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

[Tinkerer]

I plotted some distance data for a 5 inch mono coil. I used a US nickel and some squares cut from an aluminum pop can for targets. I need to improve my setup but I think the trend is correct. The coil volts is measured volts/ gain. If it doesn't make sense let me know.

Thank you for the excellent work and graph.
3 of your targets are characterized by high conductivity, skin effect and surface area. The Nickel is a short TC target due to its lower conductivity. It is also much thicker so the eddy currents can penetrate much deeper. It would be very interesting to find out what the relationship is between the surface area of a skin effect target and the surface area of the coil field.

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Using the calculator above, I get the numbers to estimate the field strength at a distance from the coil. If we also factor in the surface area of the skin effect targets we might get some interesting numbers.

[Waikiki_Sweep]

Thank you for interesting discussion!

By the way I have some interesting facts about EMI noise:

- Most powerful source of EMI noise in your home are luminescent light bulbs. They emitting like thunderstorm. Get rid of it or turn it off when working with your metal detector.

- Interesting that water shields EMI noise completely. I am so happy to see it because I am searching in salt water only.

- If you point your coil center to nearby TV or radio station in your town you will see blurring decay curve of your RX signal on oscilloscope because if high frequency interference so keep coil in flat position.

- If you want to get rid of EMI noise completely and work to adjust your detector at home just made "8" shape testing coil. It cancels all noise and magnetic fields around you.

- In QED project you can see LME49990 opamp with very low noise (0.9nV/√Hz) http://www.ti.com/lit/ds/symlink/lme49990.pdf it is four times less than ne5534 noise at 3.5 V/√Hz

[Waikiki_Sweep]

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html
Using the calculator above, I get the numbers to estimate the field strength at a distance from the coil. If we also factor in the surface area of the skin effect targets we might get some interesting numbers.

Sorry I can't find that calculator by the link.

At lease I have found a formula to calculate field strength with distance from the coil:

Graphic illustration of decreasing field with distance from coil for favorite Aziz coil sizes 10", 20", 40" would be very educational.

[Tepco]

http://hyperphysics.phy-astr.gsu.edu...curloo.html#c1

[Waikiki_Sweep]

http://hyperphysics.phy-astr.gsu.edu...curloo.html#c1

Results are amazing !!!!!

Comparison of two TX coils making same magnetic field strength: 0.628 Gauss.
First coil 8" diameter (10 cm radius) 10 Amperes current.
Second coil 16" diameter (20 cm radius) 20 Amperes current.
You can see that to keep same magnetic field strength you need only twice more current to increase diameter of coil two times ( Linear !!!!! )

magnetic field strength

depth 8"coil - 16"coil -- gain --- Graphic illustration of achieved gain
(cm)

0 ---- 0.628 --- 0.628 --- 1 : 1
10 --- 0.222 --- 0.45 ---- 1 : 2 ------
20 --- 0.056 --- 0.222 -- 1 : 4 -------
30 --- 0.02 ---- 0.107 -- 1 : 5.4 ------
40 --- 0.009 --- 0.056 -- 1 : 6.2 ------
50 --- 0.0047 -- 0.032 -- 1 : 6.8 ------

Can you see it !!!!!!!

If you just rise TX diameter twice from 8" to 16" and increase current of impulse in TX two times from 10 Amperes to 20 Amperes
buy longer impulse or thicker wire or higher power supply voltage you can create almost 7 times
stronger magnetic field on the depth 50 cm.

7 times gain for signals from your targets!!!
A fantastic depth increase!!!

It is hobbits "wrong way" only because respectful corporations will be avoiding anyway to build ridiculously looking huge coils with half meter TX even if it gives huge depth benefits.

Goodbye monocoil

[Tinkerer]

Waikiki, you got it.
Now take a 40cm diameter, 300uH, TX and a 20cm diameter 450uH RX. You get good pinpointing good sensitivity and good depth.

If you feel you want to spend some time experimenting, I am trying to put a team together for designing a PI with GEB and target ID, based on TC of target. Also FE discrimination of course.

[Aziz]

I'm getting ultimate rich now! *LOL*

Hi guys,

I have good news for gold prospectors on heavy iron mineralized gold fields.
(I'm taking 10% of all the gold finds for my effort. *LOL*)

I'm referring to a 10 inch and 20 inch coils (all normalized to 300 µH).

10 inch mono coil -> 20 inch mono coil:
If we use a 10 inch mono coil and change it to 20 inch mono coil, we get approx. 2.7 times more EMI noise (if present).
If we operate the coils at z=2 inch (typical operating height of the coil above ground), we get approx. 1.4 times more ground response from the uniform iron mineralized ground.
The EMI noise might be absent but the increased ground response on the gold fields may limit the coil size.
The ground response factors are calculated from the relation of induced magnetic flux (RX) at z=2 inch.

