Frequency

[Cletus]

I have heard some say that the lower the frequency the more depth you get. then I hear others say that you
want a higher frequency for depth. figured I would ask the experts.
I can not run my Deus on 12 here at home. I think it is the invisible dog fence that blows it up.
so I do most of my testing in 18 but most of my actual hunting in 12.

 

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[Cletus]

ok so I think I got this frequency thing figured out. the lower the frequency the more depth you get. but the lower in frequency
your detector will miss out on some small silver or small targets . so 18 would be the best for small silver or small targets but 4, 8. and 12 would go deeper. Correct ??

 

[Iron Buzz]

Think of how far the deep bass notes come from that car at the stoplight. At that distance, you can't hear the higher notes, can you?
The Navy uses low frequency to communicate to submarines.

Caveat: The above part, I am certain about. This part is simply my mental image of what may be going on:
High frequencies are more likely to pick up a smaller target because the waves are smaller. There are more waves to the inch, therefore more likely to hit a piece of metal that is also smaller than an inch.

 

[Loco-Digger]

Here is some info I found online.

Low frequency detectors are better at detecting highly conductive metals like silver. The longer waves are also better at penetrating the ground so they can lead to getting greater depth. But it is not as good at finding objects that are small or have low conductivity. This kind of metal detector will typically be in the range of about 5 kHz and below.

High frequency detectors are better at detecting objects with low conductivity like gold and iron. They are also better at detecting small objects making it easier to find things like small gold nuggets or small pieces of jewelry. But the higher the frequency, the less depth it can get and the more sensitive the metal detector is the mineralization in the ground. This could make the detector useless in some soil conditions if the frequency is too high. This kind of metal detector will typically be above about 15 kHz.

Most metal detectors fall somewhere in the middle of these extremes. This allows these general purpose metal detectors to be better all-around by getting most of the benefits of each extreme while minimizing the disadvantages of each. This type of metal detector will typically range from about 5 kHz to 15 kHz.

So what is the best metal detector frequency? Companies that make metal detectors have been trying different frequencies for years in order to find the best metal detector frequency. But the problem is that there is no “best” frequency. There are only ranges of frequencies that are better for different purposes. Knowing this will help you to choose a detector that works best for your needs.

 

[Cletus]

Loco-Digger so according to this 8khz to 12khz would be a all around good setting for the best of both worlds.

 

[Calabash Digger]

yep 8 and 12 I prefer 12 but have hit some nice silvers with 8 and only use 8 if I need to quite the machine down or getting interference from hunting partners.

 

[Loco-Digger]

I use my dues in the 12 khz mode mainly, it's nice to have a choice in frequency though.

 

[Iron Buzz]

I use my dues in the 12 khz mode mainly,....

Why?

 

[Jeff H]

Your best frequency can also depend on your ground's mineralization strength. On weakly mineralized soils 4 kHz will give you the most depth. On highly mineralized soils you will want a higher frequency. Like Loco says, your best choice of frequency can depend on your target metal. Those low frequencies will hit better on the high conductors. So if you for instance you are in weakly mineralized soil, with little trash and are looking for Large Cents (high conductors), 4 kHz will likely be your best bet. But hunting conditions are not often ideal. You have trash, EMI, etc. to deal with. You may have small targets and low conducting targets mixed in with the big targets and high conductors. Often 12 or 8 kHz is a good compromise to have a chance at hitting most of the targets.

I'll often hit a site starting with 8 or 12 kHz, then finish it up with 18 kHz. I am always amazed at the buttons I will find with 18 kHz that I had missed with the lower frequencies. I almost always even go to 4 kHz and will pull some extra deep goodies. There is no one frequency that will find it all.

 

[vferrari]

I run 8 mostly with 12 as my backup. Like to compare TID readings by shifting frequency on a captured target. Choosing between 8 and 12 is probably just a personal preference thing - basically the choice there being whichever of the two has the lowest EMI chatter. I agree with the Jeff H and Loco-Digger on the advantages of the low and high frequencies on max depth vs. small/low conductive target sensitivity and how soil mineralization can be compensated for by frequency setting. Just another great attribute of the Deus.

 

[Loco-Digger]

Why?

Personal preference, I used to swing a machine that ran at 5.9 khz that had a DD coil and then the F75 w/DD coil which runs at 13 khz and have found more good targets with the higher freq than than the lower freq. Just my experience. So since I liked the F75's performance at 13 khz, I just went right to the 12 khz with the XP Deus. I hunt mainly in Ohio and it works for me.

 

[Geochem]

Your best frequency can also depend on your ground's mineralization strength. On weakly mineralized soils 4 kHz will give you the most depth. On highly mineralized soils you will want a higher frequency. Like Loco says, your best choice of frequency can depend on your target metal. Those low frequencies will hit better on the high conductors. So if you for instance you are in weakly mineralized soil, with little trash and are looking for Large Cents (high conductors), 4 kHz will likely be your best bet. But hunting conditions are not often ideal. You have trash, EMI, etc. to deal with. You may have small targets and low conducting targets mixed in with the big targets and high conductors. Often 12 or 8 kHz is a good compromise to have a chance at hitting most of the targets.

