An impossible and insane thought ???

[mcgov51]

Large coils generally offer greater depth but have great difficulty seeing smaller targets.

Small coils see small targets very well even in amongst the trash but often lack depth.

Wouldn't it make sense for the next phase of coil design to be . . . to supercharge (Boost)
the performance of the smaller coils to 2X or 3X normal for much greater depth.

With their small footprint it should not create an excessive feedback situation but just to be
safe and if it proves to be necessary . . .

Why not experiment with built-in noise traps/filters, right onto the Coil RX itself
OR in-line (adjustable or even programmable) along the coil cable before it even reaches the PCB?

I know if my stock 5 X 8 coil were BOOSTED today to 2 X as deep I could currently and probably
adjust for any increase of noise and still run smooth enough . . . without a filter but . . .
Any more than that . . . i.e. 3X ??? . . . I would probably need design help. Who knows ?

A design such as this would be a real game changer.

In-Line Noise traps and filters have been around for 80 years or more.

Seems like most MD Manufacturers have been very lazy and slow? to adapt these improvements.
A lot of MD's models are still in the Very Expensive and Glorified Stud Finder Category ( ML Excluded )
. . . even after 80 years of evolution . . . but that's just me.

[Pennypacker]

Boosting your depth to 2X requires significantly more power.
http://en.wikipedia.org/wiki/Biot%E2...stant_velocity

Too much power can even hurt your ability to separate targets from iron etc.

A real advancement would be a detector that used less power.

[Skippy]

Quote:"boosting... smaller coils to 2X or 3X normal for much greater depth". If you boost gain by 2X, your raw airtest depth increases by about 13%, 3X gain gives about 22% airtest depth increase. In-ground, depth increases will be smaller, between +0% and the air figure. There possibly is some slight advantage to be had, particulary for the smallest coils, eg. a 5 inch one fitted to a machine optimised for a 9 inch coil. But it depends on the machine's design, and the ground. Very mild ground is more likely to give worthwhile results.

[Carl-NC]

Sorry McGov, but you have an overly simplistic view of the problem, which leads to a non-solution. In-line filtering ain't the solution. We're trying to hear someone whisper a secret while standing next to a locomotive that is blowing its air horn. Filter the air horn, and the whisper is gone, too.

It's easy to say, "In order to get twice the depth, all we have to do is double the depth." The hard part is doubling the depth.

[mcgov51]

No comment.

[bbsailor]

The answer in theory is to consider this in free air to not introduce the variability of ground. Consider the pulse path from the coil to the target as 2 to the 3rd power increase and then consider the eddy currents in the target making the return trip back to the coil or another 2 to the 3rd power increase for a total of 2 to the 6th power or an increase of 64 times.

To double the depth in free air you need to increase the power by 64 times. That is why if you know something about the size, metallic composition and total decay time of your primary sought targets, you can work backwards to optimize the: (1) coil size, (2) coil current and power, (3) RX gain and (4) maximize the signal to noise ratio for the best response for that particular target. As solid date devices improve performance with lower noise, you can obtain incremental depth improved advantages of a few percent.


Eric Foster optimized a pulse machine to seek out silver bars underwater and was using very, very, very large TX currents and RX delays of 250 us up to over a millisecond (1000us). http://www.geotech1.com/forums/showt...ht=ferric+toes

bbsailor

[mcgov51]

bbsailor - At least that gives me something more to consider, that I didn't know before.
The idea is to simply stimulate a dialogue amongst the best of the best that visit this site.

64 times the power you say . . . OK? maybe that is why ML uses a lot of high powered Lith Ion . . . ?
I have said, with my simplistic mind, years ago . . . it takes more power and a Matched / Optimized
TX & RX SET to do the job.

Noise - Is it not true, using todays technology, that just about any noise can be isolated and Boosted or SUPPRESSED.

Let's hear some daydreams from some of the lifelong MD Design Engineer types. I say,let the experts talk, not the salesmen.

What's possible?
Todays technology is still in its infancy but advanced enough that I can say . . . I have seen many examples that prove . . .
If you can dream it then it can be done. Especially when it comes to basic electronics.

[Pennypacker]

bbsailor - At least that gives me something more to consider, that I didn't know before.
The idea is to simply stimulate a dialogue amongst the best of the best that visit this site.

64 times the power you say . . . OK? maybe that is why ML uses a lot of high powered Lith Ion . . . ?
I have said, with my simplistic mind, years ago . . . it takes more power and a Matched / Optimized
TX & RX SET to do the job.

Noise - Is it not true, using todays technology, that just about any noise can be isolated and Boosted or SUPPRESSED.

Let's hear some daydreams from some of the lifelong MD Design Engineer types. I say,let the experts talk, not the salesmen.

