Understanding The Metal Detector Coil Signal.

Thank you for the information!

The more you know about your detector, the better you can make it work for you.

Thank you! It makes sense now!

Josh

ToddB64 - Excellent post! Good job and thanks.:

Not a brag but I retired from 20 years in RF microwave and I just fail to see the highlight in the peaks and metal detecting, except to say that the lowest frequency here will give more depth due to the lengthened time cycles, but you will suffer from more EMI though. Every aspect of frequencies with coils naturally have trade offs. I'm just saying that Rf signals are fun yet very diverse. jm2c

ToddB64: said:

Hi All !

Attached are a two graphic images showing sinewaves of two different coil signal frequencies. These sinewaves are representative of the coil signals produced by single frequency metal detectors like my White's Classic II 6.5 kHz frequency (wide peak-to-peak distance) and my Tesoro Compadre's 12 kHz (more narrow-compressed peak-to-peak distance).

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Good info. First, to clarify, the White's Classic II, like all the other Classic models and most White's models from about 1980 to the XLT in '94 and on to the XL Pro around 2000, were initially labeled as 6.592 kHz. Later they rounded that to 6.59 kHz and then rounded it to 6.6 kHz. It wasn't 6.5 kHz.

The Compadre, as you noted, operates at 12 kHz which was just about where the noted engineer, George Payne, determined an optimal 'general-purpose' Frequency to be for detection ability from lower-conductive gold jewelry to US Nickels and on up the Copper Cents and assorted Silver coins. That 'optimal' 'general-purpose' Frequency was 12.5 kHz.

ToddB64: said:

As you see from the graphic illustrations, when the Frequency increases, for example from 6.5 kHz to 12 kHz, the peaks become more compressed, resulting in more sinewave peaks per inch.

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Okay, but for most forks that doesn't answer much with regard to their detector performance. If you connect it with a design engineer's description it basically translates to this:

Usually the lower-frequencies may have a little better responsiveness to higher-conductive coins, such as silver or copper, and they might also handle some ground mineral better and possibly provide a little increase in detection depth.

The other extreme from a ±5 kHz operating Frequency, such as those operating with a Frequency of about 20 kHz to 25 kHz, may have a little better responsiveness to lower-conductive coins, such as a US Nickel, as well as small, thin Gold Jewelry, etc. And they might also handle some ground mineral better and possibly provide a little decrease in detection depth.

So the 'ideal' general-purpose operating Frequency would be 12.5 kHz, and since the late '70s and on to the present, the bulk of my favorite detectors have operated in the 10 kHz to 15 kHz Frequency range, making 12.5 kHz right in the middle so I am varying just 2.5 kHz either-side of 'ideal'. There is a good reason or this, and proven for me as the years and time afield have passed by.

ToddB64: said:

This information should help metal detecting newbies understand some of the effects of higher coil signal Frequencies and add to their knowledge base.

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Perhaps, but in-the-end there is a lot more to consider when we try to determine what brings about some of the best performance afield, and that really gets down to the size and shape of the search coil, and the 'type' of coil, such as a Concentric or Double-D.

Most Hobbyists do not have a clue about what a 'proper' side-view of the EMF is of those two coil types, or how different metals affect the EMF, such as a same-size and shape Ferrous or Non-Ferrous object.

A lot of things come into play to make our detector choices serve us well. And then we have to consider Circuitry design, how detectors process the ground and target signals, Recovery Speed, ability to provide quick-response and fast-recovery in heavily littered conditions, Discrimination design, etc., etc., etc.

Monte

ToddB64 said:

As you see from the graphic illustrations, when the Frequency increases, for example from 6.5 kHz to 12 kHz, the peaks become more compressed, resulting in more sinewave peaks per inch.

This information should help metal detecting newbies understand some of the effects of higher coil signal Frequencies and add to their knowledge base.

ToddB64

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Well...it's a good depiction of what frequency means, but how it affects finding stuff ... Monte provided a very good layman's explanation of that.

The phrase "peaks per inch" is interesting...let me delve into that. If you swing a 6 foot arc every 2 seconds, which is 3 ft/s, which is 36 inches every second. The 12KHz machine had 12 thousand peaks every second. So 12 thousand peaks every 36 inches. 36 inches/12000 peaks = 0.003 inches per peak. For the 6.5KHz machine the peaks are 0.0055 inches apart. Obviously we aren't detecting for anything remotely that small. Because the frequency is so high, the "peaks per inch" is irrelevant.

Like Monte said, it's really about how different metals and different soils respond to different frequencies that matters. Notice he used the phases "a little better" and "a little decrease" due to changes in frequency used. Mostly it's about getting your coil over the target, and the frequency used may get you that extra little bit of useful information.

Good information. I'll add the reason for this is lower frequencies penetrate better through any kind of medium, air, fog, dirt etc.

Here's a fun fact while we're on this topic. In the visible spectrum, the order from low to high frequency is Red, Orange, Yellow, Green, Blue, Violet. Next time you see a rainbow remember ROYGBV and notice the order of the colors.

Another fun fact: The human eye is most sensitive to green. An astute observer might wonder why then on traffic lights red was chosen for stop and green was chosen for go. If human eye is more sensitive to green then why not use it for stop which is more critical? Aside from red indicating danger in most parts of the world, it's also because red is a lower frequency and penetrates better in poor visibility conditions (fog, snow, rain etc) better than the higher frequency green which is when it's most important. Next time you're driving in poor weather and see a traffic light way off in the distance you might notice that red is easier to see than when it changes to green.

All very interesting AND over my head. My degree was in Economics. I suppose I could try and explain which detector is the better investment...but I won't bore ya.

Actually, I'm waiting for the machine that literally SCREAMS in plain English..."IT'S NOT A PULL TAB, IT IS GOLD. - DIG STUPID!"

Monkeys Uncle said:

All very interesting AND over my head. My degree was in Economics. I suppose I could try and explain which detector is the better investment...but I won't bore ya.

Actually, I'm waiting for the machine that literally SCREAMS in plain English..."IT'S NOT A PULL TAB, IT IS GOLD. - DIG STUPID!"

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I'm waiting for the x-ray goggles where you can just look at the ground and see the targets .

NickelPlate said:

Here's a fun fact while we're on this topic. In the visible spectrum, the order from low to high frequency is Red, Orange, Yellow, Green, Blue, Violet. Next time you see a rainbow remember ROYGBV and notice the order of the colors.

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Don't forget Indigo! ROYGBIV And don't start with the "they removed indigo". As long as Pink Floyd's Dark Side of the Moon has all 7 colors, that's what's official! Hahahaha.