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Steve Herschbach

VLF Concentric Vs DD Coils

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Back on the old AMDS Adventure Forum Jim Hemminway and I discussed the DD vs concentric option for the F75. Jim eventually got the concentric and made a great report, which I am reproducing here.

Jim Hemmingway


Hi Steve…

Yep, I recently I bought a 10” elliptical concentric coil for my basic F75. I was curious to see how the 10” concentric would perform over my ground [ Fe3O4 = 0.3% Ground Phase = 85ish ] compared to the stock 11” DD coil, with a view to prospecting applications where possible. I also wanted to compare its vulnerability to EMI. As you know, concentric coils of similar size to DD coils are noticeably less vulnerable to EMI, but are more susceptible to ground minerals. More important, concentrics improve sensitivity to smaller targets if ground minerals permit. That said, many prospecting areas have a ground mineral magnetic strength such that DD coils of similar size improve ground mineral penetration over concentric coils. BTW Steve, mentioning these generalities is directed at readers following along, and certainly not at you.

The 11” stock DD measures roughly 7 ½” by 11 ½” compared to the concentric elliptical at 5 ½” by 9 ¾”…quite a difference looking at them side-by-side. Aside from anything else, the smaller coil will slightly improve maneuverability in tight places between rocks… yet still provide comparatively good coverage.

I don’t leave for prospecting for two weeks yet, so the information presented below is based on checking out targets in the patio test garden where EMI is quite high at this time of year. Not real world conditions, as these are disturbed ground targets where we can expect somewhat less by way of depth and more erratic target ID responses… with most non-ferrous targets beyond a few inches depth either residing in the iron range or all over the map. Despite those factors, we can still conduct meaningful, relative side-by-side depth / sens comparisons. Below follow some observations…

(a) F75 Test Results

In the highly sensitive discriminate JE mode set to small iron elimination (12 or 13)…I saw no discernible difference in depth results over copper pennies down to 10 inches. I felt there was a very slight decrease in signal strength over the 10” Canada nickel but it wasn’t much. Solid lead “nuggets” at 2.2 grains, 4.5 grains, 0.5 grams all buried at three inches and a 1.0 gram solid nugget at four inches all responded with similarly good solid signals, with a slight improvement over the 11” DD on the smaller nuggets. All test nuggets were solid “can’t miss ‘em” signals. JE discrimination mode, for those unfamiliar with this unit, delivers an audio boost [for lack of a more suitable description ] to its signals compared to the more modulated all-metal mode, and this is quite evident on small nuggets and larger deep targets. It leads some users to believe it goes deeper than the all-metal motion mode, but that is not the case and particularly not so as magnetic ground mineral strength increases.

In the all-metal motion mode the signal response was stronger and more distinct compared to the stock 11” DD over all targets including a 10” nickel. The signal strength improvement was most profound on the two smaller nuggets and to a lesser extent on the 10” nickel.

There was no measurable target ID improvement over iron targets because these pretty much ID as iron with the 11” DD in this testbed, as mostly do non-ferrous targets beyond a few inches depth.

Instability from EMI sources was only marginally reduced in JE discriminate mode but more significantly reduced in the all-metal motion mode. That surprised me. It certainly was not reduced by nearly the same magnitude as occurs with the smaller 6” elliptical concentric. But in all-metal motion mode it made the difference between running the unit at max sensitivity (99) with a bit of low level chatter as compared to reducing the sens level to 80 for an equivalent chatter level when using the stock DD coil. This may not sound like much, but it represents the difference between hearing faint signals or missing them altogether.

I was pleased by these results. Some were anticipated, but I did not anticipate the depth results or the crispness of the all-metal motion mode signals on all targets in this ground.

