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VLF Punching Deeper


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As a rule do the lower vlf frequencies punch deeper than the higher ones, say 4.8 verses 14khz?

But what is the trade off? Are some frequencies better for silver coins? How does iron enter into this?

Need to understand how this all fits together!

Thanks for any and all answers.

 

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Yup.... 

As a rule the lower freq. do go deeper and are better suited for relic hunting.

They are less affected by ground mineralization also. also the lower freq. gets better battery life.

The trade off is the lower freq. is less sensitive to gold and small items.

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I from practical results in the field consider low frequencies to be better for large/strong/thick targets and high frequencies to be better for small/weak/thin targets. For instance a U.S. silver quarter is a strong high conductive target that does well at lower frequencies.

However, in Europe very thin hammered silver coins are common, and these may be cut in half or quartered in the old days to make change. Even though these targets are silver, a high conductor, they are small and thin and so actually do better at high frequencies.

Therefore I personally do not think much in terms of what the items is made of (gold, silver, low conductor, high conductor) but in terms of size. The larger the items are, the better off you are with lower frequencies as there is no problem detecting the target, and lower frequencies reduce ground effects that work against you.

I would consider dime size and larger items as "large" targets. As targets get smaller than dime size, they respond better at higher frequencies. The smaller they are, the higher the frequency required to get a decent signal from them. However, the higher the frequency, the more the ground and hot rocks respond. It is this sensitivity to ground effects that tends to "blind" high frequency detectors and result in depth problems in bad ground. High frequencies air test just fine; it is in the ground where the issues occur.

With that in mind, when looking for large items I tend to lower frequencies and when hunting very small items I go to higher frequencies.

There is of course more to it than that. Here are some technical details and references to more in depth information......

George Payne was one of the engineers who patented many of the basic concepts used in VLF detectors to this day. Here is an excerpt from his article at http://jb-ms.com/Baron/payne.htm (2002):
 
"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."

So that sets up the basics for air tests. The problem is we have to deal with the ground. Lower frequencies tend to be better at reducing ground issues, while higher frequencies light up both the ground and hot rocks lower frequencies might ignore. You are looking for a frequency which best lights up a target while minimizing ground effects. All in all mid frequency machines in the 10-20 kHz range offer good compromise solutions, above 20 kHz tends to be the realm of specialized prospecting detectors, and under 10 kHz the realm of the "coin detector".

Some prior posts with more details:

Frequency Changes ( Effects ) While Running All Metal

Selectable Frequency And Multiple Frequency

Why High Frequency? The Effects Of Different Frequencies On A Small Gold Target (Video)

Optimal Frequency For Coin Hunting (Monte)

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1 hour ago, johnedoe said:

also the lower freq. gets better battery life.

As a general rule high frequency detectors will have better battery life. This can be seen directly with machines like the Nokta Impact and White's V3i, where the lower frequency settings more quickly consume battery power than the high frequency settings.

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Second question how recovery speed is connected with frequency. In Impact user manual, we get information about that we need to work with 20kHz to get best performance... right?

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Higher frequencies aid in recovery speed.

From the Nokta Impact Owner's Manual page 24:

Frequency

IMPACT offers 3 operating frequencies — 5kHz, 14kHz and 20kHz— to suit different target
and soil types. Based on the frequency selected, the detector's detection performance for different types
of targets will vary. The list below includes, but are not limited to, different types of targets
that correspond to each frequency:

5kHz: Large ferrous and non-ferrous objects
High conductive coins
Medium or relatively small targets in non-mineralized ground without iron trash
Ferrous masses and militaria

14kHz: General use
Small coins
Different size coins in medium-highly mineralized ground

20kHz: Small coins with different conductivities and thin large coins
Gold coins, rings, small jewelry , sheet iron, foil
Small targets in iron trash

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Rege,

Steve has mentioned shape of the target as being important.  I have to agree.  It is not just a 'size' thing but shape.  

I mostly use a 3030 (It has a graph.) on the beach these days but in the past I used an SE Pro which is also multi frequency and a White's 6000 DI Pro Coinmaster. (It's meter shows probable coins.) Now I sometimes use the Gold Bug Pro (It just has numbers.) to find a few coins in the desert.  With all of these detectors size matters and I always say a man made shape matters most in what I hear.

One problem on the beaches now is the corroded zinc penny.  It can crackle back at you showing multiple target signals.  It can sound or look like a penny or other objects.  Copper wire or copper strips can look like quarters or higher silver.

Another problem on the beach would be chains.  They have multiple facets and disperse a signal to a point that it is not heard back by the coil.  When I used to use my 5000 on the beach (a PI) I have a find story about finding a fine chain with a diamond while digging a coin.  I later put the chain on the ground by itself and could not see it with the detector.  I found the jewelry by accident.

When I do to the beach any more I don't have the energy (sore shoulder) or time to dig everything. (If you have a PI you dig it deep and you dig it all.) Iron tones, bobby pins and other trash on the 3030 are on the bottom of the graph.  I'm so confident of it most of the time I don't dig.  It is the same way with the top left for bottle caps.  You just know what it is without digging it.

Good luck with your use.

Mitchel

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Yes, I have dug bobby pins with my PI, in the Virginia red dirt at 8".  That is a chore for sure.

 

 

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Some current single frequency detectors:

Fisher 1280X - 2.4 kHz
Fisher F2 - 5.9 kHz
Fisher F4 - 5.9 kHz
Fisher F5 - 7.8 kHz
Fisher F70 - 13 kHz
Fisher F75 - 13 kHz
Fisher F19 - 19.2 kHz
Fisher Gold Bug - 19.2 kHz
Fisher Gold Bug 2 - 71 kHz

Garrett Ace 150 - 6.5 kHz
Garrett Ace 200 - 6.5 kHz
Garrett Ace 250 - 6.5 kHz
Garrett GTI 2500 - 7.2 kHz
Garrett Ace 300 - 8 kHz
Garrett Ace 400 - 10 kHz
Garrett AT Max - 13.6 kHz
Garrett AT Pro - 15 kHz
Garrett AT Gold - 18 kHz

Minelab Go-Find 20 - 7.7 kHz
Minelab Go-Find 40 - 7.7 kHz
Minelab Go-Find 60 - 7.7 kHz
Minelab Gold Monster 1000 - 45 kHz

Tesoro DeLeon- 10 kHz
Tesoro Cortes- 10 kHz
Tesoro Silver uMax- 10.6 kHz
Tesoro Outlaw - 10.6 kHz
Tesoro Compadre - 12 kHz
Tesoro Tiger Shark - 12 kHz
Tesoro Cibola - 14.5 kHz
Tesoro Vaquero- 14.5 kHz
Tesoro Tejon- 17.2 kHz
Tesoro Lobo - 17.5 kHz

White's TreasurePro - 8 kHz
White's TreasureMaster - 8 kHz
White's Coinmaster - 8 kHz
White's MX5 - 14 kHz
White's MXT - 14 kHz
White's MX Sport - 14 kHz
White's GMT - 48 kHz

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11 hours ago, Steve Herschbach said:

As a general rule high frequency detectors will have better battery life. This can be seen directly with machines like the Nokta Impact and White's V3i, where the lower frequency settings more quickly consume battery power than the high frequency settings.

Steve.....

You are absolutely right about that .... I get a little dyslexic sometimes.. or is that just a little absent minded which comes with my advancing years....:blink:

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