I am quite confused when it comes to kHz signals from coils, and what metals sound off in that range. Some say 6-7 khz is a good range for coins and 18 is good for gold, brass, lead. I would LOVE to see a chart explaining what frequencies or whatever, various metals signal on certain coils. Then I could figure which coil would be good for relics, which for gold, which for coins etc. You know what I mean. All these various kHz numbers in different coils gets confusing.
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kHz and different metals
- Thread starter upnorth
- Start date
Uncle Willy
New member
Your detector generates the khz's not the coil. The coil just transmits and receives the signal sent out by the detector. The coil,by its size and shape, determines the depth and the shape of the signal entering the ground. Changing coils solves nothing except determining depth and ground coverage or dealing with mineralization as with the DD coil. That's why they make special detectors for hunting gold as they all run at a higher khz.
Bill
Bill
Hi sf,
I am NO expert but will have a go at an answer seeings that no-one else has...
my understanding is :
Low Khz ( say under 5 ) more sensitive to silver - but will also detect other metals.
High Khz ( over 12 ) sens. to iron, copper & gold - " " " " " "
Medium Khz is middle of the road and what most standard coils come from the factory with as its a good all rounder.
Ace 250 coil is in the range of 7.0 - 7.5 Khz or thereabouts I think...thats what I would call medium.
Some new top end machines run multi frequencies with the one coil...you can choose a Khz or run in multi Khz .
Hope this may help.
T59
I am NO expert but will have a go at an answer seeings that no-one else has...
my understanding is :
Low Khz ( say under 5 ) more sensitive to silver - but will also detect other metals.
High Khz ( over 12 ) sens. to iron, copper & gold - " " " " " "
Medium Khz is middle of the road and what most standard coils come from the factory with as its a good all rounder.
Ace 250 coil is in the range of 7.0 - 7.5 Khz or thereabouts I think...thats what I would call medium.
Some new top end machines run multi frequencies with the one coil...you can choose a Khz or run in multi Khz .
Hope this may help.
T59
Uncle Willy
New member
Yeah I been trying to get Garrett to come out with a multi-frequency machine that could be set to hunt any kind of metal anywhere.
Bill
Bill
Mick in Dubbo
New member
While it's true that the detector is what sends out the signal, the coils (antenna) are tuned to the wavelength of the signal sent out.
The frequency (as I understand it) is the number of modulations per second. ie 3kHz is number of waves per second that is sent out. 18kHz is 18 times per second eta. The end result is, that the higher number means that you in effect, have a higher resolution (like the number of pixels on a camera). In effect, making the detector more sensitive to smaller targets eta. (In the same way a 15 megapixel camera will have a clearer picture than say a 2 magapixel camera.) The down side of a higher kHz, is that, because of the more waives, the height of the signal is proportionally smaller, which means less depth.
If you use say a 3kHz coil on a suitably set up detector, you will have a coil that will go deeper (perhaps a couple of extra inches) and will ignore small bits of iron a lot more easily, BUT, because of that lower resolution, you should swing it a bit slower as the lower resolution can result in you missing some targets.
Gold likes higher frequencies while silver like lower frequencies. It seems that the ideal frequencies for coins ranges from 3kHz to 15kHz, Gold is usually higher, as frequencies lower than this, generally isn't sensitive enough to be practical. There are always exceptions to this, as I there can be other factors (unknown to me, but I've seen it) that can effect this.
Mick Evans.
The frequency (as I understand it) is the number of modulations per second. ie 3kHz is number of waves per second that is sent out. 18kHz is 18 times per second eta. The end result is, that the higher number means that you in effect, have a higher resolution (like the number of pixels on a camera). In effect, making the detector more sensitive to smaller targets eta. (In the same way a 15 megapixel camera will have a clearer picture than say a 2 magapixel camera.) The down side of a higher kHz, is that, because of the more waives, the height of the signal is proportionally smaller, which means less depth.
If you use say a 3kHz coil on a suitably set up detector, you will have a coil that will go deeper (perhaps a couple of extra inches) and will ignore small bits of iron a lot more easily, BUT, because of that lower resolution, you should swing it a bit slower as the lower resolution can result in you missing some targets.
Gold likes higher frequencies while silver like lower frequencies. It seems that the ideal frequencies for coins ranges from 3kHz to 15kHz, Gold is usually higher, as frequencies lower than this, generally isn't sensitive enough to be practical. There are always exceptions to this, as I there can be other factors (unknown to me, but I've seen it) that can effect this.
Mick Evans.
