9" Concentric vs 6" DD LF coils

[mojotrout]

so far my stock 9 inch MF coil seems to be doing ok. I have hit a few trashy areas that have been problematic but in most spots it seems to sift through the stuff in the ground pretty well. I am going to get a LF coil and was wondering what depth differences people are seeing between the Digger 6" and the 9"coil in 3kHz. For that matter, what differences to you see between the MF and LF 9" coils? I realize that the 6 incher should do better separating in trash and perhaps in highly mineralized ground, but Im wondering strictly about depth in moderate ground between these three coils with silver being the primary target.

Thanks for any info you might be able to provide!

 

[Old Longhair]

The difference is primarily one of target response. George Payne did the research years ago that showed that the best response to a silver dime was generated with a transmit frequency of 2.7kHz. Typically, high conductors like silver and copper respond better to lower frequencies, while low conductors like gold give a better response to higher frequencies.

In choosing a different frequency, you also need to know how the Xterra target bin segment widths are programmed relative to the various frequencies.
Target segment bin widths are narrower and the lower end of the detection scale and wider at the high end with low frequency coils The inverse is true with high frequency coils. So a silver quarter for example, will give a response that will neatly fit in a segment with a low frequency coil, but might give a response that is too big for one segment with a high frequency coil. The effect of having it be a bigger response for the bin width, is that it is likely to "bleed over" into an adjacent bin on either side, causing it to indicate a "jumpy" ID with multiple tones, making positive ID much more of a challenge (<-read "you'll dig more trash chasing that quarter"). MF coils have the most uniform bin widths across the entire detection range, making them the "most honest", and a good general purpose use frequency. (check out the illustration below)

Now, understanding all of this, go read Randy's four part blog article titled "Which Xterra Coil Is Best For Me".

Which X-TERRA coil is best for me? part 1
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-1
Which X-TERRA coil is best for me? part 2
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-2
Which X-TERRA coil is best for me? part 3
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-3
Which X-TERRA coil is best for me? part 4
http://www.minelab.com/usa/treasure-talk/which-x-terra-coil-is-best-for-me-part-4

 

[mojotrout]

Thanks for the info LH. So are you saying that there is no appreciable difference in depth of detection on silver coins between these three coils and that it is the quality and "honesty" of a target response that differs most between them? That will be interesting to see once I settle on which 3 kHz coil to purchase. It seems that if there is little or no difference in depth, then the 9 inch 3 kHz coil is obsolete as it doesnt do anything the 6 incher (except little more coverage) cant while the 6 incher will separate better and do better in highly mineralized soil. If there is a significant difference in depth, how much? How deep can I expect to get a silver dime with each of these coils? I know the 9 inch 7.5 kHz coil will do at least 6 inches because I have one buried that deep that it hits well. It will actually hit it with the coil two inches above the topsoil.

So many questions! Thanks for the help!

 

[Old Longhair]

Now you see, lower frequencies also penetrate the ground better in mild conditions. So hitting a dime @ 6" isn't a problem with the 6" 3kHz coil.
Don't get me wrong. Until we got the two Coiltek 3kHz coils, the 9" LF virtually lived on my machine. And for depth & coverage where separation or ground conditions aren't a big concern, it would out perform the 6". The 9" actually does a nice job of filling the gap between the 6" and 15"

 

[argyle]

The biggest problem I've found with the 6" 3khz coil on a 705 Mojotrout, is that the sensitivity has to be dropped so much due to severe falseing of tone and visual, in comparison to the 9" concentrics.
A week ago I was down to 19 with the Digger one morning, usually I can run 22-24 with that coil.
By dropping so much sensitivity, depth for the silver suffers greatly and really negates it's use.
In fact I wouldn't betting I'm getting double the depth with a 6" 7.5khz concentric over the 3khz Digger in many of the trashy areas.
The 9" 7.5 kHz seems to gell so well on the 705 compared to the 9" low freq.
Out of the two 9" concentric coils you've mentioned, I firmly believe the Stock 7.5khz a more stable coil and equal in depth on silver. I've always been able to run the stock at a higher sensitivity than the 9" 3khz.
In theory you'd think it would work the other way round. The digital world is fast becoming a changing place for detector theory.
For a 6" deep seeking silver hunter, when a smaller coil is needed for heavily trashed spots, I think the 6" 7.5khz concentric to be the hottest and deepest silver and copper hunter I've ever used.