(B) Comparison to Goldbug2

At the time I was running these tests, I was thinking that the Goldbug2 also uses elliptical concentric coils. They are very effective over this ground, surprisingly so on small shallow targets but comparatively less effective on deeper nuggets and nickels than is F75. But then…that is a result of high operating frequency over fairly mineralized ground. I ran some comparisons between these units and identical size coils…both the 6” and 10” elliptical concentrics. I should mention here that the F75’s 5” DD coil performs similar to the 6” concentric elliptical on depth /sens to both small nuggets and larger deeper targets such as nickels, although my in-ground tests indicate the 6” elliptical has a slight advantage on small stuff in this ground.

Both units were initially adjusted to max sensitivity and GB2” was tested in both Normal and Low Mineralization modes. Of the two modes, Low Mineral mode …a slower autotune rate… gave the best signals overall, so that’s what I used for the comparison. The GB2 audio boost feature was not used in these tests. It dramatically enhances signals, but it was not used in these tests because it is not possible to hunt mineralized ground here at max sensitivity and with that feature turned on. It results in constant annoying false ground signals with slight changes in coil elevation over the ground. The F75 threshold was set to “0”…one détente step above minus (-1) one…barely audible… with audio pitch set to mimic the Goldbug2’s audio pitch more closely…at least for my hearing.

The Goldbug2 does not see a 10” nickel with the 6” elliptical coil and barely detects it with the 10” elliptical. The F75 delivers a modest but solid signal with the 10” coil and a light signal over it with the 6” coil. F75 gives a much stronger signal than Goldbug2 over 6” and 8” nickels regardless which coil size is compared. The F75 delivers a stronger signal on each of the small nuggets described above regardless whether the 6” or 10” elliptical concentric coils are compared…and this is even more pronounced when comparing the 10” coils. Both units respond quite well to the nuggets, but the F75 has a very decided edge in signal strength.

With these results in mind, I buried a 1.1 grain lead nugget at an estimated one-and-a-half-inches and a half-grain lead nugget at an estimated half-inch depth and no less. Comparing the small coils, the Goldbug2 delivers a much more discrete signal on the 1.1 grain nugget from any sweep direction than does F75, making it easier to hear. Both units signal modestly over the 0.5 grain nugget, but the F75 delivers a louder, easier-to-hear signal. Comparing the 10” coils, both units signal lightly on the 1.1 grain nugget, but Goldbug2 gives a more discrete signal. Both units signal better over the 0.5 grain nugget but again the F75 delivers a louder easier-to-hear signal than does Goldbug2. To put an accurate spin on these results, overall the Goldbug2 has the edge on these tiny nuggets… but it’s a very close thing.

As a separate point, the Goldbug2 signals on these tiny nuggets even with the sensitivity reduced down to a setting of 5ish on a scale of 0 to 10. The F75’s sensitivity could not be reduced nearly so much…say about 90ish on a scale of 0 to 100…before it effectively loses these tiny nugget signals. F75’s sensitivity to these tiny nuggets is highly gain-dependent.

The comparison between these units comprises many tests over several weeks. All testing was done wearing NuggetBuster headphones with the volume set to the same level by using the “clicks” on the volume control. Signal strengths and gain levels needed to hear them sometimes changed slightly on different days but in a relative sense remained the same. High residential EMI is the culprit, as no rainfall was experienced during that droughty period in July.

Something readers should keep in mind is that the F75 is more vulnerable to high residential EMI increasing background chatter compared to Goldbug2 at max sensitivity settings used in these tests. This makes it more difficult to hear weak signals from these tiny nuggets. That will change when searching prospecting environs away from EMI sources. By comparison here, the Goldbug2 is much, much less vulnerable to EMI, a result being that even the weakest signals are easier to hear against it’s very quiet, smooth threshold hum. Don’t underestimate the effect of EMI. It can really reduce air test depths compared to in-the-ground depths and that does not just apply to PI units.

No meaningful comparison was done in discrimination mode over any of the targets. The GB2 iron discrimination is fixed at a level slightly above small iron elimination in order to deal with hotrocks. This results in very poor, usually broken signals over most disturbed ground targets in this testbed. So, there was just no point. I can relate that using the 10” elliptical coil, the F75’s JE discriminate mode set as high as “14” will give a modest two-way signal over the 0.5 grain nugget at one-half inch… and will detect a 12” deep Jefferson nickel in this ground. Raise the discrimination setting to “15” and these targets are gone….similar to GB2 in discrimination mode.

