Targets frequency response

[pebe]

Hi Mikebg,

I understand the following sentence:"The fundamental time constant,
respectively, the fundamental cutoff frequency, depend on the square of the eddy diameter. "

In the time domain, like PI, we have a few test targets for testing the TIME CONSTANTS (TC) and the detector's ability to detect targets with these time constants.
A square of 1" of Aluminium foil, of the thin household kind, has a TC or time constant of about 10us. If we cut this square in half, the time constant is reduced to 5us.
The time constants are affected by the conductivity of the target. An example is nickel that is often used in alloys for modern coins and stainless steel, that shortens the TC.
High conductivity = long time constant, but the thickness of the target also affects the TC.
Targets with less that 0.5mm thickness show a short time constant. Example is an Aluminium soda can. It gives a high amplitude response because of it's large surface area, but the TC is very short compared to a Aluminium block of the same volume.
Magnetic targets have a longer TC. Some types of detector use this feature to differentiate or discriminate FE targets. However, thin rusty pieces of steel sheet, have short TC's, so this type of discrimination is not very good for these targets.
With the TINKERERS_V1 the TC of a target behaves much like a cutoff frequency in the frequency domain. This is why some degree of target identification can be gleaned.

Tinkerer

I am familiar with RC and LR time constants (TC), but don't see how they are relevant in your text. Are you referring to 'time delay' or am I missing something? And why 'fundamental'?

[Tinkerer]

Mikebg, Tinkerer and other Gurus

The purpose of this thread is to try to identify an unknown target by some response properties that may help deduce an unknown target's identity. Because targets may lay in the ground in many different orientations, it is possible that some target responses may be ambigious requiring the target to be dug up to actually see its identity.

I am mostly a beach hunter as I live near the New Jersey Shore. I want to share some experiences and ask the active members of this discussion thread to comment and independently verify by doing a few quick tests.

I have experienced the following. When I locate a target on the beach, I visually pinpoint the target location, then move the coil about 2 inches off to the side and sweep the coil slowly over the target listening for the sound level in my headphones. I increase the sweeep speed about 3 times faster than the slowest sweep speed and listen for an increase in signal level. Most of the time the increase in signal level indicates a ferrous target like a washer.

If this technique could be verified, it may provide another tool in using coil sweep speed changes to help better deduce the identity of an unknown ferrous target. This should be tried with and without Ground Balance methods being activated.

Thanks for the interesting discussion.

bbsailor

Hi bbsailor,

thanks for sharing your experience with us.

Magnetically permeable targets, concentrate the field lines of the earths magnetic field and also change the vector of the field.
Sweeping the coil through the earths magnetic field will generate a current in the coin. If the field lines are parallel to the coil sweep, this current is practically 0.
Near the equator, in an area without iron ore bodies, the earths magnetic field lines are practically parallel to the surface of the earth.
In higher latitudes the angle increases. Near iron ore bodies the angle can be near 90 degrees.
But if the field lines are perpendicular, the coil current can be appreciable.
The faster the coil is swept across the field, the stronger is the coil current.
Even an object as small as an iron washer can concentrate and distort the earths magnetic field lines by 90 degreed in it's immediate vicinity.
Therefore, an iron target can give an increased response when the sweep speed is increased.

The following post about nulling the earths magnetic field response, has some bearings to this phenomenon.
http://www.geotech1.com/forums/showp...0&postcount=22

All the best

Tinkerer

[Qiaozhi]

