Interchanging Metal Detector Coils?

[Professor Hester]

I am different!  I want to know why and what makes it work, what makes it different.  I want to know the technical differences between products.  The manufacturer can give a detector a name and claim that his detector is the best for detecting for gold.  Sorry, manufacturer's claims come in one ear and out the other. 

Give me the facts and let me decide.

I am going through the process of de-coding manufacturer claims and guess what.  It is real hard to just find the facts.  Right now I am searching for a list of the operating kHz frequencies of all metal detectors.  Let's face it, a coil just creates a magnetic field and the only variables on any coil is the kHz frequency and the size of the coil.  At least those are the two variables that I have found so far.

I really need to know whether the kHz frequency is determined by the operating frequency for the Detector control box or is it controlled by the operating frequency of the coil?  It makes sense to me that when you wind the copper wire into a coil that you should be able to determine the coil's  kHz frequency by the number of winds of the copper wire, when you are making the coil.  So, I am thinking that the coil actually determines the frequency that is built into the control box of the metal detector.  Am I right?

What I need now, is for someone who knows more about the industry to step up to the plate and provide a list of the kHz frequencies of the metal detector coils that are currently on the market.  Any two coils that has the same kHz frequencies, whether it is made by the same manufacturer or not, should be interchangeable. 

A magnetic field is just a magnetic field.  The operating kHz frequency, does it affect the magnetic field?  If so, I need another expert to tell me whether different frequencies get better results, depending upon what the prospector is searching for. 

Please, do not respond to these questions unless you have scientific facts to back up what you say.

If anyone can set me straight and help me get the facts I'm looking for, I would appreciate your time and effort.  Don't worry, any response you give will not offend me.

Thanks,  Professor Hester

[Ivansgarage]

I don't know if this will help, but I have been winding electric motors for a long time, for variable frequency. 

In all my understanding the turns or the wire size does not have anything do to with frequency. frequency is all handled by the controller. My reasons for wire size and turns is to handle the difference in current and voltage.

So in a nut shell the wire size and turns would be designed for the voltage and current of the coil.

 

http://ivanbennett.com/forum  all about motor winding coils............. 

 

Ivan

 

[snakejim]

The operating frequency is selected by the manufacturer based on what he wants the detector to do; and on several things such as the conductivity of the items being searched for and in how mineralized the ground is to be searched.

There is all ways a trade off as low frequencies go deeper on high conductive items in bad ground; but they lose sensitivity to low conductive items such as small gold nuggets. Early detectors built for coin hunting  and some early gold detectors were 5khz to maybe 7 or 8 khz such as the very early White's Gold masters. Then there was the 15 khz Garrett A2B Nugget detector which worked pretty well until the original Fisher Gold Bug came out at 19khz, which was even better. Then came the newer White's Goldmasters at 48 to 50khz and then the Fisher Gold Bug 2 at 71khz, which is still today the highest frequency nugget detector on the market.

I just received a new Gold Bug 2 yesterday, as a matter of fact.

Not all coils running at the same frequency are interchangeable on other detectors. Each coil is tuned to the specific circuits of the detector it is designed for. The frequency may be the same, but the resistance and or impedance will be different.

Even though the original Gold Bug of 1982 was 19khz and the newest Gold Bugs of 2009 are 19khz they are not the same and the coils will not interchange at all. Coil designs have changed as well as the move to digital circuits, and away from analog circuit design.

Here's a link to essays written by a friend of mine who has been a designing engineer of metal detectors for many years.

http://www.fisherlab.com/hobby/dave-johnson-essays.htm

Another good link: Steve has used more detectors than any one I can think of for gold hunting.

http://www.detectorprospector.com/gold-prospecting-guides/steve-guide-gold-nugget-detectors.htm

Hope this helps.

What do hope to gain with this knowledge? Are you thinking of buying a detector?

[Steve Herschbach]

http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=info.dat

http://www.geotech1.com/cgi-bin/pages/common/index.pl?page=metdet&file=projects.dat

http://www.talkingelectronics.com/projects/200TrCcts/MetalDetectors/MetalDetectors-1.html

http://inside.mines.edu/~jamcneil/TPT_MetalDetector.pdf

http://www.minelab.com/__files/f/11043/METAL%20DETECTOR%20BASICS%20AND%20THEORY.pdf

http://mobile.kellycodetectors.com/buy/960-6000226

In a nutshell, as Jim has said, just because two coils operate at the same frequency does not mean you can swap them around on different detectors. Sometimes it can be done but often time circuit differences prevent it.

Low frequencies have less issues with ground mineralization but are less sensitive to low conductive/small targets like small gold nuggets. High frequencies hit low conductive targets better but have more ground and hot rock issues. It is a trade. From George Payne whose work forms the basis of many detectors made today at http://jb-ms.com/Baron/payne.htm

"The x and r target signals are frequency dependent and obey very predictable characteristics when the operating frequency changes. We know that the x component decreases as the operating frequency decreases. Above a certain frequency the x component reaches a maximum. The r component acts differently. It is maximum at one particular frequency and decreases if you go up or down in frequency. We call the special frequency at which the r signal is maximum, the target’s “-3db” frequency. It also turns out that at the -3db frequency the x signal is one-half of its maximum value. This special frequency is unique to each target and is different for different target.