Alan Applegate
New member
I wish is was as simple as it appears here. Alas, it is not.
While the frequency of operation can be important, so is the size of the coil, and the amount of power applied to it. And obviously, the receiver is important too. All of these, in combination, determine how deep, and how sensitive any detector is to any given metallic mass. Change any one, and you effect the other parameters too, especially battery life.
The actual makeup of the the metallic mass (ferrous, non-ferrous), its shape, its size, all have an effect as well. A good example is a ring laying parallel to the detector's coil. Since it's a closed circuit, the detection depth is much greater than it would be if it had a gap in its circular shape. Further, if you rotate the ring perpendicular to the coil, the detection depth is even less. If you understand how electromagnetic fields interact with one another, this is an obvious scenario.
The question is then, is frequency all that important, all else being equal? Think about this. The old BFO machines typically operated between 200 kHz, and 500 kHz. Yet, under nominal conditions, they'd detect a quarter 6 to 8 inches deep using an 8 inch coil. This isn't much different than today's TR machines. Before you trash this thought, it's important to remember the other parameters involved.
I'm often amused with the mythical hype given to some specific designs as if they're the panacean answer to the holy grail of metal detecting. All too often as a result of the placebo effect, the mythical belief turns into pseudo fact. This scenario isn't limited to metal detecting either. They're just as much of it, if not more, when it comes to amateur radio antennas; a very similar product from a technical aspect.
The bottom line is, there isn't any free lunch, and there isn't just one, all-encompassing, universal parameter.
Alan Applegate, K
While the frequency of operation can be important, so is the size of the coil, and the amount of power applied to it. And obviously, the receiver is important too. All of these, in combination, determine how deep, and how sensitive any detector is to any given metallic mass. Change any one, and you effect the other parameters too, especially battery life.
The actual makeup of the the metallic mass (ferrous, non-ferrous), its shape, its size, all have an effect as well. A good example is a ring laying parallel to the detector's coil. Since it's a closed circuit, the detection depth is much greater than it would be if it had a gap in its circular shape. Further, if you rotate the ring perpendicular to the coil, the detection depth is even less. If you understand how electromagnetic fields interact with one another, this is an obvious scenario.
The question is then, is frequency all that important, all else being equal? Think about this. The old BFO machines typically operated between 200 kHz, and 500 kHz. Yet, under nominal conditions, they'd detect a quarter 6 to 8 inches deep using an 8 inch coil. This isn't much different than today's TR machines. Before you trash this thought, it's important to remember the other parameters involved.
I'm often amused with the mythical hype given to some specific designs as if they're the panacean answer to the holy grail of metal detecting. All too often as a result of the placebo effect, the mythical belief turns into pseudo fact. This scenario isn't limited to metal detecting either. They're just as much of it, if not more, when it comes to amateur radio antennas; a very similar product from a technical aspect.
The bottom line is, there isn't any free lunch, and there isn't just one, all-encompassing, universal parameter.
Alan Applegate, K
Uncle Willy
New member
I'd like a tector with adjustable,multiple frequencies, from 0 to 100 so one could fine tune it for any metal in any situation anywhere.
Bill
Bill
Alan Applegate
New member
Well, Bill, that might be nice, but as I pointed out, changing frequencies alone won't necessarily change anything except the complexity. As I said once before, there is no way to perform a double blind test of a metal detector. If there was a way, quite a few myths would be quickly dispelled, not the least of which is the importance of operating frequency. Knowing what I know about the subject, I suspect the difference between say 5 kHz, 10 kHz, and 15 kHz is so slight as to go unnoticed, especially so if the other components are not nominalized to the frequency in question.
Assume you could perform a double blind test (you can read about blind tests here: http://en.wikipedia.org/wiki/Blind_experiment). You would also have to normalize the testing media, and that's rather hard to do. Temperature, moisture, coil height above the surface, coil movement speed, etc., all would have to be controlled. Without all of the variables set in stone, any outcome would have a subjective variance, or basically what we have now between the various factions supporting their pet brand/model/frequency.
I don't mean to drag this out, but the real truth is, the vast majority of the differences between one brand/model/frequency, and another, has more to do with the operator, than the device itself.
Alan Applegate
Assume you could perform a double blind test (you can read about blind tests here: http://en.wikipedia.org/wiki/Blind_experiment). You would also have to normalize the testing media, and that's rather hard to do. Temperature, moisture, coil height above the surface, coil movement speed, etc., all would have to be controlled. Without all of the variables set in stone, any outcome would have a subjective variance, or basically what we have now between the various factions supporting their pet brand/model/frequency.