The attachment below contains two impedance diagrams. In the left diagram, which is normalized, are shown OM signal across a coin and signal ON from a piece of coin cut in half as case 2 above in posting. Phases of the signals are properly presented in the normalized diagram, but not amplitudes. For proper display of amplitudes, is given the right diagram in which the frequencies and amplitudes are not relative, and each point means KHz.
Suppose that a coin has a cutoff frequency fc = 5KHz and we have a CW (Sine Induction) TX illuminating it with f = 5KHz. Then the relative frequency is F = f / fc = 1 and we receive a signal 0M with phase lag 45 deg, as shown in normalized diagram.
When the coin is cut into two halves, as in case 2 of the upper posting, the eddy diameter is twice smaller. The cutoff frequency fc is inversely proportional to square of eddy diameter. That means the new cutoff frequency is 4 times higher, ie fc = 20KHz. But TX continue to illuminate it with f = 5KHz. Then the relative frequency of case 2 is F = f / fc = 5 / 20 = 0,25 and we receive a signal 0N having phase lag less than 45 deg. A small phase lag is good when you need to suppress the signal from ferrous ground, which has a phase advance almost 90 degrees. Synchronous demodulators discriminate best when the phase difference is close to quadrature.
However, the work LF region is a disadvantage when targets are buried in conductive nonferrous ground because the ground signal also has a phase lag instead phase lead. Therefore, in saline beach is better TX to work in HF region to get closer to quadrature phase difference between the target signal and ground signal.
When TX works in the LF region of targets, the RX receives considerably weaker target signal. The left figure shows that ON <OM, but in reality this difference is much greater. If you do not use normalized impedance, we get shown the right picture, where the frequencies and amplitudes are not relative, but real. The reason for the weaker signal from a small eddy diameter is in addition due to the fact that the received signal depends on the mutual inductance between the RX loop and eddy loop in target. As smaller the eddy loop, the less is the mutual inductance.
So far we have examined the case of cut coin, but the experiment does not need to cut coins. There are coins of different diameter, made of the same metal.
Everything said here for "eddy diameter of target", goes for gold nugget. In a large gold nugget, TX illuminates it in HF region, where small - in LF region. In what region must work TX, depends on EM properties of ground. So the next posting will contain an explanation of grond signal as normalized impedance.

In short, I presume you are saying that you need to use a higher TX frequency in order to find small targets such as gold nuggets.

[mikebg]

Using of Normalized Impedance

The attachment below contains two impedance diagrams. In the left diagram, which is normalized, are shown OM signal across a coin and signal ON from a piece of coin cut in half as case 2 above in posting. Phases of the signals are properly presented in the normalized diagram, but not amplitudes. For proper display of amplitudes, is given the right diagram in which the frequencies and amplitudes are not relative, and each point means KHz.
Suppose that a coin has a cutoff frequency fc = 5KHz and we have a CW (Sine Induction) TX illuminating it with f = 5KHz. Then the relative frequency is F = f / fc = 1 and we receive a signal 0M with phase lag 45 deg, as shown in normalized diagram.
When the coin is cut into two halves, as in case 2 of the upper posting, the eddy diameter is twice smaller. The cutoff frequency fc is inversely proportional to square of eddy diameter. That means the new cutoff frequency is 4 times higher, ie fc = 20KHz. But TX continue to illuminate it with f = 5KHz. Then the relative frequency of case 2 is F = f / fc = 5 / 20 = 0,25 and we receive a signal 0N having phase lag less than 45 deg. A small phase lag is good when you need to suppress the signal from ferrous ground, which has a phase advance almost 90 degrees. Synchronous demodulators discriminate best when the phase difference is close to quadrature.
However, the work LF region is a disadvantage when targets are buried in conductive nonferrous ground because the ground signal also has a phase lag instead phase lead. Therefore, in saline beach is better TX to work in HF region to get closer to quadrature phase difference between the target signal and ground signal.
When TX works in the LF region of targets, the RX receives considerably weaker target signal. The left figure shows that ON <OM, but in reality this difference is much greater. If you do not use normalized impedance, we get shown the right picture, where the frequencies and amplitudes are not relative, but real. The reason for the weaker signal from a small eddy diameter is in addition due to the fact that the received signal depends on the mutual inductance between the RX loop and eddy loop in target. As smaller the eddy loop, the less is the mutual inductance.
So far we have examined the case of cut coin, but the experiment does not need to cut coins. There are coins of different diameter, made of the same metal.
Everything said here for "eddy diameter of target", goes for gold nugget. In a large gold nugget, TX illuminates it in HF region, where small - in LF region. In what region must work TX, depends on EM properties of ground. So the next posting will contain an explanation of grond signal as normalized impedance.

[Qiaozhi]

Eddy Current Experiments

The following information is provided as a summary of the recent experiments on eddy current generation in a coin used as a metal detector target.

This exercise was started when mikebg posted a claim that Carl's article on "Induction Basics" contained an incorrect explanation for the effect where an open round target, such as a ring, gives a stronger response than a solid target, like a coin. See post #128, and figure 1 below.

My first reaction was that mikebg's assumption (which I have termed "The Concentric Ring Theory") was incorrect. The main reason being that this theory would appear to predict a solid round target has a better response than an open round target. Now, as any metal detectorist knows, this is not the case, and (as stated previously by Carl) the ring can be detected 2 to 2.5 cm farther away than the coin. In order to prove "The Concentric Ring Theory" was indeed incorrect, I started a series of experiments, with some interesting results.

