Narrow detection field

Anyone noticed a narrower detection field (less than the full length of the coil) with the black LTD?
By narrow I mean, not the full length of the 11" coil on coins from 4-5" deep.
Not getting an audio until the coil is 2" over the target.

For what its worth.......I sent my T2 to First Texas to be examined about 1 1/2 weeks ago.........one of my concerns was the stock 11" DD coil's detection pattern. Following is from my previous forum post here:

"The problem is that the strength of detection does not appear to be equal along the length of the coil. If one were to draw 5 imaginary horizontal lines accross the bottom of the coil, the results are as follow using a copper U.S. penny at 6" from the coil bottom, to circumvent the DD tendency to misread objects if to close... approx 3" from coil..... The area I am measuring is the area covered by the raised spine on back of coil, not the flat areas at the very edge of the heel & toe sections.

(Air Tested)
1. The middle section (2/5th's of area) have a detection strength of lets say, 100%.
2. The heel section (1/5 of area) is 80% strength of the middle section.
3. The toe section (1/5 of area) is 60% strength of the middle section.

Along with the diminished audio strength (toe & heel) is also a diminishment in correct VDI reading."

Dave Johnson himself (I was shocked), fully examined the unit after the tech had looked at it. I spoke with Dave J on the phone yesterday regarding the above. He told me that what I perceived as a issue above, is how the 11" DD normally operates. I was under the impression that the DD coil would have full detection strength along the full length of the coil from what I read in the past about DD coils. So, at this point, I will defer to Dave J, as he knows a hell of alot more than I do......LOL.
Hope my experience helps you a bit.

If that is true and I take Dave at his word also, then I must learn to overlap about 1/2 the coil length in order to not miss anything that might be deeper than 6".

I came to the same conclusion regarding the overlap too

That's because your thinking has been influenced by advertising images such as this:

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That's not how physics works in reality.

Oh, and here is the concentric version.

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I would love to see a diagram that really portrays the field of a DD...........anyone know of one? Or perhaps describe the field if no diagram is available.

((( Was this a private PM.....or did I not answer this phenomenon on this forum ))).

In any case.....yes those graphics are very misleading. Physics does NOT work in 'straight lines'...........straight down from the coil.......IRT coil footprint(s). Not concentric......nor the DD coil.

Overlapping sweeps is very important.

I think in the past on this forum, the DD coil field pattern was discussed, but AFIK, no one actually described the pattern a DD produces eg..knife blade, semi-balloon shape, etc. For as many of these DD coils as are in use, a accurate description of the coil field pattern would be most helpful. Why is this info so hard to acquire?

Take the capital letter "D"......point it downward on the ground..............and sweep.

.



Edited 1 time(s). Last edit at 06/15/2011 05:06AM by TerraDigger.

That diagram I think was made to descibe the ML DD.
I think the side view is fairly accurate in that the very bottom will radius off on the edges.
The front view however isn't right since that the coil will pickup on the left and right side
at shallow depths.
And I believe the narrowness on front view is way overexaggerated in how thin the field is.

A well known detectorist did a test with the F75, using sweep strokes liken to using a
vacuum cleaner and stated that it wouldn't pickup a zinc penny using it like that.
Now I don't know if he had a bad coil or exactly how he conducted the test
but I don't think that really matters anyway since I don't believe anyone would
use a detector like that.
I wrote him since he believed the ML Explorer would do the same thing but
mine signaled on the zinc anyway.
I have no idea if he was scrubbing the coin or had the coil lifted off the ground.

Like I said though, I don't think that matters, I am only pointing out that FT seems
to have a different design or perhaps processing the signal has some effect.
Seems like I read somewhere that the one coil is not on the same plane as the
other in their design but I have no way of knowing if this is true of not.......
maybe an old wives tale for all I know.

Pehaps some can do this test to see if he was correct.
Or perhaps the fast recovery has to be taken in consideration.

I wonder what the pattern would be with a retangle shaped coil!





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Also since the Minelab Etrac has a larger D shaped coil the detection pattern would have to be different from the Fisher..



Edited 1 time(s). Last edit at 06/15/2011 04:24PM by markg.

The detection zone of a search-coil is a combination of the field strength from the transmitter coil reaching the target, and the signal picked up from the target by the receive coil. On a DD coil assembly, the target will see the strongest field when it is directly beneath the centre of the transmit coil. However in this position, it is some way off from the centre of the receive coil, so the net result is less-than-optimal detection of the target. Likewise, the receive coil will detect a target best when it is under the centre of this (RX) coil, but that means it's some way from the TX coil, so the same result occurs. In the centre, below the overlap zone is the best compromise point - not too far away from either coils' 'sweet-spot', so the 'round-trip' from TX-to-target-to-RX gives the strongest signal.

This also explains why DD coils with straight parallel central sections behave little different to ones with curved central sections. The dominant feature is the general elliptic shape of each coil - having a slightly flat bit or a curved bit on one side makes little difference. If the target is some-way front or back from the central 'ideal point', it is further from the centre of both TX and RX coils, and hence gives a weaker signal to the detector.

The straight or curved profile of the central sections has a small effect on the amount of coil overlap you would need to obtain a null (induction balance), and this can affect the complete coil assembly performance, in terms of shape of the detection zone, and the amount of ground pickup.

Thank you to everyone for helping me on this.
Pimento, are you saying that the shape of the field is roughly like the elongated shape of a 'marquis' cut diamond? The enclosed area where the 2 coils are overlapping?



Edited 1 time(s). Last edit at 06/16/2011 05:32AM by TerraDigger.

That is correct.

Yes, I would say that's a pretty good representation ( I had to look 'marquise' up, thank you Google). I would guess that DD's that have two semi-circular coils would have a less elliptic shape. One day, I'm going to do a proper computer simulation of search-coil designs. One day......

Correct. And this is to say............a elliptical DD coil will generate a narrower/tighter EM footprint................vs...................a round DD coil.

I am guessing that the First Texas 11" DD produces 'marquise' shaped field and the 5" DD has the 'wider' field. Now, just for theory's sake........the little 5" seems to have more depth (not factoring the higher sens the small coil can be set to) than the 11", if one ratio's the sizes vs depth. Are the circular DD's deeper and the ellipticals a narrower footprint in regards to design intent?

Pimento - I should have Googled 'marquise' for the correct spelling

It would be nice to say that the 11" DD coil can detect a dime at 22"...................................and the 5" coil can detect a dime at 10". The 11" coil can not perform in such a capacity; yet, the 5" DD coil can indeed detect a dime at 10".

A concentric (round) DD has a larger/wide (wiper-blade beam) footprint..........subsequently, will ascertain greater depths........with identical control panel settings.
A elliptical DD coil has a much narrower wiper-blade beam footprint..........subsequently, will ascertain more restricted depths.

This is "in clean ground". If dirt is heavily mineralized..........or excessive masking///trashed-out...........the tighter footprint coil will "see between" to greater depths. "Depth" .... vs .... "Effective Depth". I defer you to article titled "Coil size myths" in Fisher Intelligence 5th & 6th edition.....on home web-page.

Will review the article again.