20 inch mono coil -> 20 inch TX/10 inch RX (seperate TX/RX coil):
We get 2.7 times less EMI noise with the smaller RX coil.
Or the same EMI noise compared with the 10 inch mono coil.
We get 2.85 times less ground response with the smaller RX coil (at z=2 inch).
Or we get 2 times less ground response compared with the 10 inch mono coil.

I'm curious, whether the gain factor of approx. 2.8 will compensate the detection depth losses from the 20 inch mono coil -> 20 inch TX/10 inch RX seperate coil. Stay tuned..

Cheers,
Aziz

[Aziz]

FREE LUNCH!!! I'm getting ultimate rich now! *LOL*

I'm curious, whether the gain factor of approx. 2.8 will compensate the detection depth losses from the 20 inch mono coil -> 20 inch TX/10 inch RX seperate coil. Stay tuned..

YEAH!, FREE LUNCH!!! Yummy yummy! Bon appétit!
I'm getting ulti-ultimate rich now! *LOL*
(And don't forget my 10% share! )

Yeah, the seperate TX/RX clearly outperforms the same size mono coil.
Be patient for the nice results please.

Cheers,
Aziz

[Aziz]

Results and Excel Table

Hi all,

this is the result of the EM simulation analysis.

Note:
Coil Current = 1A, Coil = 300 µH, Frequency = 1 MHz, Target = 20 mm diameter wire ring, target response along the coil center axis (Z-axis). I take the high frequency induction (1 MHz), because I don't want to see a lot of zeros in the response figures. Also notice, that large coils will have low target response along the center-axis. Small targets will be usually detected on the coil edge (not taken into account). We are focussing to the depth detection only.

The detection depth improvement is dependent on the noise floor level (SNR) of the front-end system. The higher the SNR or the lower the noise floor level, the higher the detection depth improvement. You usually have somewhere your noise floor level (where the targets can't be detected and the noise chatter begins). If you draw a horizontal line at this level, you can see the possible depth improvement for the coil and specified target (20 mm diameter wire ring).

The increased EMI noise and ground response hasn't been taken into account for the 10", 20", 30" and 40" mono coil comparison. So large coils will usually have a larger noise floor level and you will have lower detection depth improvement at the end.

Now to the plots:






When I make use of the 2.8 times benefit (low EMI/low ground response) I get better results with seperate TX/RX coil.


This is the Excel file for your convenience (zipped):
CoilComp6(CoilCMP3)-01.zip


Cheers,
Aziz

[Davor]

OK now, and how about some off-axis Tx that is also not in the same plane and at an angle, like some kind of quasi-orthogonal setup. I'd say that you'd be able to equalise response for a large span of depths, while simultaneously the large Tx would not be clumsily on your way when scanning the surface.

[Aziz]

OK now, and how about some off-axis Tx that is also not in the same plane and at an angle, like some kind of quasi-orthogonal setup. I'd say that you'd be able to equalise response for a large span of depths, while simultaneously the large Tx would not be clumsily on your way when scanning the surface.

Hi Davor,

this is in principle possible (both IB and PI type). You have to make the TX coil very large and have to operate it with much more current however (but if you double the TX coil current, you get four times more headache *LOL*). IB type would be quite critical and the coil would be very heavy to make it stiff and stable. The average TX coil height is quite high and it makes it not much efficient and attractive for us.

I for one would use the "Top-Hat"(c)(r)(tm) IB AI coil (induction balanced, anti-interference coil). This is the "World's Best Coil Ever"(c)(r)(tm). *LOL*
Well, it is using the same principle like the seperate TX/RX coil design.

Cheers,
Aziz

[Silver Dollar]

While I was working in underground radar we made a setup with 5 antenna's and did some experiments
with steering the beam. We could fire individual TX's and listen to each RX. We had to time correlate the
data but were getting some good results. Not sure how far the field will go on a MD though...

The "TOP HAT" sounds good but a bit much to lug around!?! Of course a 5 coil detector would be tricky too!

We also made an EM device with 3 coils but the case was Royalex and after a while in the sun would grow 3/8"
it threw the cal way off!

[Davor]

Aziz, I'm not sure a slightly bigger Tx coil that is off-axis, and thus not providing any friction against grass, can be much of a problem. What I expect it to do is a bit more of the good stuff, ironing the shallow responses and keeping the good deep responses. It does not have to go far away from Rx as in 2box, but only as much as the favourable ironing effect is observed.

[Funfinder]

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

It is not just a question of size!
If the electronic can't filter out the real from the wrong signals it results in huge depth losses.

[Aziz]

I'm Desperate & Down!

What's happened again?

The forum-trolls and sceptics do not criticize the seperate TX/RX coil design. I'm again missing the statement, that it won't work. Meanwhile, I'm fed up eating all the "free lunch".

There are more obvious benetifs with the seperate TX/RX coil design.

Cheers,
Aziz,
got fat with free lunch *LOL*