The Concentric Ring Experiment

This experiment was designed to confirm that a set of concentric rings would have an improved response as more rings were added, thus demonstrating that "The Concentric Ring Theory" predicts something different to reality. The rings were constructed from 22SWG (0.71 mm) tinned copper wire with diameters of 3.7cm, 3cm, 2,5cm and 1.5cm. They were arranged as shown in figure 2 below. The RX amplitude was recorded for different combinations of the rings. It was a simple matter to show the best response was obtained by using all 4 rings, whereas any other combination gave a lower response.

The Coin Experiment

The same measurements were then made using an original coin plus a number of modified coins. See figure 3 below.
Now this is where it gets interesting...

It is clear from the measurements that the ring (#2) has a lower amplitude than the original coin (#1). This was contrary to expectations, and raises the question as to why the ring is detectable at a greater distance from the coil. The answer can be found by measuring the phase-shift of the RX signal for these two targets. In this case it was discovered that the ring (#2) produced a greater phase-shift than the original coin (#1). Therefore, although the RX amplitude is lower, the phase-shift is greater.

It was also found that coins 3, 4 and 5 gave progressively lower responses than the original coin, and the cut ring gave an extremely poor response.

How can we explain these results?

Obviously there are other factors, apart from the RX amplitude, in operation here that cause the ring to have an enhanced response in the real world. For a VLF detector the ring produces the greatest phase-shift in the received signal. For a PI the ring allows the eddy currents to persist for a longer period of time. In the concentric ring experiment the combination of rings A+C actually provided the greatest phase-shift. This could be explained by considering the individual targets as a tuned circuit. As we know, slightly adjusting the value of either the RX inductor or capacitor will cause the initial TX-RX phase relationship to change. Much the same situation must also be occurring for the various targets. With reference to figure 4 below, let's now consider the cut coins.

Coin #3 gave a slightly reduced amplitude response when compared to the ring (#2) or the original coin (#1). This is consistent with the idea that the outer eddy current ring(s) have been cut. Also note how the responses of coin #4 and #5 are markedly reduced when compared to coin #3. This is consistent with all the rings being cut , thus forcing the eddy currents to form alternate paths within the two halves of the coin, as shown in figure 4 below.

Conclusion

Although the eddy currents are generated in the target object as a number of extremely small intersecting vortices, the superposition of these eddy currents results in cancellation, such that larger eddy current rings are formed. The experiments have shown that the overall shape of the target has a significant effect on the received signal for both magnitude and phase, and a dramatic demonstration is given by the cut ring (#6), where the eddy currents are restricted to moving in small circles defined by the thickness of the ring.

Unfortunately I am failing to understand mikebg's description (most likely due to the language barrier) but we do now seem to agree on the way the eddy currents are formed in the target. Initially I expected the eddy currents to almost completely cancel in the coin interior, leaving a single strong circulating current around the circumference, but the experiments indicate otherwise. I believe Carl's modified diagram, shown in figure 5 below, is correct. Note how the arrows get smaller as you move towards the center of the coin.

This was an interesting set of experiments, even if I did manage to blunt a number of drill bits in the process.

I hope everyone else found it as interesting.

[bbsailor]

Though experiment - Mentally damaged coins.
In the attached figure using a single term "eddy diameter" of the target. This is the largest recorded
diameter circle in the outlines of the target when it is viewed in the direction of the magnetic field.
The eddy diameter of halfspace is equal to diameter of TX loop. The fundamental time constant,
respectively, the fundamental cutoff frequency, depend on the square of the eddy diameter.
What happens with cutoff frequency when we cut the coin?
In frequency domain, if we reduce twice eddy diameter, the cutoff freqency will increase 4 times. This
is done by cut coin in half, as shown in case 2.
Let's see how this looks on Normalized Impedance plot and on a not normalized Impedance plot.
To be continued.
normalized.

Mikebg, Tinkerer and other Gurus

The purpose of this thread is to try to identify an unknown target by some response properties that may help deduce an unknown target's identity. Because targets may lay in the ground in many different orientations, it is possible that some target responses may be ambigious requiring the target to be dug up to actually see its identity.

I am mostly a beach hunter as I live near the New Jersey Shore. I want to share some experiences and ask the active members of this discussion thread to comment and independently verify by doing a few quick tests.

