the relationship between the coil diameter and detect distance in PI metal detector?

[lou0908]

hi
I want to know the approximate relationship between the coil diameter and the induct distance which the PI metal detector can detect. Are there some related theoretical knowledge about this relationship I mentioned above. if so, can you recommend or explain some to me, thank you very much .
regards

[Waikiki_Sweep]

Sure with bigger coil you can get deeper targets. But size of the target is important too. Coin and coca can will be detected from three - four times different distance.

If you seeking for coin size target overage size of coil is near 12 inches and depth up to 12 inches. (Motion detectors)

For big targets (cannon ball) it may be a one meter coil with depth up to 2 meters or so. (Non motion detectors.)

Also metal composition of target may change the detecting depth. Gold and other low conducting metals require short delay to be detected from longer distance.

Also interference of the ground or water, especially salt water or EM interference turns rising size to be useless and in such conditions smaller coil may have better signal to noise rate and as result seek deeper.

Coil with bigger size may create a weaker magnetic field and make that distance shorter if you did not increased energy of impulse. (High voltage, longer impulse, lower resistance or wires)

You can imaging that spreading of magnetic field around coil (or find a pictures made by Aziz). You can see that field circling around wires. If coil small those circles are small too. Field is not spreading far away.
You just can not send that magnetic field away like radio wave or laser beam. That is a problem why to go deeper you need bigger coil.

And second problem is a noise that masking signal while you need to amplify your signal in 1,000,000 times or more to go deep enough.

All that metal detecting science is applied to fight that problems.

It is my, practical, point of view but I am sure many members of the forum are ready to give you mathematical formulas to describe that dependence of size and distance.
So you can ask them or wait if someone will take a minute to write it. Also you can find that already explained on the forum because members talking about it for last ten years.

[dfbowers]

The optimum coil size for a particular object is one where the object is detected at 1/2 the coil diameter. In other words, a coil larger or smaller will not detect the object any deeper. Eric Foster posted a graph some time ago that showed the relationship.

[Waikiki_Sweep]

"1/2 the coil diameter" is probably worst case scenario. Properly assembled detector with good coil makes it "up to one diameter".
Also it depends for what we searching for. Small gold chain you can rob on coil and get no beep at all.
By reading findmall forum you can find some another formula that depth for Minelab Excalibur is "a coil diameter plus two inches".
Also depth on air and in real ground may be significantly different.

[Reg]

Waikki Sweep,

I think you misunderstood the half diameter concept. The general rule is if some particular object can only be detected to a depth equal to 1/2 the diameter, then selecting a different size coil won't allow you to detect that object deeper. If that object can only be detected to less than 1/2 the diameter, then using a larger coil will usually result in less depth of detection.

If you can detect an object deeper than half of the diameter, then a larger coil will probably allow for greater depth of detection. Here is the depth chart mentioned.

Reg

PS: Here is Eric's explanation on how to use the graph.

Here are the curves I have used for many years. As Robert said, the range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.
If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it won’t be much, i.e. going from 11in to 8in will only make 0.5in difference but below 4in diameter, the range will drop rapidly.
Now, suppose with the 11in coil, you can detect an object at about 12.5in (B on the vertical scale. This indicates that the coil is not an optimum size for that particular object. If we carry on up the curve (direction of arrow) we can see that by using a 20in coil, we could gain another 2.5in (C). The curve peaks at 15in with a 30in coil. But the extra inch gained hardly makes such an unwieldy coil worth while.
Other factors come into play of course. The curves assume that the number of turns and the coil current is the same in all cases; which it isn’t necessarily. For the same inductance value, a smaller coil has more turns, which counteracts to some degree the loss in range. Also a smaller coil will pick up less electromagnetic noise, earth’s field noise and ground effect, which make for a smoother threshold.
The end result is, that with a small nugget that can be detected at between 5 and 7in with the 11in coil, so that it is on the top part of the curve, an 8in coil may well give a similar range. That is not to say that smaller coils do not have other advantages. Small coils and probes are very useful in rocky areas or searching in undergrowth. They have less drag too for water hunting, and less pickup from mineralised soil or conductive sea water plus better signal separation on close or multiple objects.
One other point regarding PI, is that the small object sensitivity is largely determined by the sample pulse delay. If an object is so small, or thin, or made of high grade stainless steel, such that all the signal has decayed before sampling takes place, it would not matter how small a coil you made, it would never be picked up.
Bed time problem for the depth junkies – If the Aquastar with a 10in coil can pick up a ring at 20in, what is the optimum size coil for maximum range, and what range could theoretically be achieved? That’s the depth at which a mere pulltab could trigger suicidal inclinations
Eric.