A Final Comment…

It’s interesting stuff Steve, everything considered. We occasionally read reports from around the country about how different units perform over varying minerals / soil types. This is my experience over my testbed disturbed ground.

The Goldbug2 performs quite well in this soil, especially the 6” elliptical coil on very tiny, shallow targets. I love that unit so much I made the effort to reinstall the box under the armcuff. Now it feels even lighter, and of course the location of the controls is permanently burned into my memory. The F75’s small nugget sensitivity is exceptional for a 13 kHz operating frequency, and this leads one to consider additional design factors that contribute to small nugget sensitivity besides operating frequency. Think what you will, but these results tell me that more extreme ground mineralization than exists here will be required to reduce the 10” concentric’s depth / sens performance on small nuggets below that of the 11” DD coil.

Take care on your trip Steve and watch out for those damned snakes. I have to get packing for a two month trip, first to the Appalachians, then to northern Ontario. We'll speak with you again early next winter if not a bit sooner.


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Thanks for fixing that link Steve. I find myself puzzled by  Reg's comments about the area of detection with concentric coils while in discrimination mode. I fail to see how increased depth can shrink the detection area from the area of the outer coil to the area of the inner coil. Other than the pregnant V detection pattern becoming smaller the deeper the target is. But that is not what I thought Reg was alluding to?

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I have Written a lot on coils I have tested and my next Novel, lol. will be Using the Concentrics on The GMT because there is a Bad lack of Coils Available For The GMT, So Now I have got the Coils sorted I will start testing them in the near future.

I use Concentric coils as much as possible and Only if the Ground Gets up around 75 on the MXTs Meter do I change to a DD, DD's have become more of a Crutch than A Tool, Mainly By companies using them to quieten down noisy Detectors, and one manufacturer Went all out to convince us we Need to have DD coils which is Absolute rubbish,

I used the 12"/300 coil on a field that had been gone over by every Metal Detector made since 1968 and I found 3 lots of Gold in one sweep of the coil and a silver pendant along with 8 Roman Coils and 1 Hammered Cut Half all from a Place that is so Iron infested that you can get 15 to 20 or more Iron signals per sweep,

The other Serious Issue is we all Crave More Depth from our Machines, So here is Part of what I wrote about Coils.

Enjoy, John


Here are Some Coil Basic's Regarding Size to Shape and Configuration, As A Very Rough Guide of what should Be Possible, In Theory??, I say In Theory because I Do Not want anyone Shouting "FOUL" But I will Post some pretty Amazing Figures that Happened During Testing,

Most Concentrics will See Coin Sized Item at the equal Depth of their size, But It is Very Common for them to see Coin sized Target AT 1.5 X their Size, But The Average Depth with Concentric Coils Is More Like 1 and A 1/3 times their size,

So with the 12"/300 = 16" and with the 950/9.5 = 12.5 to 13" and the 5.3 (6") = 8 to 9" (10 inches is NOT Uncommon)

Now to Equal those Depths using a DD you will need A Much Larger Coil Because A DD will Find Coin Sized targets Equal to their Size and the Bigger DD coils go The More Sensitivity they Loose,

So if you use A 12x10 coil that has a possible Depth on coins at around 11", (Add the 12 to the 10 and Divide by 2).

And the Same with A 8x6 =14 Divide by 2 = 7 inches, the 15x12 has A Possible Depth of 13.5" and the 18x15" has A Possible Depth of around 16.5", Now In THEORY this all sounds Really Good But DD's Never Produce these Figures In The Ground Where As Concentric Coils Do Or Very Close to them Figures,

Now Compare the Concentric Figure's to those of the DD's and See How Much bigger A the DD Coil Needs to be to match Those of the Concentric. Not to Mention the Weight But Most of All the Cost,

We All Know that Air Tests are of Little Value but he's one I did with a Dealer Who Used another American made LTD model of detector he had fitted with the 18x15 Coil, He was so bent on Making my MXT 300 Look Stupid that he fired it up and set it flat out in the Shop that it was falsing that much, That it was Impossible to tell what was A Signal and what was EMI.