What's possible?
Todays technology is still in its infancy but advanced enough that I can say . . . I have seen many examples that prove . . .
If you can dream it then it can be done. Especially when it comes to basic electronics.

Was somebody trying to sell you something?

I'm pretty sure Carl and bbSailor are experts in their respective feilds, I find your demands kind of offensive and do not nurture innovation.

Perhaps we can get back on topic and you can discuss how your "filter" could be implimented, perhaps on an existing platform to aid in cost.

Are you implying that your filter will have the benefit of discrimination without the nulling effect that occurs with conventional discrimination? Am I understanding you correctly?

[mcgov51]

Peace. If you don't want to contribute to this dialogue then ignore the post.

[Dave J.]

The "rule of 64". It applies to both PI and VLF-IB. The transmitter is a magnetic dipole that follows a 1/3 power law. So is the metallic target itself, so going out and coming back is a 1/6 power law. Once you're no longer in close proximity to the loop (say farther away than its diameter), in order to double the "air depth" of detection for a given receiver requires 64 times the transmit voltage.

For a given transmitter, that's 4,000 times the power. Take a typical handheld metal detector with transmitter power consumption of 100 milliwatts. Now we're at 400 watts, the searchcoil will melt in about 3 seconds.

Those 3 seconds wouldn't do you any good anyhow, because the ground signal (which is usually the limiting factor in how deep you can find targets) came back 64 times as strong, so you couldn't find anything any deeper than you could before anyhow. The "locomotive blowing its air horn" problem that Carl explained. Now it's sticks of dynamite going off a foot from your ear. The fact the whispering target is now shouting won't matter.

Back in the early 1980's, I was happy to get 7 1/2 inches air test out of the 1260-X. Not bad for a motion discriminator of that era. Nowadays with the Tek and Fisher SE's the air test limit is about twice that. Every now and then I ask myself how I doubled air test depth while actually reducing power consumption, rather than running the thing on a truck battery and having a leaf blower aimed at the cooling fins on the searchcoil. The answer: half an inch at a time, fifteen times in fifteen different ways.

Other engineers face the same physics limitations and make progress that same way, sometimes including fatter batteries but not even Minelab runs 'em on truck batteries. This is why there are so many companies in the business, and when it comes to finding buried metal, no one company beats all the rest.

There are search conditions (rather common in fact) where the lowly BH Tracker 4 (air test about 6-7 inches) in the right hands will deliver usable performance in the same league as machines costing over $1,000, including ours (Teknetics and Fisher).

Can the dictatorship of "the Rule of 64" be overthrown? Well, yeah, proton precession mags defeat it, but they don't detect nonferrous metals. ECDO's (commonly used in pinpointers and industrial metal detection applications) overthrow it, but lack the benefits of induction balance or of time domain separation between transmitter and receiver. Plus try adding "features"-- so much for that! Gardiner (no relation to our engineer with a similar name) back in the 70's was the master of ECDO technology (even did multifrequency!), but having defeated the Rule of 64 wasn't enough for him to make any impact on the VLF induction balance mainstream despite the fact that VLF induction balance back then was pretty primitive.

--Dave J.

[Carl-NC]

McGov, if you have a particular filter in mind, then post the info and ask, "Would this help?" Or if you have a suggestion for supercharging coils 2-3X, then post the suggestion! Wishing up a solution to doubling the depth is something the LRL guys do, and now their devices can detect a single coin a mile away. If you read through the 12 or so years of Geotech posts, you won't find anyone who has even proposed a 2X increase in depth, beyond toting around a car battery.

I get similar wish-solutions from customers quite often. From an actual email a week ago:

"You take the White’s Dual PI and add a few needed features to it like Discrimination, Sensitivity and Threshold..."

As if all I need to do is add a knob, label it DISC, and suddenly the SMPI will discriminate. As I said, this is what the LRL guys do. Metal detector designers are at a disadvantage, cause we have to make it actually work.

[Aziz]

Oh boys!,

I have made the "rule of 64" obsolete. The "rule of 64" is a limes value (approaching 64) for detection distances > coil radius/diameter. It is achieved in the infinite distance to the coil.
Aziz

[Davor]

Gardiner ... back in the 70's was the master of ECDO technology...

I'm not familiar with the acronym ECDO. I was not around long enough apparently.

[Qiaozhi]

Oh boys!,

I have made the "rule of 64" obsolete. The "rule of 64" is a limes value (approaching 64) for detection distances > coil radius/diameter. It is achieved in the infinite distance to the coil.
Aziz

Even if the "rule of 64" does not apply for distances less than the coil radius / diameter, how is that of any benefit if you can already detect a target at a distance beyond the coil diameter?

[Dave J.]

I'm not familiar with the acronym ECDO. I was not around long enough apparently.

Eddy current dampened oscillator. Metal in the field of the coil absorbs energy, reducing the amplitude of oscillation.

--Dave J.