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Offline tabdogTopic starter
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« on: October 21, 2009, 03:56:44 am »
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You ever wonder what operating frequency has to do with metal detecting. George Payne introduced the VLF metal detector around 1980 while at Bounty Hunter. He also made the more important improvements on the design.

This is what he had to say,

The target signal returned to the receive coil can be thought of as composed of two components, one we call x and one we call r. The polarity of the x signal (its direction) tells us if the target is ferrous or non-ferrous. The r signal has only one polarity. Also, the ratio of the x and r signal tells us the target?s phase. In addition, the signal magnitude (which relates to sensitivity) of both x and r are a function of operating frequency.

A VLF detector by its very nature is only designed to respond to the r signal and ignore the x signal. Since the ground reaction primarily produces a x signal in the receive coil the VLF detector does not pick-up the ground but only responds to the r signal of the target. Therefore, the VLF detector only needs the r signal for proper operation.

However, for discrimination we need to measure the x signal and the r signal to determine what the target is. Since we are using the x signal then we have to contend with the resultant ground signal pick-up.

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.

All of the discussion so far pertains just to ?r reading? VLF detectors. If you now add in the discrimination requirement if gets really confusing. Remember, to obtain discrimination we need to read both the x and the r signal components. As I said the best response to the x signal is not the same as the r signal. We need to be at an entirely different frequency for x. Generally for best discrimination we need to have an operating frequency well above the targets -3db frequency.

As you can see the ideal frequency for each target is different. In addition, for best performance the operating frequency to read x should be different from the frequency to read r. The best we can do is reach a compromise frequency. Generally we can say that high frequencies are best for low conductivity targets and low frequencies are best for high conductivity targets.

Tabdog thinks that 6 or 7 kHz is a compromise for all around hunting, if you are after coins and other things. But for lower conductive targets, 10 kHz or higher works better. That is where Tabdog likes to be. He likes his small golden goodies.

George Payne suggested that for most hunting that a Frequency of around 14 to 17 kHz would be a good compromise. He must not be that big on silver dimes and so on. But they will still be detected even if the depth suffers a little.

Now days there are metal detectors that operate at more than one frequency. Garrett Whites, Minelab and some others have introduced these type of machines. Some have two frequencies and some have more. This is an attempt to have the best of both worlds. They are also more expensive and some what harder to learn.

Most starter detectors are single frequency.

So, I suggest thinking about this when you make your next detector purchase.

But, I do not discount reports by other users.

If all this seems too much to cope with,

There are some starter detectors that have good reviews. That means that people had fun with them.

That is the deciding factor after all.

So yawl have fun out there, and tell me how you done.

I will be interested to see anyones adventures on the road to treasure.

Happy Huntin,

Tabdog




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« Last Edit: October 21, 2009, 04:01:02 am by tabdog »
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Offline tenclaw
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« Reply #1 on: October 21, 2009, 07:50:55 am »
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 Clapp  good post there tabdog but my little brain was scrambled with x and r by the time i got to 4th paragraph. guess i'm too lo tech cause i'm still in trouble with dial up. hows that thing work??? (hope you don't mind a little fun being poked at you) but did enjoy that.thanks for the tune -up---tenclaw Wise Detecting and keep swinging... in your opinion, what is best detector for the gold chains?? my sovxs just won't read'em...

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Offline homefire
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« Reply #2 on: October 21, 2009, 08:39:29 am »
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More Keen Info:

General Rule of Freq is the :

Lower will penetrate Earth better.  Responds to larger or More Electromagnetically Conductive targets better.  (Silver, Iron Ect)  Better Depth.

Higher will be Adsorbed or Impeded by Earth.   Responds to Smaller Less Electromagnetically Conductive targets Better.  (Gold, Aluminum, Ect)  Not as Good on Depth.

Gold Rings / Chains!

Take a brass washer and do a air test and check were it will be detected.

Now Using a pair of wire cutters, snip the washer making a clean cut in one location. Insure the cut is separated and ends are not in contact with each other.  Open end and Washer is no longer a ring.

Re due the Air Test!

What happened to your detection distance?


A closed loop or Ring of metal will cancel a portion of the returned signal produced by your transmit signal.