I don't mean to drag this out, but the real truth is, the vast majority of the differences between one brand/model/frequency, and another, has more to do with the operator, than the device itself.
Alan Applegate
Uncle Willy
New member
Well I've been at this for 45 years and I didn't learn detecting or about detectors from Wickepedia. Double blind tests work fine if one is testing a new drug or medication but doesn't pan out too well with detectors due to a plethora of variables.. If that was the case then there would be no need for different frequencies and all detectors would be universal in operation - running the same frequency for all types of metal and all types of conditions. As more new detectors pop up on the scene we'll probably see multiple frequency detectors much like the new White machine. Minelabs already run at multiple frequency and Garrett's Infinium PI operates with 96 frequencies ( but neither with adjustable frequencies ) so apparently these companies have invested a lot of time and money for nothing. Back when Compass was the detector to own they ran at 100 khz through a DD coil and found a ton of stuff.
Bill
Bill
Mick in Dubbo
New member
I was hoping someone with a bit more knowledge could fill in a bit more info.
Mick Evans.
Mick Evans.
Alan Applegate
New member
Mick, I tried to answer your question in part in another post: http://www.findmall.com/read.php?32,1132106
Technically, I'm not a neophyte, and I do understand how electromagnet waves work. It really doesn't matter whether we're talking about audio frequencies, or microwaves, or anything in between. They all follow specific, and well established rules.
The issue here is the frequency of operation. It shouldn't make much difference what the frequency is, if the wave length is significantly longer than the size of the object inserted into the coil's field. You might present a case wherein the soil's makeup might effect the penetration depth of some specific frequency, but I doubt it. There are a couple of things multi frequencies could do, however.
If we used largely disparate frequencies (certainly not a few kHz), we could use the slightly different signals from multiple frequencies to determine what the object is by using interferometry techniques. The problem is, detecting the difference would require much more processing power than is possible in a hand carried device.
As I said in the other post, the most important part is the operator.
Alan Applegate
Technically, I'm not a neophyte, and I do understand how electromagnet waves work. It really doesn't matter whether we're talking about audio frequencies, or microwaves, or anything in between. They all follow specific, and well established rules.
The issue here is the frequency of operation. It shouldn't make much difference what the frequency is, if the wave length is significantly longer than the size of the object inserted into the coil's field. You might present a case wherein the soil's makeup might effect the penetration depth of some specific frequency, but I doubt it. There are a couple of things multi frequencies could do, however.
If we used largely disparate frequencies (certainly not a few kHz), we could use the slightly different signals from multiple frequencies to determine what the object is by using interferometry techniques. The problem is, detecting the difference would require much more processing power than is possible in a hand carried device.
As I said in the other post, the most important part is the operator.
Alan Applegate
TURNMASTER
Active member
In tests I have done with my Xterra 705, I find that my 9" 7.5kHz CC hits most targets tried 1" deeper than my 5x10 18.75 kHz DD. Used on the same machine and using the same procedure to GB, Noise cancel and set the Sensitivity. Remember the Xterra series changes Freq with different coils.
Yet I still prefer to hunt with the 5x10 DD. I feel it handles the range of soils here better and in some places gets better depth with better separation overall.
I find that my machine behaves differently with the different coils used.
Currently I have 3 coils at 2 freqs
9" 7.5 kHz CC
5x10 18.75 DD
6" 18.75 DD (this coil rocks in the trash)
I have a 10.5 DD at 7.5kHz on the way, and intend on learning the machine with the new coil.
The difference between the two 18.75 coils is minimal except in moving about in the trash. I "feel" the DDs GB better, based on the numbers the machine will GB at for a given site.
I do not use the High Freq coils to their potential as I have not been out prospecting with them yet.
Both of the HF coils will pick up VERY tiny pieces of metal, frustrating at times.
Posts in the prospecting forum (by D.T., I believe) suggest the different coils and Freqs react differently to different sizes and types of nuggets.
As others over in the Xterra forum will testify to, frequency does make a difference. If it did not then VLFs designed for Gold would not run the freqs they do (15 to 20 kHz and up). Doesn't the GB 2 run at 70 kHz? It seems to me the better coin and relic machines run around 5 to 8 kHz. Certainly a machines electronic setup for signal processing makes a difference, and so does the operator.
I make my living as a CNC machinist. I have run machines from various manufactures and they are not all created equal. Even different machines from the same mfg. can differ greatly and excel greatly at differing tasks within the same size range.