I wouldn't worry too much about the heavily mineralized ground affecting anything but a low frequency DD, as I've found the concentrics excel on heavier ground and can carry a higher sensitivity load.
They just aren't like the concentrics of old.
And for the different types of ground we both hunt, and same goes for nearly all XTerra users, I think we pretty much all hunt the same ground types in our different places .....Mushy conductive deep ground. Ground of grass then dirt then stone at the 7" mark. Red clay, grey clay. Hardpacked dirt directly over gravel. Slightly mineralized- heavy mineralized ect. and I feel the 7.5khz concentrics match our units the best for deep and stable operation.

I leave the DD's on the XTerra's for shallow work only now. They are missing too much at depth.

 

[Old Longhair]

As evidence that soil conditions DO vary greatly, I offer that my experiences aren't reflected in your post at all.

As for the Digger coil, I routinely dig targets at 6-8", even in heavy trash, with sensitivity settings typically between 15-26, without notable loss of depth. It is a HOT coil! Excessive falsing is something that I most commonly associate either with swinging too fast, changing coil direction over a junk object, or sensitivity inappropriately set.

 

[mojotrout]

very informative Argyle, thanks a bunch. It appears that my best route would be to see what types of sensitivity I can run with the 3 kHz 6" vs. the 7.5 kHz 6" to see if I can actually take advantage of the lower freq. coil. If i can run a high sens, say, in the mid 20's, with the Digger, it would serve my purposes well. But if I cant run it w/o turning the sens down into the teens, I'm better off with the 7.5/ 6", or just keep plugging along with the 7.5/ 9".

Ive always wondered about the frequency/conductivity theory and just how much practical, real world difference it makes. Some swear by it, others don't think it makes much a difference at all, given all of the other variables that have much more impact on depth of detection.

 

[mojotrout]

Now you see, lower frequencies also penetrate the ground better in mild conditions. So hitting a dime @ 6" isn't a problem with the 6" 3kHz coil.
Don't get me wrong. Until we got the two Coiltek 3kHz coils, the 9" LF virtually lived on my machine. And for depth & coverage where separation or ground conditions aren't a big concern, it would out perform the 6". The 9" actually does a nice job of filling the gap between the 6" and 15"

Do you think there is a big enough difference between the LF 9 and the MF 9 to justify having both? So far my MF 9 seems to run pretty well in my dirt (low to high 20's phase #, with low to mid 20's for Sens.). Argyle's experience suggests no real depth difference in his conditions, with the MF 9 being more stable. I was wondering what you've seen on your hunting that made you abandon the MF 9 and go with the LF 9 as your go-to coil.

 

[argyle]

As evidence that soil conditions DO vary greatly, I offer that my experiences aren't reflected in your post at all.

As for the Digger coil, I routinely dig targets at 6-8", even in heavy trash, with sensitivity settings typically between 15-26, without notable loss of depth. It is a HOT coil! Excessive falsing is something that I most commonly associate either with swinging too fast, changing coil direction over a junk object, or sensitivity inappropriately set.

My point about soil conditions all over the world Rick, is that we all have varying ground types within a few miles of each detecting spot. So in essence we all detect in the same ground types. Dirt is Dirt. I'm not a believer of a certain detector or coils not coping in someone's area because the soil is of a different make-up to that of another State and so on.
Only the most severe ground is a catalyst.


I really liked my first 6" Digger coil, I found it not only a great hot and deep seeking coil, but the best junk identifier in the coil mix. I bought a second one pretty quick and set it up on another 705.
The ever occurring bottle-cap ID problem dissapeared the day I started using one. The caps can come in solid and un-changing directly into the same bin as our two best copper/gold coins, the only hope till the Digger came out was sizing, which works fine once you're used to it. But there was always the worry of missing a double coin drop using sizing alone.
The Digger coil is far and away the best at this, it really shines in it's ability to clip-off on these type of junk signals, and at a good depth too, not just in the first three inches. I'm sure you've found this as well.

My beef with it is the sensitivity drop needed, and not just on the odd occasion but in the main.
As you know the 6" 7.5khz concentric is a really hot coil too, in fact hotter I believe, but it doesn't need such a high drop of sensitivity as does the Digger. Something to do with the 3khz correlation with the processor? I'm not sure what it is.

Some days running 25-28 are fine and the Digger can get to it's true depth and hits hard. Loves deep silver and high copper content coinage!
But I do know that when I'm in the low 20's and high teens in sensitivity, which is most days, I'm lucky to be hitting coinage at 3 to 4" in any ground type with the Digger. And that's simply not good enough when a 6" concentric or 6" high freq DD can be hitting at their full depth.

 

[Old Longhair]

At the time, a the only 7.5kHz DD and the only available 3kHz fit my needs best.