[Waikiki_Sweep]

Thank you for the explanation. Now I get it. I have to agree with that empiric graph build on great experience!

But also I can tell that graph probably based on situation when you changing only coil and not adjusting anything in electronic part.
Probably when you install bigger coil you creating wider but weaker magnetic field because same power spreads on bigger area.

So it is good idea to correct strength of magnetic field by pumping much more power into impulse for bigger coil.
If you able to keep same strength of magnetic field with bigger coil field goes proportionally deeper as you can see it geometry spreading down.
So it will give you more depth for same coin size target. Sure noises and ground conditions will oppose but it different for different spots.

I remember I have tried bigger coils and was dissapointed by decreasing of depth. But cranking the impulse length I have make it much better and preferring big coils now.

Also big coil has a big advantage that you can cover much area in one sweep so you can get more items/per hour
It is very good for over-hammered by competitors huge ocean beaches where you can run 20 minutes to hit first target.

Just try to make a coil light weight because bigger coils tend to be heavy and get you tired fast.

Also I am using alternative geometry of the coil: 17" long 9" wide. Because that long coil has a smaller area than let say 17" round coil in comparison
so I do not need to pump super power to keep magnetic field strength and it has more turns for same 300uH and gets less noise as result of less total area.
That coil helps me to cover area better as I usually making 17" steps.

[Waikiki_Sweep]

Also configuration with big TX coil pumped with high power and sending magnetic field to the great depth and small RX coil that has more turns and gets less noises may be a good solution to get much deeper if depth is needed more than coverage. I am not working in this direction because of my local conditions but probably it is good idea to build a "Deep One" some day

[mohandes]

The optimum coil size for a particular object is one where the object is detected at 1/2 the coil diameter. In other words, a coil larger or smaller will not detect the object any deeper. Eric Foster posted a graph some time ago that showed the relationship.

hi
can you put this graph or tell more
regrads

[Aziz]

EM Numerical Simulation

Hi all,

that's really an interesting question to investigate it further. When I find more free time (in a few days hopefully), I'll make an EM numerical simulation for a 10", 20", 30" and 40" diameter round mono loop coil for a 20 mm diameter closed loop round wire target.

I'm too lazy (& out of blue pills ) to make the theoretical and analytical math model right now.
Stay tuned.

Cheers,
Aziz

[Aziz]

Hi all,

well, I'm going to make the coil size/detection depth comparison for a fixed (TX) coil inductance (normalized to 300 µH) and a fixed (RX) EMI noise induction contribution (N*A = const, where N=turns count, A=coil flux surface area = pi*R*R, where R=coil radius). You know, that small coils will pick up less EMI noise and hence we can increase the gain of the amplifier or setup the threshold to more sensitivity. Small coils will have more turns count for a given inductance as well.

I'll make the coil simulation on seperate TX/RX coils in my software. The TX coil will be normalized to 300 µH (fixed). The RX coil inductance depends on the EMI noise induction. As we get less EMI noise with small coils, we can rise the gain and this is made via more turns count in my coil software. This is just a workaround for a mono coil simulation. But makes my simulation life in the matrix much easier. My reference coil will be a 10 inch round mono loop coil.
Note: The ground mineralization & ground noise won't be taken into account.

This numerical coil simulation comparison has never been made before. But it is quite accurate to take these effects into account.

So stay tuned...
(Where are my blue pills now? I hope a lot of beer would suffice too... )

Cheers,
Aziz

PS: Food for the forum-trolls/ML jihadists:
Who is the "World's Best & Smartest Coil Guru"(c)(r)(tm) ever? *LOL*

[Waikiki_Sweep]

Great God!!! Finally I got the attention of Best Coil Guru in the Universe!!!

OK. Magnetic field is not spreading as a radio wave or laser beam. Probably even kids knowing it.
You know how fast magnetic field strength decreasing with distance from the coil.
That is why we have so small signal from target so we have to amplify it in millions times.

TX signal quickly decreasing on the way down than target signal drastically loosing power on its way back to coil.

But you can see that coils with bigger diameter has larger magnetic fields going deeper. You can not increase RX or MONO coil size without limits
because you need it do detect targets of particular size, also noises, ground effects and earth magnetic field will give you a lot of troubles.
Let say that RX or mono coil have to be near 10" for coin size targets.

