3 Khz Coil, hot on dimes?

[pplinker]

Got a 3 khz for my 705 and was trying it out today. Found 6.29 in clad at a couple of ball fields and i found a higher percentage of dimes than i ever have. Usually there's more pennies but i found 28 dimes, 24 pennies, and 13 quarters and zero nickels. It also seemed like the TID was almost always 38 for the dimes, whereas the DD 18.75 would id them at 40 or 42.

 

[Digger]

Congrats on the finds. As I've been saying since I field tested the 3 kHz coil several years ago, it is a dime killer! Lower frequency coils are just plain better at hitting solid on those high conductive silver coins. And even though I wish they made a 3 kHz DD in a small coil, the 3 kHz concentric does just fine if the trash isn't too concentrated. HH Randy

 

[pplinker]

Thanks Digger! I looked under FAQs and found the following that you wrote: (Is this the comparison you're referring to? If there's another, i'd like to read it.)

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The X-70 offers the user the capability of using any of three frequencies of coils. They come in 3 kHz, 7.5 kHz and 18.75 kHz. As a rule of thumb, the higher the frequency, the better it responds to lower conductive targets. The lower the frequency, the better it responds to higher conductive targets. So, theory has it that the 3 kHz coil will be the best on silver and copper coins, whereas the 18.75 should prove to be the jewelry hunters choice. Right in the middle is the one I use the most if I am hunting an area with modern trash. The 9-inch concentric stock coil, operating at 7.5 kHz. The 7.5 kHz series is considered to offer the best discrimination characteristics in trash infested environments. Kind of an all-round coil for coin shooters. I have used the lower and higher frequency coils and found the higher frequency coil to be a bit more sensitive. But, this required me to lower the sensitivity setting on the X-70. As a coin shooter, I had to ask myself, If I have to lower the sensitivity of the detector to accommodate the higher sensitivity of a coil, what is the point? Again, it is suppose to be an excellent choice for jewelry hunters, beach hunters and nugget hunters. I haven't found that I need to diminish the sensitivity as much when I use the 10.5 DD coil at 18.75. I know the windings of a DD coil are designed to cancel much of the interference that might affect concentric designs. But, the additional weight of the 10.5 DD does seem to be a bit more awkward to swing for any period of time. I have a couple sites that the 3 kHz concentric has been good to me, finding old silver on each outing. In fact when I hunt in areas with little or no modern trash, the 3 kHz coil is the one I like the best. Like when I am hunting amongst those nasty old square cut nails. It seems to me that the 3 kHz coil does a better job of providing a solid low tone on the ferrous objects, allowing me to concentrate on those higher signals. But, as with this entire article, that is just my opinion.

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[Beale]

Congrats on the nice hunt! I Good work, Beale.

 

[xwyokid]

Dimes most every time ring right at 38 for me also. I too wish they made a smaller 3KHz.

 

[Digger]

That one, and a few others.
http://www.findmall.com/read.php?55,1147157,1147288#msg-1147288

http://www.findmall.com/read.php?55,692506,692523#msg-692523

http://www.findmall.com/read.php?55,421944,421944#msg-421944

http://www.findmall.com/read.php?55,460148,460188#msg-460188

http://www.findmall.com/read.php?55,497015,497015#msg-497015

http://www.findmall.com/read.php?55,638405,638631#msg-638631

 

[pplinker]

Thank you Digger, i read everything in the links, and took some notes.

Penny

 

[David]

Got a 3 khz for my 705 and was trying it out today....It also seemed like the TID was almost always 38 for the dimes, whereas the DD 18.75 would id them at 40 or 42.

I do not have a LF(3 khz) Concentric but I do want one. But one point I want to make is, I find the TID numbers seem to be higher for copper pennies(and maybe the same applies to silver dimes) while using my round 10.5" HF(18.75) DD as compared to my 9" MF(7.5 khz) Concentric.

Does any one else notice that a HF DD has higher TID numbers on copper and silver coins than a MF or LF Concentric?? If so, why is this?

Also I think some ideal coils that need to made for the X-Terra's are both a LF => 6" Concentric and DD, and an ellipitical 10" x 18" LF DD for larger homestead-farmers field coverage. The X-70's Instruction Manual does say in plural, LF coils, yet only one is made, it does not make sense. I give Minelab alot of credit for great products, but I do not know what is wrong with them(Minelab) as they should get their act together on LF coils for the X-Terra's, because for the best performance on silver, clad, and copper coins, along with the best iron rejection-ID, the LF coils are best for these tasks and we need more LF coils made, as the vast majority of people are coin hunters and also like to knock out iron. Come on Minelab, we need some more LF coils made, are you listening??

 

[TURNMASTER]

I think I have noticed this ID thing. Others, on the forum here have said as much also.

 

[TheMarshall]

Got a 3 khz for my 705 and was trying it out today....It also seemed like the TID was almost always 38 for the dimes, whereas the DD 18.75 would id them at 40 or 42.