 

[Old Longhair]

My beef with it is the sensitivity drop needed, and not just on the odd occasion but in the main.
As you know the 6" 7.5khz concentric is a really hot coil too, in fact hotter I believe, but it doesn't need such a high drop of sensitivity as does the Digger. Something to do with the 3khz correlation with the processor? I'm not sure what it is.

Some days running 25-28 are fine and the Digger can get to it's true depth and hits hard. Loves deep silver and high copper content coinage!
But I do know that when I'm in the low 20's and high teens in sensitivity, which is most days, I'm lucky to be hitting coinage at 3 to 4" in any ground type with the Digger. And that's simply not good enough when a 6" concentric or 6" high freq DD can be hitting at their full depth.

I believe that we are pretty much on the same page, but looking at it a bit differently. I agree that 3kHz is not appropriate for certain conditions, and that a 6" MF would be wonderful, and that in it's absence the 6" concentric is a good option.

As far as to why, I think that it has to do with a programming entanglement involving the effort to display and respond with ID's closest to what was established, combined with whatever conductive mineral content there is. And I say specifically conductive minerals as opposed to magnetic minerals, because 3kHz gets better response from high conductors, which I also believe contributes to the reason that we often see coin targets that ID higher than normal. High iron areas suffer from both types of mineral "contamination". Most other soil types have a variation that's within a range that tracking can deal with. But, I believe that conductive variations are even greater, Involving salt among other trace metals and oxides, and often are why a coil that works great in Michigan farmland might not work in a coastal area (just as an example).

JMHO, and worth what you paid for it.

 

[argyle]

Well I think your humble opinion is a good one Rick and on the money

Be good to have a Minelab tech take us through the correlation of how the XTerra's behave between the 3 freq's, sensitivity and conductivity on the forum in a practical way. But between a Shearer and a Farrier he may have to dumb it down a notch

 

[argyle]

very informative Argyle, thanks a bunch. It appears that my best route would be to see what types of sensitivity I can run with the 3 kHz 6" vs. the 7.5 kHz 6" to see if I can actually take advantage of the lower freq. coil. If i can run a high sens, say, in the mid 20's, with the Digger, it would serve my purposes well. But if I cant run it w/o turning the sens down into the teens, I'm better off with the 7.5/ 6", or just keep plugging along with the 7.5/ 9".

Ive always wondered about the frequency/conductivity theory and just how much practical, real world difference it makes. Some swear by it, others don't think it makes much a difference at all, given all of the other variables that have much more impact on depth of detection.

That 9" 7.5 kHz coil you have Chris, is the most deep and true stock coil I have seen on a TID detector. I purchased a 3khz 9" but saw no advantage for deep silver/copper and it needed a touch lower sensitivity setting, so I do not use it. It also didn't seem to do much better with small junk, so the purchase price didn't match my expectation.

As you've already worked out, a smaller coil is needed for your heavy junk work. And if ID'ing bottlecaps and twist-tops are a concern for you, I guess they are for all of us, the Digger shines there. For isolating each target with that DD design, and junk handling information, along with a low freq pure depth for silver/copper, they are a great small coil.
I thought you should at least be aware of the sensitivity cut that has to take place in comparison to the other two 6" coils.

In relation to the three 6" coils for heavy junk work, we all have a favorite I guess. Mine is the 6" concentric as it isolates targets well in junk (the inner ring is pretty small), it is the deepest of the three and more of a ID stable all-rounder.
The Digger is my must have for cap ID, it really rules in that regard, the 18.75 kHz DD does a good job at that as well, but the 6" concentric lacks there because the inner ring see's the full cap. And if notching is brought in to bear, the processor places the cap very quickly into a good solid segment.
I can't take the 3 kHz Digger for powerline work, the EMI and sensitivity issue leave it with no depth. But for some reason the 6" 18.75 DD does best here and really shines. Go figure.


Yeah, it's a funny thing about our XTerra's and how the frequency, conductivness and so on fits in with this 'fake' digital way of operation. Although fake is probably the wrong word and seems a bit harsh. Maybe 'digital interpretation' would be better.
I don't believe we have a 3 kHz, 7.5 kHz and 18.75 kHz. Or a true Ground Balance. Merely the digital interpretation of them. But it's an interpretation I like!

 

[mojotrout]

You guys are great. Where else can you get two knowledgeable guys politely discussing a newbie's question in such an informative manner? Nowhere that I know of. This forum is simply fantastic.

Thanks to both of you for your time and thoughts. I will likely get the 6" Digger first, as it appears to be the best compliment to my current coil, then the 6" MF concentric, then the 10x5 HF. Then I will have to stay in bed for a week because of all of the plasma I had to sell to afford them!

 

[therover]

It's a great coil and it ergonomically fits perfectly on the 705.