But you can increase TX coil size with less difficulties. Only weakening magnetic field is a problem for bigger TX coils but you can pump it with Amperes.
If it becomes slow probably additional electronic can short coil in proper time to makes decay faster.

If you can imaging TX coil that has infinite size and create magnetic field same strength as a 10" MONO coil and that field is
uniform in shape you will see that field going down to any depth without decreasing (sure it is only theoretical step for the illustration)

So now we have only RX signal decreasing with distance not TX.
As result all the amplification, noise, e.t.c. are the same as for 10" monocoil but because deeper targets are excited by global TX field and returning stronger signals you can detect it on extra depth.

Illustration:

It was theoretical because to create infinite uniform magnetic field same strength as 10" monocoil gives you you will need infinite energy source and have other technical difficulties.

But for practical realization you can just make TX coil some bigger than 10" RX and pump more current into it to keep same magnetic field strength deeper.
As result you will have deeper detection for particular target depending on size of TX coil you build and TX power you can use.

If you have good power source why not to use that idea to get several more inches in detection depth?

[Aziz]

Great God!!! Finally I got the attention of Best Coil Guru in the Universe!!!

OK. Magnetic field is not spreading as a radio wave or laser beam. Probably even kids knowing it.
You know how fast magnetic field strength decreasing with distance from the coil.
That is why we have so small signal from target so we have to amplify it in millions times.

TX signal quickly decreasing on the way down than target signal drastically loosing power on its way back to coil.

But you can see that coils with bigger diameter has larger magnetic fields going deeper. You can not increase RX or MONO coil size without limits
because you need it do detect targets of particular size, also noises, ground effects and earth magnetic field will give you a lot of troubles.
Let say that RX or mono coil have to be near 10" for coin size targets.

But you can increase TX coil size with less difficulties. Only weakening magnetic field is a problem for bigger TX coils but you can pump it with Amperes.
If it becomes slow probably additional electronic can short coil in proper time to makes decay faster.

If you can imaging TX coil that has infinite size and create magnetic field same strength as a 10" MONO coil and that field is
uniform in shape you will see that field going down to any depth without decreasing (sure it is only theoretical step for the illustration)

So now we have only RX signal decreasing with distance not TX.
As result all the amplification, noise, e.t.c. are the same as for 10" monocoil but because deeper targets are excited by global TX field and returning stronger signals you can detect it on extra depth.

Illustration: [ATTACH]27823[/ATTACH]

It was theoretical because to create infinite uniform magnetic field same strength as 10" monocoil gives you you will need infinite energy source and have other technical difficulties.

But for practical realization you can just make TX coil some bigger than 10" RX and pump more current into it to keep same magnetic field strength deeper.
As result you will have deeper detection for particular target depending on size of TX coil you build and TX power you can use.

If you have good power source why not to use that idea to get several more inches in detection depth?

Hi Waikiki_Sweep,

yep, I have been saying the benefits of split TX/RX coil for long time now. You are one of the few, who has recognized the benefits. (All the others: keep sleeping, don't change things, keep sleeping... )

The latest EM numerical simulations even show, that a large mono coil (20") isn't performing better than a splitted TX (20")/RX(10") coil with equivalent EMI noise induction. The splitted TX/RX coil has additionally a better pin-pointing feature and will have less ground response too.

(Thermo-nuclear melt-down!!!!! Ooops, keep sleeping guys...)
Cheers,
Aziz

[Aziz]

Missing My Forum-Trolls!

Oh man!,

where are my special friends when I need them? I'm missing them so. They don't motivate me anymore.
I'm missing such like:
"No, it won't work!"
"No, there aren't any benefits!"
"No, you fail!"
"No, it's a copy of ML's IP!"
"No, you can't beat ML!"
"No, forget it!"

What's happened to my special mates?

4212

[Waikiki_Sweep]

Yep. Sleeping on the edge of coma.
I bet engineers knowing that but companies just don't need it. Hooking a monocoil is a costs saving solution.

Why not to continue to make a profit by selling 20-30 years old technology and advertise it as a "life style" to metaldetectorists.

Also battery life (AA) is so precious (I am not even telling about 9V) nobody wants to switch to Li-Ion for example to kick it with more power.

[Silver Dollar]

Well I just want the best I can make for the Detector I am building so I went with a center tapped RX on my VLF.

I'm still not clear on how to implement a PI coil. I made a twisted pair teflon coated loop so I could center tap it.

Are you suggesting a separate loop for the TX? I know various things were tried on different threads but I'm still

foggy on the best way to go here...