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View Full Version : F75 Using the 10" Elliptical Concentric????



Jim Hemmingway
06-04-2011, 10:05 AM
Steve have you played with this set-up at all? Been considering the 10" elliptical. Wondering about better response to small stuff, improved iron discrimination, and more stable operation in high EMI locations. I know DDs have their place...but admit I've never really cottoned to 'em much.

Incidentally, I have tried a ferrite core EMI reducer but don't see any noticeable benefit here.

Jim.

Steve Herschbach
06-04-2011, 11:53 AM
Hi Jim,

No, I have not. Interesting read on F75 coils at http://www.dankowskidetectors.com/discussions/read.php?2,4402,15603

My solution to F75 EMI has been to break out my X-Terra 705. Though I do need to try the 5" round DD based on the referenced thread.

Jim Hemmingway
06-07-2011, 10:00 AM
Yep I’m familiar with that particular thread having posted to it several times. For prospecting I’m not all that concerned about target separation. I own both the 5” DD and the 6” elliptical and the latter is my personal preference because it does have a wider electronic footprint. In my ground it gives ever so slightly better depth / sens to small stuff.

For general searching in high EMI areas I’d like to try the larger 10” elliptical to see if it will perform suitably. It seems reasonable to expect that it could be run at a higher sens level than the stock DD and acquire more stable operation over my ground. In any case Steve, it’s just a thought…I suppose I’ll have to buy one and check it out. Many thanks...

Jim.

Steve Herschbach
06-07-2011, 10:44 AM
Ironically I was offered one to test awhile back but turned down the opportunity because it was still winter here!

Steve Herschbach
08-17-2011, 11:31 AM
So Jim, did you ever get one? If so, what do you think?

Jim Hemmingway
08-18-2011, 11:31 PM
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.

(c) 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. You will be in my thoughts and my very best wishes go with you. :)

Jim.

Steve Herschbach
08-19-2011, 06:46 AM
Thank you Jim for the very in-depth report. Looks like I need a 10" concentric. Best of luck to you also on your outings!

auminesweeper
09-02-2011, 04:18 AM
Thanks Jim, another great post, I have heard simular problems with the F75 over here in the UK.
and thanks to you I now know how to combat these issues, Some folks have had simular problems with the T2
as well, But most of their problems is through lack of Testing,

Thanks for sharing your findings Jim, and be safe and enjoy ya self, take care.

John