the phenomena.

The 'at the edge' response is to be expected, because the target is crossing the TWO most intense areas of the coils total flux pattern.

At those points, the (alternating) flux is travelling downwards initially, turns, and then 'heads upwards, in a very tight circular manner. (Reversing every cycle)

Hence the signal produced, mimics this 'double transit' as it passes through these zones, located close to the outer edges of the coil.....Mattr.UK.

So TX, Cody and I concur in that the initial statement is not absolutely correct.

p.s Hope the diagram helps.

[attachment 20560 coin2.jpg]
 
you can hear the strength of the target if you listen to it,you can tell if it's under center of the coil.................
 
Does putting the Expl II in pinpoint change the coil from a DD coil with hot strip to a mono coil with a pinpoint pattern ? If so, does that mean deeper signals in pinpoint mode ?
 
No, double D and mono refers to the design of the coil. It is a double D wound coil. Think of two letter Ds back to back for a double D and a circle inside of a circle for a mono.
 
I understand they are two different designs but they must change something to make it PINPOINT. When you push the pinpoint button does it change the DD coil arrangement to make it "appear" to be a mono coil with a cone shape search pattern ? If so, does that mean deeper signal search while using it in pinpoint mode ? I'd like to know because of interest in nugget hunting. I've read that it's best to use a mono coil with a cone shape pattern for nugget hunting then with a DD coil. Thanks...
 
No, they coil does not appear as anything but a double D. Pinpoint is a non motion all metal mode. There are differences of opinions as to what zero discrimination and true all metal are but what is primary is that no motion is required.

The Explorer pinpoint has no motion all metal for pinpoint. It is often posted that it helps to lift the coil off the ground to "shrink" the target response. I think this is effective in some instances and after we get use to the Explorer. However, the Explorer has an auto circuit that does this for us and the Manual suggest we do not lift the coil as this in conjunction with the audio "shrink" circuit can cause us to have problem pinpointing. I think no understanding correct pinpointing may account for users that prefer to not use pinpointing.

I have attached an illustration that may help to pinpoint a little better. The first sweep in pinpoint only tells you that there is a target SOMEWHERE in the hot strip. We know after the first sweep that the target is in a line from front to back of the coil and a couple inches or so wide. The second sweep at 90 degrees from the first is done using a fixed point on the coil. I use the point just in front of where the shaft connects to the coil. The second sweep locates the targets closer to the exact location of the target. You have read about the signal seem to crawl around in the ground. That is as we sweep many times locating the targets the auto circuits shrink the targets size that is seen by the electronics so we can get to the exact center of the target. A larger shallow target is worse than very deep small ones in that the auto shrink circuit will often show the actual center to be a foot from where it is first detected.

On very deep faint targets if we they drop out while pinpointing then move the coil to the side so the circuits will reset and go back to the target and it will come back in if it is in fact a good target.

I guess this sounds involved but actually it only takes 10 to 15 seconds to pinpoint any target within a few inches after we use the machine for a while.

As a general rule a mono coil is easier to pinpoint because the electromagnetic field is cone shaped. There are advantages, as you know, to each design. A double D has very good noise rejection and is good at working in a between co-located targets. A mono has slightly better depth but is subject to noise induction. I have no problems with mono or double D and use both but on different detectors.
 
I carefully plotted the response of a silver quarter on my Quattro with the stock 10.5" coil. I was surprized that the field shape looked more like a mono coil than a DD coil. Why?
Rick
[attachment 21251 f-r-grid.JPG][attachment 21252 l-r-grid.JPG]
 
That is the same pattern I have understood and with the mono being a little more cone shaped. Electromagnetic lines of force enforce or repel as you know. The lines of force expand outward from the coil in a circular path and are "pushed" together as they complete the circle and are compressed through the hotspot or strip. A mono has a shape that comes down to a cone something like a dixie cup with a dull rounded tip.

Nice illustration and plot.
 
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