I do not have a LF(3 khz) Concentric but I do want one. But one point I want to make is, I find the TID numbers seem to be higher for copper pennies(and maybe the same applies to silver dimes) while using my round 10.5" HF(18.75) DD as compared to my 9" MF(7.5 khz) Concentric.

Does any one else notice that a HF DD has higher TID numbers on copper and silver coins than a MF or LF Concentric?? If so, why is this?

Also I think some ideal coils that need to made for the X-Terra's are both a LF => 6" Concentric and DD, and an ellipitical 10" x 18" LF DD for larger homestead-farmers field coverage. The X-70's Instruction Manual does say in plural, LF coils, yet only one is made, it does not make sense. I give Minelab alot of credit for great products, but I do not know what is wrong with them(Minelab) as they should get their act together on LF coils for the X-Terra's, because for the best performance on silver, clad, and copper coins, along with the best iron rejection-ID, the LF coils are best for these tasks and we need more LF coils made, as the vast majority of people are coin hunters and also like to knock out iron. Come on Minelab, we need some more LF coils made, are you listening??

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Hello David.

In answer to your query highlighted in red:-

The reason for the higher TID values on mentioned coins, is because of the higher frequency involved.

The current induced into a coin depends on 3 primary factors.

(1) The conductivity of the metal.

(2) The area, thickness and coin 'attitude' as 'seen' by the search-head.

(3) The frequency involved.

So as far as your question goes, (1) and (2) are 'Constant' as 'seen' by any search-head, but the FREQUENCY differs.

The higher the frequency, the higher the 'rate-of-change' in the searching magnetic flux, and so the higher the eddy current flow...WITHIN LIMITS.

The limiting factor being, when the increasing ' a.c. resistance ' or 'reactance' rises faster than the frequency causing it.

For ANY target, there is an optimum frequency, which is dictated by a formulated relationship of ALL three categories listed.

I'll stop there David, for the ultimate proof requires a mathematical formula, and simply quoting it is not a problem, but it has little meaning unless the reader has the background knowledge to interpret the abbreviated script involved............Hope what I've briefly explained, is of some help.

Here is an idealised example of implementing the formulae in graphical form, for an article to be published here in the UK Treasure Hunting Mag., about the Whites 3 frequencies and their relationship to coin metals ....The Minelab X-Terra 705 with its multi-head-frequencies, is to follow soon

You can mentally place the three Minelab frequencies in their appropriate places to get a general idea of their response level to a 2 mm thick silver disc........TheMarshall

[attachment 160866 2mmsilver.jpg]​

P.S.

One important fact in these matters, is not only which frequency best matches the target material, but equally important is what frequency best 'makes the trip' through the intervening SOIL to reach the target.

The lower the frequency, the better it penetrates the soil. So the optimum frequency for detecting a coin at depth.....varies with depth.

So high frequencies are best for shallow coins, but as depth increases, the frequency should be lowered to combat the ground's mineralisation......All complexed stuff.

 

[David]

I see, thank you very much for that extra effort to help out Marshall! I basically understand, but the technical stuff is a bit hard.

"The reason for the higher TID values on mentioned coins, is because of the higher frequency involved."

So the reason is because a HF coil has a different(higher) TID number than MF or LF coils. The DD vs Concentric has very little to do with it.

The best response level of 100%, being the optimum frequency for the silver coin, is 3.4 kHz (LF).

7.5 kHz (MF) is running at about 75%.

22.5 kHz (HF) at about 25%.

AND also the lower (about) 3 kHz frequency (LF) is suppose to be deeper as well. So the optimum frequency for a normal sized sliver(and copper) coin is around 3 kHz, for both the best response and most depth. Also with the added benefit of the 3 kHz having best iron identification and rejection.

Low frequencies are king(the best) for most normal sized silver and copper coins, in all respects. But the exception is I have heard that for very small mini sized, like very thin and tiny 1/4 cut hammered European(UK) sliver coins(especially sitting on edge-or angled sideways), a higher frequency is better for them. Because HF's have more sensitivity to a very small mini sized objects, HF's will pick up all very tiny objects better, no matter what metal they are made of, either silver or gold. (Being better than LF's), HF's are best to use when purposely searching for gold jewelry-especially small gold rings(that hold diamonds) & fine gold chains, and also for very small gold nuggets.

I would like to see a graph of a test on a normal sized gold coin. (Like on a US quarter-eagle $2.50 or half-eagle $5 gold coin.)

Would the 17 to 22 kHz (HF's) frequencies have the best % response and/or depth on a full sized round gold coin ?? (This would give a person some idea-in relation to a gold ring.)

 

[Furious T]

Thanks Marshall! That was great, and pretty easy to understand. You simplified well what we know is a pretty complex and convoluted topic. Thanks again.