I have experienced the following. When I locate a target on the beach, I visually pinpoint the target location, then move the coil about 2 inches off to the side and sweep the coil slowly over the target listening for the sound level in my headphones. I increase the sweeep speed about 3 times faster than the slowest sweep speed and listen for an increase in signal level. Most of the time the increase in signal level indicates a ferrous target like a washer.

If this technique could be verified, it may provide another tool in using coil sweep speed changes to help better deduce the identity of an unknown ferrous target. This should be tried with and without Ground Balance methods being activated.

Thanks for the interesting discussion.

bbsailor

[Tinkerer]

Though experiment - Mentally damaged coins.
In the attached figure using a single term "eddy diameter" of the target. This is the largest recorded
diameter circle in the outlines of the target when it is viewed in the direction of the magnetic field.
The eddy diameter of halfspace is equal to diameter of TX loop. The fundamental time constant,
respectively, the fundamental cutoff frequency, depend on the square of the eddy diameter.
What happens with cutoff frequency when we cut the coin?
In frequency domain, if we reduce twice eddy diameter, the cutoff freqency will increase 4 times. This
is done by cut coin in half, as shown in case 2.
Let's see how this looks on Normalized Impedance plot and on a not normalized Impedance plot.
To be continued.
normalized.

Hi Mikebg,

I understand the following sentence:"The fundamental time constant,
respectively, the fundamental cutoff frequency, depend on the square of the eddy diameter. "

In the time domain, like PI, we have a few test targets for testing the TIME CONSTANTS (TC) and the detector's ability to detect targets with these time constants.
A square of 1" of Aluminium foil, of the thin household kind, has a TC or time constant of about 10us. If we cut this square in half, the time constant is reduced to 5us.
The time constants are affected by the conductivity of the target. An example is nickel that is often used in alloys for modern coins and stainless steel, that shortens the TC.
High conductivity = long time constant, but the thickness of the target also affects the TC.
Targets with less that 0.5mm thickness show a short time constant. Example is an Aluminium soda can. It gives a high amplitude response because of it's large surface area, but the TC is very short compared to a Aluminium block of the same volume.
Magnetic targets have a longer TC. Some types of detector use this feature to differentiate or discriminate FE targets. However, thin rusty pieces of steel sheet, have short TC's, so this type of discrimination is not very good for these targets.
With the TINKERERS_V1 the TC of a target behaves much like a cutoff frequency in the frequency domain. This is why some degree of target identification can be gleaned.

Tinkerer

[mikebg]

Eddy Diameter

Though experiment - Mentally damaged coins.
In the attached figure using a single term "eddy diameter" of the target. This is the largest recorded
diameter circle in the outlines of the target when it is viewed in the direction of the magnetic field.
The eddy diameter of halfspace is equal to diameter of TX loop. The fundamental time constant,
respectively, the fundamental cutoff frequency, depend on the square of the eddy diameter.
What happens with cutoff frequency when we cut the coin?
In frequency domain, if we reduce twice eddy diameter, the cutoff freqency will increase 4 times. This
is done by cut coin in half, as shown in case 2.
Let's see how this looks on Normalized Impedance plot and on a not normalized Impedance plot.
To be continued.
normalized.

[mikebg]

LF region

Using the Normalized Impedance plot
The advantage of Normalized Impedance plot is that its shape is almost identical for all forms of nonferrous targets and nonferrous halfspace (ground, salt water). There is a difference, which
is illustrated in the attached figure, but it is not essential for design purpose. We need to determine in what area of the spectrum can be increased phase difference between
signals in order to improve discrimination and to suppress the signal from the ground.
Disadvantage of Normalized Impedance is that it not shows the real value of the magnitude and
frequency, but only relative to the maximum magnitude and relative to own cutoff frequency. This
shortcoming creates discomfort when in common drawing two targets are presented.
I will explain this in the next posting as damaged a few coins, but only virtually. Who does not
believe the results of virtual coins can actually damage :-).
Now to explain what is shown in the figure below. It is a comparison between the shapes of the normalized
impedance at 4 nonferrous solids: bracelet (ring), cylinder (coin), sphere (gold nugget) and
halfspace. They are scaled so that to coincide its LF region (below cutoff frequency). You may know what is the curve of the bracelet (ring), because occurs Moreland's effect.