I'll often hit a site starting with 8 or 12 kHz, then finish it up with 18 kHz. I am always amazed at the buttons I will find with 18 kHz that I had missed with the lower frequencies. I almost always even go to 4 kHz and will pull some extra deep goodies. There is no one frequency that will find it all.

Excellent post, sharing your Deus learned "targets verses frequencies!"

 

[Iron Buzz]

Your best frequency can also depend on your ground's mineralization strength. On weakly mineralized soils 4 kHz will give you the most depth. On highly mineralized soils you will want a higher frequency. Like Loco says, your best choice of frequency can depend on your target metal.

I'm one of those people that has to know why. WHY do low frequencies not work as well in highly mineralized soils? ("I don't know" is a fair answer... just for me, knowing why helps me to take an idea and expand on it. It also helps me to remember.

 

[Iron Buzz]

Personal preference, I used to swing a machine that ran at 5.9 khz that had a DD coil and then the F75 w/DD coil which runs at 13 khz and have found more good targets with the higher freq than than the lower freq. Just my experience. So since I liked the F75's performance at 13 khz, I just went right to the 12 khz with the XP Deus. I hunt mainly in Ohio and it works for me.

Thanks. I will say, however, that the bolded part is much like the logic for a fisherman's favorite lure. You use it 90% of the time because you catch 90% of your fish with it. On the other hand, its hard to argue with success!

 

[vferrari]

I'm one of those people that has to know why. WHY do low frequencies not work as well in highly mineralized soils? ("I don't know" is a fair answer... just for me, knowing why helps me to take an idea and expand on it. It also helps me to remember.

Is that because you are an engineer? I am, and doctors hate me because I need the "why" answer and it has to be based on logic and physics. ha!

 

[Jeff H]

vferrari, even though he asked me, I am more than happy to let you give Iron Buzz a detailed technical answer as to WHY 4 kHz works better in weakly mineralized soils

 

[vferrari]

vferrari, even though he asked me, I am more than happy to let you give Iron Buzz a detailed technical answer as to WHY 4 kHz works better in weakly mineralized soils

OK. I am inclined to give the "I Don't Know" answer, followed closely by "Because Andy Sabisch said so" but will try to actually answer the question with psuedo-science. First lets separate the issues. Frequency vs. Depth and Frequency vs. Mineralization. Also, remember that metal detectors are radiating radio signals (electromagnetic waves) that travel at around the speed of light regardless of frequency. Since the the speed/velocity of any waveform equals the product of the wavelength times the frequency, if the velocity is constant the wavelength has to go down proportionately as the frequency goes up. So if you know the frequency of an electromagnetic wave, you know the wavelength. The speed of light will change slightly in mediums other than a vacuum but it is insignificant for the materials we are talking about here (i.e., soil).

Frequency vs. Depth - lower frequency signals tend to penetrate further in material more dense than air, in general. That is because denser material is comprised of particles that tends to scatter high frequency RF signals (where the smaller wavelength of the higher frequency signals approaches the size of the scattering "particle") causing greater attenuation, and hence less penetration. This is also the property that makes high frequency signals more suitable to detecting small, less conductive targets because most of the waveform energy is absorbed by the smaller target if the wavelength is small which makes it more detectable. Longer wavelength (i.e., lower frequency) signals tend to lose less energy when bouncing of small conductive objects.

Frequency vs. Mineralization: Think of highly mineralized soil as consisting of more ferro-magnetic particles that would tend to scatter the small RF signals even more than non-mineralized soil similar to increasing density. Therefore, the lower frequency signal would penetrate better under these soil conditions since the mineralization would tend to exacerbate the signal attenuation of the higher frequncy signal.

These are generalizations and are subject to other effects, but I think it illustrates the point.

Bottom line you can make the following generalizations but as with all metal detecting advice - your mileage may vary because there are just too many variables to account for :

Generally use lower frequencies for max depth.

Generally use lower frequencies for high conductive targets.

Generally use low frequencies in highly mineralized soils.

Generally stay away from the highest frequency in trashy conditions.

Generally use high frequencies for low-conductive, small object detection.

Like I said, 8 and 12 khz are great starting, middle of the road frequencies that will cover most situations. Switch to the extreme frequencies for the above situations if 8 and 12 are not cutting the mustard for you or you have highe EMI at those frequencies.

Hope this helps.

 

[Jeff H]

Excellent explanation vferrari. I couldn't have explained it better myself. Actually, I couldn't have explained it at all without doing a LOT of homework first.

 

[rick67]

Ouch!

 

[Geochem]

Influence of soil properties

Great write up on frequency, vferrari.

As an addition, the soil properties have an impact on how electromagnetic waves (EM) travels through soil. Soil is a triad primarily of solids, liquid, and air, with solids being organic and inorganic.

EM travels better through air than soil, due to as vferrari mentioned, the damping/attenuation of EM in soil, since soil is an arrangement of particles poorly touching each other with pores of air and often water filming the particles and filling a degree of pore spaces.

The below figures were taken from a EM transmission in soil medium paper. It showed transmission reduction is mostly due to EM frequency and soil water content. With water taking over dominance of reducing EM travel above about 15% bv. The circled regions apply most to MDing.