Anyway I Got A Large 50% Silver Coin from their Finds Tray and the Best that other machine could do was 18 to 21 inches, So then It was My Turn with My MXT 300 with the Factory 12" Coil, I tried it at the Pre Set (9.5 ish) and then turn it up to above 10 and when it started to False I backed it off just enough so it Behaved it's self, and the Distance it picked up that coin was 27" inches.

So when you Compare the Depths I have quoted and relate them to the Air Test In the REAL WORLD you can work out How much Bigger A DD has to be before it Equals Whites 12"/300 Coil, So to try and equal it you will need at Leased the 18x15 but we Know that Dog Don't Hunt, so the Next and Only Option is the 21x18" or an 18" Round DD.

Like I said earlier the 12"/300 Coil is about as Good as A Coil gets and you would need the 18" to match it and Only if the Mineralization is High will you see any Advantage with using the DD Coils Listed, But Whites also Make the 4x6 DD, 6x10 DD, 10" DD and The 15" MXT Max DD?? (Not Sure if that is A DD or Concentric)

Now to get anymore Depth you either Need A PI or remove the top layer of soil

The 12"/300 is A Lot easier to handle than the 18" DD and did/does come standard on some machines so no extra costs are involved,

Anyway, These are my Views based upon my Tests and having used such Coils, If your Soil is Reading up around the Early to Mid 70s then you are Bordering on DD Territory and then you need to ask your self whether if it would be A wise Investment, along with Do I Buy the Whites 2D 10" coil or A 12x10" But Here's the Clue, UNIFORMITY, END QUOTE.

If you wish to read the whole of my Ramblings you can find it Here,


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Which one Paul ?

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Hey Steve, what is your thoughts on this, I think he was saying to pretty much stick with the stock Whites coils, if I read that right. I was all ready planning on other coils for the TDI-SL.



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On 6/11/2015 at 7:15 PM, Ivansgarage said:

Hey Steve, what is your thoughts on this, I think he was saying to pretty much stick with the stock Whites coils, if I read that right. I was all ready planing on other coils for the TDI-SL.



When It Comes to TDI Coils, Miner John/RazorBack  are about the Best So Far from what I have seen,

In them posts I was referring to Whites VLF Machines, Excluding the GMT because there are No After market Coil for it apart from the Ones Jimmy S has,


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Thanks John  and what coil do you recomend  gold prospecting. TDI-SL



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Thanks John  and what coil do you recomend  gold prospecting. TDI-SL



Their 9X5 Folded Mono  BLACK COIL and the 7.5X12 ?? RazorBack  (White Colored Coil)

And For A Bit More Depth Their 14" Mono, you have two choices there because they Make One Spoked and a Solid Version.


Here's the Link, http://razorbackcoils.webs.com/apps/webstore/


Hope This Helps,,  john

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Hey Steve, what is your thoughts on this, I think he was saying to pretty much stick with the stock Whites coils, if I read that right. I was all ready planning on other coils for the TDI-SL.http://forums.whiteselectronics.com/showthread.php?73748-The-Reason-I-Use-Whites-Products


I think John answered that already.

Using coils made by a manufacturer for the detector they made is pretty cheap insurance and not a bad idea in general. I usually do that myself unless there is a size I want the manufacturer does not make. Then I go after market.

In general though people have probably figured out I have no brand loyalties. I just try stuff out and use what works for me. You never know which company will produce tomorrow's gem, and the best companies make a turkey now and then.

Johns experience has made him quite down on Detech coils. Other people really love them. I have three (yes, I bought them) and as far as I can tell they are just coils. No better or worse than other coils I have used.

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