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

[green]

I made two more plots. The coil level signal plotted against distance for a nickel with a 5 and a 9.5 inch diameter mono coil. The 9.5 inch plotted at two current levels. The average current for the 9.5 inch is the same as the 5 inch coil. I've seen you should be able to detect a nickel at 15 inches more than once on this site. I'm not there yet. The curves indicate I need a noise floor less than one micro volt. I'm wondering if that is typical or are my amplitude measurements not correct. The other is a time constant plot for some different targets, all plotted at one inch above the coil. Should I expect the difference at one inch to be the same at other distances?

[Tinkerer]

I made two more plots. The coil level signal plotted against distance for a nickel with a 5 and a 9.5 inch diameter mono coil. The 9.5 inch plotted at two current levels. The average current for the 9.5 inch is the same as the 5 inch coil. I've seen you should be able to detect a nickel at 15 inches more than once on this site. I'm not there yet. The curves indicate I need a noise floor less than one micro volt. I'm wondering if that is typical or are my amplitude measurements not correct. The other is a time constant plot for some different targets, all plotted at one inch above the coil. Should I expect the difference at one inch to be the same at other distances?

Thank you for the great graph and all the work you put into it.
http://www.miscel.dk/MiscEl/miscel.html .........this calculator is very useful for calculating the coils
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html ............and this calculator is very useful for calculating the field strength at a distance from the center of the coil. Note, you must use the number of turns.

For detecting a nickel at 15" I suggest using a 15" diameter coil.

[Aziz]

The "GOLD-Coil"(c)(r)(tm)

Hi all,

I'm naming the seperate TX/RX concentric co-planar PI coil (large TX, small RX) the "GOLD-Coil"(c)(r)(tm).
It is especially designated for GOLD prospecting on highly (iron) mineralized ground.
It goes deeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep.

(Ok, my special mates in Oz needs some cooling. Freeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeze! )

Cheers,
4212

[6666]

(Ok, my special mates in Oz needs some cooling. Freeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeze! )


Thanks Aziz, today is the 4th day in a row of 42'c +

[green]

Why such a big difference? I measure 3.5 mv at 1 inch for a 20 mm ring with a 5 inch coil. Aziz predicts 100 mv at 2 inches for a 20 mm ring with a 10 inch coil.

[Waikiki_Sweep]

Hi all,

I'm naming the seperate TX/RX concentric co-planar PI coil (large TX, small RX) the "GOLD-Coil"(c)(r)(tm).
It is especially designated for GOLD prospecting on highly (iron) mineralized ground.
It goes deeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeep.

(Ok, my special mates in Oz needs some cooling. Freeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeze! )

Cheers,
4212

That is boring really. Nobody objects my concept of BIG TX / Aziz "GOLD-Coil"(c)(r)(tm).

Do I have to move from defense to prosecution side???

- Dear Jury Members,

What if highly powered big TX coil will magnetize all that iron and turn it into huge permanent magnet?

[Aziz]

Why such a big difference? I measure 3.5 mv at 1 inch for a 20 mm ring with a 5 inch coil. Aziz predicts 100 mv at 2 inches for a 20 mm ring with a 10 inch coil.

Green, don't forget that I'm using a very high frequency induction (1 MHz). You can't compare the simulation model with your measurements. All my coil simulations models are made for 1 MHz to avoid a lot of leading zeros.
Aziz

[Tepco]

What you can expect in reality is: if “end range” detection of gold ring is possible at 30cm using 10in coil, two coil system, 10+20in and doubling peak current will provide some 36-37cm detection in same configuration, measured, not calculated. May vary slightly for different object shapes, sizes and materials. For ring sized objects, over 20%, but not more than 30% improvement.

[Aziz]

That is boring really. Nobody objects my concept of BIG TX / Aziz "GOLD-Coil"(c)(r)(tm).

Do I have to move from defense to prosecution side???

- Dear Jury Members,

What if highly powered big TX coil will magnetize all that iron and turn it into huge permanent magnet?

The coil concept isn't really novel. It's indeed very very old. It is merely based on "Ground Loops" and we (on the other forum) have discussed it to death. And this concept can't be patented. *LOL*
Aziz

[green]

Green, don't forget that I'm using a very high frequency induction (1 MHz). You can't compare the simulation model with your measurements. All my coil simulations models are made for 1 MHz to avoid a lot of leading zeros.
Aziz

Thanks for the reply. If I look at your 10 inch diameter curve at 1 inch and 15 inches there is a little over a 1000 to 1 decrease in signal. If I divide 2 mv I get for a nickel at 1 inch by 1000 I get 2 uvolts. a little over what I was predicting by extending with a straight line. It looks like if I use your curve the micro volt noise floor is about right for a nickel at 15 inches with a 10 inch diameter coil.