BFO, VLF or PI
I suspect, in a few years you will see PI machines with some of the ID performance the better VLFs have now
Technologies vary and so do opinions.
HH
Jeff
P.S. Some of my thoughts are based on ignorance.
Yet I still prefer to hunt with the 5x10 DD. I feel it handles the range of soils here better and in some places gets better depth with better separation overall.
I find that my machine behaves differently with the different coils used.
Currently I have 3 coils at 2 freqs
9" 7.5 kHz CC
5x10 18.75 DD
6" 18.75 DD (this coil rocks in the trash)
I have a 10.5 DD at 7.5kHz on the way, and intend on learning the machine with the new coil.
The difference between the two 18.75 coils is minimal except in moving about in the trash. I "feel" the DDs GB better, based on the numbers the machine will GB at for a given site.
I do not use the High Freq coils to their potential as I have not been out prospecting with them yet.
Both of the HF coils will pick up VERY tiny pieces of metal, frustrating at times.
Posts in the prospecting forum (by D.T., I believe) suggest the different coils and Freqs react differently to different sizes and types of nuggets.
As others over in the Xterra forum will testify to, frequency does make a difference. If it did not then VLFs designed for Gold would not run the freqs they do (15 to 20 kHz and up). Doesn't the GB 2 run at 70 kHz? It seems to me the better coin and relic machines run around 5 to 8 kHz. Certainly a machines electronic setup for signal processing makes a difference, and so does the operator.
I make my living as a CNC machinist. I have run machines from various manufactures and they are not all created equal. Even different machines from the same mfg. can differ greatly and excel greatly at differing tasks within the same size range.
BFO, VLF or PI
I suspect, in a few years you will see PI machines with some of the ID performance the better VLFs have now
Technologies vary and so do opinions.
HH
Jeff
P.S. Some of my thoughts are based on ignorance.
Alan Applegate
New member
I'm not so sure Jeff. I've put a great deal of thought into this of late, and I think the next advancement will come from DSP (digital signal processing). While PI might be used, the real deal will be interferometry. It requires more than one frequency, but if you do it correctly, assuming you have the processing power to do so, you'll be able to distinguish the material the target is made of, its size, and of course its depth. The bugaboo is the requisite power source. Thankfully, CPUs are getting more powerful, and less power hungry. Batteries are getting better too. Whenever it comes, the one given attribute; it won't be cheap!
Alan Applegate
Alan Applegate
TURNMASTER
Active member
I am not realy a techie, but I am capable of using tools correctly. I was kinda thinking these guys are a good part of the way there with the PI machines. The next step In my highly uneducated opinion is learning to interperate the siglnals and process the peak and drop off in a fasion that gives a more solid indicator of composition.
Shootin in the dark
Jeff
Shootin in the dark
Jeff
Uncle Willy ;
a few years ago i ran across an hombre who probobly has been detecting longer than anyone here . started out just after ww11 with a mine detector . thru the years he has pretty much tried them all . finally realizing they all had their shortcomings he decided to perfect his own . after untold thousands of $ spent on r&d he finally had it . wanted to go into production , prototypes had been made . then he found out he couldn't patent it !!! it fell outside the gov. freq. limits or output power or something of that nature . i dont remember all the details anymore , it's been about 12 years since i've seen him . he is internationally known and you probobly know him already . lives in the northeast . works claims up in the 40 mile district . it would be nice if you were too contact him and get his take on things . i doubt he is going to hand over a full schematic after spending all that money for free . but i'm sure that some pointers in the right direction would be pleasureable . i dont dare post who he is here and have him covered up with questions , but if you desire to talk to him i can email you his name and address .
Best Regards .
a few years ago i ran across an hombre who probobly has been detecting longer than anyone here . started out just after ww11 with a mine detector . thru the years he has pretty much tried them all . finally realizing they all had their shortcomings he decided to perfect his own . after untold thousands of $ spent on r&d he finally had it . wanted to go into production , prototypes had been made . then he found out he couldn't patent it !!! it fell outside the gov. freq. limits or output power or something of that nature . i dont remember all the details anymore , it's been about 12 years since i've seen him . he is internationally known and you probobly know him already . lives in the northeast . works claims up in the 40 mile district . it would be nice if you were too contact him and get his take on things . i doubt he is going to hand over a full schematic after spending all that money for free . but i'm sure that some pointers in the right direction would be pleasureable . i dont dare post who he is here and have him covered up with questions , but if you desire to talk to him i can email you his name and address .
Best Regards .