[mikebg]

Editor wanted

Qiaozhi and Ivica,
Look again at the right side of the figure in posting # 134. It represents cross section of a cylinder made of magnetic field. This is the shape of the magnetic field in the volume of the coin when a DC TX current suddenly stops. How should eddy currents flow in the coin to form such a cylinder? How eddy currents are needed and how should they be distributed in coin volume to form such a cylinder?
Fourier (1768-1830) has decided to heat a similar task in 1807, ie more than 200 years ago. REMI group has already given me almost all of the draft decision valid for eddy currents. It says rediscovering of EMI! They did not accidentally invented the name of the group to begin with "RE". Once rediscovered eddy currents, they will reinvent even metal detecting :-)
Now faced with the task before me to translate in English, mathematics text, explaining the decision of the differential equation given in posting # 149.
But I have so little linguistic knowledge in this area, it is necessary to seek help from the participants in this forum. The mistake I made with the spelling the name as Quiaozhi is nothing compared to the errors which I will do in a mathematical text.
Someone who is familiar with mathematical terminology in English, must edit my translation. Please, who wants to edit a text written in mathematical language Pseudoenglish (worse than Chinglish), he gave me his email.

[Qiaozhi]

Hi Qiaozhi. Good work!

Do you think can be used technology of coil selector/rejector for this purpose? Read the text of this magazine on Google books. Year: 1955.

http://books.google.com.py/books?id=...age&q=&f=false

Coin selector/rejectors are using the same principle that you see when dropping a magnet down a copper pipe. They induce a magnetic field in the coin and measure how long it takes to slide down the incline. It's probably not that useful for these tests.

I am working on a full summary of my experiments, that I intend to post here. It's starting to get a little confusing for anyone reading this thread with all the different posts. I will try and make it clear and simple with lots of diagrams.

[Esteban]

Hi mikebg,

So far you have failed to convince me that the concentric ring theory is correct ... BUT, I may have just convinced myself that it could be so.

Read on ...

In order to disprove the concentric ring theory I constructed a number of rings using some tinned copper wire (22 SWG, 0.71mm) with diameters of 3.7cm, 3cm, 2.5cm and 1.5mm. Let's refer to these rings as A, B, C and D.

I then performed some measurements using these rings by monitoring the output of the metal detector's sample gate in the GEB channel. Combinations of rings were tried so as to duplicate sets of concentric rings. The results were as follows:

* A = 75.8 mV
* A+B = 81.3 mV
* A+B+C = 79.2 mV
* A+B+C+D = 78.8 mV

As you can see, the maximum response was achieved with A+B, whereas I would have expected the response to increase as each subsequent ring was added. After some further experimentation, using different combinations of the rings, I found that the combination with the highest response was in fact A+C (87.5 mV). Things initially appeared very strange until I realized this was actually a measurement error, as far as this test was concerned. Instead of monitoring the output of the GEB sample gate I should really be monitoring the output of the DISC channel. By monitoring the GEB channel I am seeing the amplitude change caused by phase-shifts in the RX signal. A more accurate result can be obtained by using the DISC channel and, although there is still some error introduced by phase-shifts, the measurements should be accurate enough for these tests. The results using the DISC channel were:

* A = 68.7 mV
* A+B = 100.4 mV
* A+B+C = 111.0 mV
* A+B+C+D = 112.9 mV

As you can see the target response increases as more rings are added. At this point I thought "OK, that's the concentric ring theory busted!". Because it is clear from real world tests that a drilled-out coin has a better response than the original, but the concentric ring theory indicates the reverse.

Then ..... I repeated the original coin tests while also monitoring the DISC channel:

1. Original 2p coin = 51.3 mV
2. 2p coin drilled/filed to a 5mm ring = 49.7 mV
3. 2p coin with 5mm cut in circumference = 48.0 mV
4. 2p coin cut through to center = 32.7 mV
5. 2p coin cut through to other side = 30.7 mV
6. 2p cut ring = 3.4 mV

By the way, the voltages shown represent only the change in voltage at the DISC output. There was a residual voltage of 72.3 mV that was subtracted from each of the measurements.

Now - what very strange result can you spot from these tests?

Test #2 (the ring) gives a lower amplitude change than the original coin, whereas we all know that the ring can actually be detected at greater depth. So what's going on? The conclusion is that the improved target response is down to phase-shift alone.

Also (now the measurement error has been eliminated) you can see from test #3 that the 5mm cut causes a reduction in amplitude. Dare I say it? ... as if some of the outer rings (of the concentric ring theory) have been cut.

I should probably repeat these experiments while monitoring both the GEB and DISC channels, then calculate the resultant amplitude vector. This would eliminate any errors due to phase-shift in the DISC channel. Stay tuned for more later.