[Waikiki_Sweep]

Sure, I saw it years before. Anything new is a well forgotten old.

I saw one guy with old PI detector having small coil and small scoop
and digging targets pushing his hand with scoop into sand almost up to the shoulder
after whole squad of detectorists armed with Whites Dualfields left the area finding nothing.

I realized that it was old PI detector with concentric coil - 9" TX and very small RX.
Still looks fantastic to me.

[PATCHES JUNIOR]

You blokes never cease to amaze me with your technology.

[Aziz]

Hi all,

we can make up a list of benefits for the "GOLD-Coil"(c)(r)(tm) (not complete):
- low EMI noise pickup
- low ground response pickup
- passive depth improvement (with large TX)
- active depth improvement with more TX current (and large TX)
- more inductance for the RX coil (reducing amplifier noise)
- thin wire for RX, thick wire for TX, overall low capacitance for the RX front-end
- low inductive coupling coefficient between TX & RX, RX front-end switch can be realised with low voltage fets, or the TX section can be driven with much more flyback voltages
- simple circuits (RX galvanic decoupled from the TX)
- magnetic field refraction low in the RX region (center) on super hot mineralized ground -> sensing deep going magnetic fields (note: super hot mineral ground is acting like a magnetic shield, particularly, when the magnetic fields aren't perpendicular to ground surface)
- improved pin-pointing (due to small RX)
- detecting small targets (due to small RX)
... and so on (there might be more, feel free to add)

Cheers,
4212

[Tinkerer]

What you can expect in reality is: if “end range” detection of gold ring is possible at 30cm using 10in coil, two coil system, 10+20in and doubling peak current will provide some 36-37cm detection in same configuration, measured, not calculated. May vary slightly for different object shapes, sizes and materials. For ring sized objects, over 20%, but not more than 30% improvement.

Increasing the TX coil diameter by a factor of 2, increases the field area inside of the coil by 4.
But we need to increase the peak TX current by a factor of 2 to maintain the same field density or target sensitivity. (also enters the rate of change in consideration, but not considered here)
The sensitivity to small targets is related to the area of the coil. Common numbers found are 2% of the coil diameter, but I have no information where this number actually comes from or how real it is. It would be useful to produce a graph to get solid numbers on that.
Skin effect.
The largest % of partial response of the targets is from the skin eddy currents. This is directly related to the surface area. We can visualize it by imagining a number of field lines cutting across the target.
What exactly is the % of skin eddy current response? We don’t seem to have any numbers yet.
Back to the coil field. When we maintain the same inductance in the coils, say 300uH, the 10” coil has about 22 turns and the 5” coil about 34 turns. Field=ampere turns, therefore we must increase the amperes by a factor of about 1.54 to get the same ampere turns at the same 300uH inductance.

It gets more interesting with the distance from the coil.

The better part of the coil sensitivity is within about the distance of one coil diameter.
We use the coil at a distance of about 2” from the soil, so we already lose a good part of this with a 5” coil, but for a 10” coil it makes little difference.
The advantage gets even bigger with the increase of depth. Using the Hyperphysics calculator, we see that the 5” coil produces a field density at 5” into the soil (2”+5” from the coil) of 0.1943 Gauss.
The 10” coil at the same depth gives us 1,0499 Gauss
Even if we only use the same ampere turns we still get 0.5240 Gauss.
One question arises: Is the relationship of the coil area/target area/sensitivity, still the same?

One more question begs for an answer: The earth’s magnetic field is about 0.5 Gauss. In some regions the earth field is nearly parallel to the earth surface. This means that with the coil field we change the field within the target from a parallel field of 0.5 Gauss to a vertical field of 0.5 Gauss and back again. We know that the changes in the field generate the eddy currents.
What happens in a region of ironstone?
In these areas the earth’s magnetic field is near vertical. The coil field is also near vertical, therefore we have not the same change of field in the target and not the same response.

Caution: I have written the above “ad hoc”, in the spur of the moment. There may be mistakes, but the intent is to add something useful to the discussion.

[Waikiki_Sweep]

While you keep coil flat and move coil in one plane Earth magnetic field makes no eddy currents in any place on globe.
To get those eddy currents from moving coil you have to change an angle to magnetic lines entering coil - tilt coil or rotate it.

Sure guy with shaking hands will generate some electricity in his coil.
So prevention of Earth magnetic field influence is still required.