Chains are made up of small rings linked end to end and react the same way.

Most detectors will pass right over a fine gold chain and never produce a response.

Higher Freq do do better at them!











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« Reply #3 on: October 21, 2009, 11:29:11 am »
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 thanks a lot homefire- good explaining--will have to try the washer(brass) thing--- i think thats what makes a good forum--different folks w/different ideas--have a good week------------ tenclaw--ex-carpenter w/all ten still intact------

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I'VE USED MANY BRANDS AND MODELS-TESORO, WHITES,GARRETT,FISHER,NAUTILUS,D-TEX,WILSON NEUMAN,ETC.
« Reply #4 on: October 21, 2009, 11:47:30 am »
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Congrats Tenclaw ol' buddy ex- carpenter--this is treasuremomo. Me and you have went on a lot of detecting expeditions together--I was with you one day when you found the 1838 half dime and I found the 1844  4-reale mexican silver coin in that ol' field behind the house. How's you and the misses doing? We need to head up to claiborne again real soon soon as the peanuts or cotton is harvested--ha! Call ol' treasuremomo when you get the chance and tell the mrs. hello for me. I've got the misses a few more japanese persimmons if she wants them--call me

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« Reply #5 on: October 21, 2009, 11:52:56 am »
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 thanks momo, glad to hear from you.

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« Reply #6 on: October 21, 2009, 12:21:34 pm »
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It's always been my experience that higher frequency machines were a whole lot better at picking up nickels, aluminum foil, and gold items. The Tesoro Cibola is a high frequency machine around 17khz I think and it will air test a nickel over 12 0r more inches--that's real good for a detector to do that--but it won't detect a silver dime as far as it will a nickel or gold item---it's the frequency that causes it to be that way.  A detector that has a frequency below ten--say 5 to 7 kh just works better at penetrating soil and finding those objects such as dimes and other objects that have similar metallic makeup. I've a;ways done better at finding gold rings with machines that at least have 10 to 12 khz through about 17khz. Of course, I've used multi frequency machines and dual frequency machines and the results weren't that much better with these units than a unit with just one good "Compromising" frequency chosen to effectively detect the most of both types of metal that it could catch with one detector. I think a 10khz or 12khz is a good all around frequency level--not too high--not too low... Detecting Great Violent

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« Last Edit: October 21, 2009, 12:30:02 pm by treasuremomo »
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« Reply #7 on: October 21, 2009, 01:32:55 pm »
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 hey momo,get busy and build one with 150 kh. when you get it built i'll help you dig,o.k.--have a good week------------  tenclaw

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« Reply #8 on: October 21, 2009, 02:09:06 pm »
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tenclaw, I have been considering getting some electronics books and maybe taking a class in basic electronics---I'd love to build my own detector some day--maybe a super dooper coin and jewelry snooper-ha! We could name it something like the MONEY PUNCHER or the Gold Snatcher-ha!!! Clapp Detecting Great Pray. We could sell them for outrageously low prices like $179.00 and folks would buy them and outshoot any of the 800 buck highly advertised units. But if it worked any less than those dowsing rods we've experimented on, we'd be in trouble and have to hock our tools just to get back to eating regularly-ha! Violent Fight Detecting

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Offline tabdogTopic starter
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« Reply #9 on: October 21, 2009, 08:49:34 pm »
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Quote:Posted by tenclaw
Clapp  good post there tabdog but my little brain was scrambled with x and r by the time i got to 4th paragraph. guess i'm too lo tech cause i'm still in trouble with dial up. hows that thing work??? (hope you don't mind a little fun being poked at you) but did enjoy that.thanks for the tune -up---tenclaw Wise Detecting and keep swinging... in your opinion, what is best detector for the gold chains?? my sovxs just won't read'em...


No, I dont mind being made fun of.

I make fun of me all tha time.

Fun is the thing that we need. But
we choose to worry and fret and be
too sereious much too often.

Save more time for fun.

I knew I may have been a little far out there.
But I have tried to answer questions about
frequency and I struggle to try to relate what
info I do know.

It is complicated and I thought that Mr Payne
could say it much better than I can.

Frequency effect actually makes sense when
he talks about it.

Happy Huntin,

Tabdog

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