The higher the conductivity of the target the higher will be the targets -3db frequency. Conversely, the lower the conductivity the lower the -3db frequency. The -3db frequency of the high conductivity target will also make the r signal peak at a high frequency, normally well above the operating frequency of the VLF detector. This will make the high conductivity target have lower sensitivity on the VLF detector because the r signal amplitude drops if we are significantly below the -3db frequency. Simply put, maximum sensitivity on a VLF detector would be if we position the operating frequency directly at the target’s -3db frequency. For example, a dime and penny have a -3db frequency of about 2.7KHz. This is where their r signal peaks and would be the best frequency for picking them up using a VLF detector. However, a silver dollar has a -3db frequency of 800Hz. Nickels, on the other hand, have a -3db frequency, where its r peaks, at about 17KHz. Targets like thin rings and fine gold are higher still. Clearly there is no one frequency that is best for all these targets. The best you can do is have an operating frequency that is a compromise."

[Reno Chris]

For someone who has so little knowledge of the operation of detectors, you ask a whole lot of questions!

Coils have many characteristics such as inductance, capacitance, etc. The design of a detector must take into account these "other " characteristics. It is not just about the kHz which a detector is designed for. It makes a big difference if a coil consists of 40 windings or 400. Additionally, metal detector sensor heads or "coils" may consist of many coils - I have seen designs with as many as 5 separate inductor coils.

I would start with just a basic understanding of coils, or inductors as they are known to those with experience in electronics.

See: http://en.wikipedia.org/wiki/Inductor

Carl Moreland's book (linked to in Steve's post above) is an excellent reference and will answer the bulk of your questions.

Since they are YOUR questions, do the work and discover the answers for yourself. Any real professor would tell you that self motivated investigation is the best way to learn any subject. When your efforts are complete, come back here and tell us about what you have learned.

[Professor Hester]

After reading your responses, I feel like I just sit around a fire-pit for about three hours listening to a bunch of experts. You know one of those conversations where you don't talk and they are talking about a subject that you don’t know anything about. It reminds me of listening to my grandfather when I was a teenager.

First, I want to thank you all collectively for trying to help a novice. I am a CPA, a Professor of Accounting, CFO but I am not an electrical engineer. What I don’t know about being an engineer would fill a library; however, if I can ever help you with an accounting or finance question, that is what my library is full of. My library is full, just with stuff not applicable to metal detectors.

Second, I apologize for my ignorance. Believe it or not, I had given it a lot of thought and my reasoning and conclusions sounded pretty good to me, that is, before you guys blew everything full holes.

I consider myself lucky to have found this forum web site. I will be reading the articles mentioned in your responses.

One of you asked me, “What do you hope to gain with this knowledge? Well, I am trying to decide which metal detector I should buy, with my limited funds. My friend owns a Fisher 75. It is a very expensive detector. His detector has a DD 11” oblong coil. I was trying to find a complimentary metal detector that might help fill some of the gaps that a Fisher 75 has. The metal detector that I was thinking about buying is the Teknetics Euro Pro with a DD 11” coil. I have read a lot of reviews on the Euro Pro, but naturally, the reviews rarely say anything negative about the product.

If anyone would like weigh in on selecting a metal detector under $300 dollars, I would appreciate your input.

Thanks guys!

[Steve Herschbach]

My only real advice is that if you stick with a First Texas (Bounty Hunter, Fisher Teknetics), Garrett, Minelab, Tesoro, or White's detector under $300 you are assured of getting a competitive model with good backup and support. That price range is saturated with units that all perform within a hair of each other. Main thing is stick with mainline manufacturers and you can't go wrong. Everyone has their favorites.

[Reno Chris]

What is your intent for your detector?

Coins and Jewelry in local parks?

Nuggets in the goldfields?

Treasure caches?

[Mike Hillis]

Professor,

I'd steer you toward the new Bounty Hunter Land Ranger Pro.    It has a great feature set for a beginner, plenty of stuff to grow with, AND it includes the Eurotek feature set among its many selections.   If you search online you can find it under $300.  

I also believe the coils will aso interchange with the F75.   Not advertised to do so but i believe the Land Ranger Pro is built on the same freq and platform as the Teknetics frat brothers and all those coils will work with the F75 and visaversa.  I could be wrong though.  (((Yes, you are going to get even more confused about coils when you find out a 7.8 freq coil will work ok on a 13khz machine but ignore that for a while)))

Check it out.  Its has a great feature set.

Link to operating manual.... http://www.detecting.com/manuals/LandRangerPro-Reader.pdf

Disc 3 is the EuroTek Pro audio feature

Good Luck,

Mike

[Mike Hillis]

Eurotek pro frequency is 7.81 kHz

Fisher F5 frequency is 7.8  kHz

Fisher F19 frequency is 19 kHz

Fisher F75 Frequency is 13 kHz.

All the coils interchange among these detectors.  I don't have any issue swapping coils between those units.

The Land Ranger Pro is 7.69 kHz.   Frequency wise, the coil would be compatible with the others listed above, HOWEVER... looking at the manual, the coil connector is not capatible.  The BH LRP has a standard BH push in connector, while the aforementioned units have a threaded connector.  So I was wrong.  The LRP coil will not interchange with the F75.

HH
Mike