The big question I have at this time is, "What could cause the eddy currents to flow in concentric rings?", if of course that is what's actually happening.

Most curious.

Hi Qiaozhi. Good work!

Do you think can be used technology of coil selector/rejector for this purpose? Read the text of this magazine on Google books. Year: 1955.

http://books.google.com.py/books?id=...age&q=&f=false

[Qiaozhi]

Hi mikebg,

So far you have failed to convince me that the concentric ring theory is correct ... BUT, I may have just convinced myself that it could be so.

Read on ...

In order to disprove the concentric ring theory I constructed a number of rings using some tinned copper wire (22 SWG, 0.71mm) with diameters of 3.7cm, 3cm, 2.5cm and 1.5mm. Let's refer to these rings as A, B, C and D.

I then performed some measurements using these rings by monitoring the output of the metal detector's sample gate in the GEB channel. Combinations of rings were tried so as to duplicate sets of concentric rings. The results were as follows:

* A = 75.8 mV
* A+B = 81.3 mV
* A+B+C = 79.2 mV
* A+B+C+D = 78.8 mV

As you can see, the maximum response was achieved with A+B, whereas I would have expected the response to increase as each subsequent ring was added. After some further experimentation, using different combinations of the rings, I found that the combination with the highest response was in fact A+C (87.5 mV). Things initially appeared very strange until I realized this was actually a measurement error, as far as this test was concerned. Instead of monitoring the output of the GEB sample gate I should really be monitoring the output of the DISC channel. By monitoring the GEB channel I am seeing the amplitude change caused by phase-shifts in the RX signal. A more accurate result can be obtained by using the DISC channel and, although there is still some error introduced by phase-shifts, the measurements should be accurate enough for these tests. The results using the DISC channel were:

* A = 68.7 mV
* A+B = 100.4 mV
* A+B+C = 111.0 mV
* A+B+C+D = 112.9 mV

As you can see the target response increases as more rings are added. At this point I thought "OK, that's the concentric ring theory busted!". Because it is clear from real world tests that a drilled-out coin has a better response than the original, but the concentric ring theory indicates the reverse.

Then ..... I repeated the original coin tests while also monitoring the DISC channel:

1. Original 2p coin = 51.3 mV
2. 2p coin drilled/filed to a 5mm ring = 49.7 mV
3. 2p coin with 5mm cut in circumference = 48.0 mV
4. 2p coin cut through to center = 32.7 mV
5. 2p coin cut through to other side = 30.7 mV
6. 2p cut ring = 3.4 mV

By the way, the voltages shown represent only the change in voltage at the DISC output. There was a residual voltage of 72.3 mV that was subtracted from each of the measurements.

Now - what very strange result can you spot from these tests?

Test #2 (the ring) gives a lower amplitude change than the original coin, whereas we all know that the ring can actually be detected at greater depth. So what's going on? The conclusion is that the improved target response is down to phase-shift alone.

Also (now the measurement error has been eliminated) you can see from test #3 that the 5mm cut causes a reduction in amplitude. Dare I say it? ... as if some of the outer rings (of the concentric ring theory) have been cut.

I should probably repeat these experiments while monitoring both the GEB and DISC channels, then calculate the resultant amplitude vector. This would eliminate any errors due to phase-shift in the DISC channel. Stay tuned for more later.

The big question I have at this time is, "What could cause the eddy currents to flow in concentric rings?", if of course that is what's actually happening.

Most curious.

[Qiaozhi]

The formation of eddy currents in the target is slightly more complicated than my simple diagrams suggest, as they assume a 2-dimensional shape. However, any currents that circulate across the top and bottom of the coin will be perpendicular to the the plane of the coil and will not contribute to the target response.

Therefore I agree that a sphere, with the same cross-section as the coin, will most likely give a similar response.

Before anyone points this out, I was talking cr*p in the above post. Of course there are no perpendicular eddy currents, as all they are all formed around the magnetic lines of flux created by the TX coil. When the coin is in the center of the coil these magnetic force lines will be perpendicular to the coin's surface, and hence the eddy currents will be created in the same plane as the coin. Which means the coin can be treated as a 2-dimensional object. Hope this makes sense.

[Aziz]

Hi Qiaozhi,

do you remember the compact coil and its inductivity?

The coin #1 is a coil with 1 winding and the winding is not densed. It will induce not much compared to a thin ring version.

Now let's think of again.
Aziz