Skin Effect

[okantex]

Hi Carl ,
I was reading about BFO theory in archives.
What is Skin Effect

[okantex]

a sentence from paragraph

moderately higher frequencies generate a better eddy current response up to the point where skin effect begins to degrade it.

[Qiaozhi]

Skin Effect

Hi Carl ,
I was reading about BFO theory in archives.
What is Skin Effect

moderately higher frequencies generate a better eddy current response up to the point where skin effect begins to degrade it.

When current with a high frequency is flowing in a conductor it has a tendency to move along the outside surface. This is called "skin effect"; and starts to become noticeable above 100KHz. At these higher frequencies the signal only penetrates the target to a shallow depth which (of course) degrades the eddy current response.

[Unregistered]

Does this skin effect only occurs in transmitter?
can this occur in recievers?
And ,is there anything that can be a cure to this effect?
for example increasing wire gauge or adding second coil in paralel?

[Qiaozhi]

Does this skin effect only occurs in transmitter?
can this occur in recievers?
And ,is there anything that can be a cure to this effect?
for example increasing wire gauge or adding second coil in paralel?

Skin effect can cause problems with high frequency transformers, but this is not the same problem that is being discussed here. As far as metal detectors are concerned the skin effect is occuring in the metal target, not the TX or RX coils. The induced eddy currents have a tendency to flow near to the surface of the object, which means that the received signal is degraded because of of the reduced level of current. Although the same thing is happening in the TX and RX coils it doesn't cause any real problems, only to increase the effective resistance of these coils, which increases with frequency. The only real solution is to lower the frequency. In a BFO design the higher frequency results in greater sensitivity, with the result that lowering the frequency to conteract the skin effect does not really give you any advantage. That's why BFOs usually operate at around 100KHz. For an I.B. design there are significant gains to be made by operating at a lower frequency, as in this case you're interested in getting as large a return signal as possible. Discrimination is also improved, because the TX signal is able to penetrate the metal target to a greater depth and hence improve the phase response of the received signal.
Of course, you could try winding your coils with litz wire to increase the TX signal, but the same skin effect problem will be present in the metal target. Therefore I doubt there would be any significant advantage in this approach.
Hope this makes sense.

[Unregistered]

Hi Qiaozhi,
thanks for explainations.
After I learned that skin effect occurs in target over 100khz,I remember words of GPR producers.they say GPR works with 100mhz ,300mhz,500mhz antennas.So why do not them effected by skin effect.they are generally told to be working through elctromagnetic fields as PI,VLF.....some with sound .

[Qiaozhi]

Hi Qiaozhi,
thanks for explainations.
After I learned that skin effect occurs in target over 100khz,I remember words of GPR producers.they say GPR works with 100mhz ,300mhz,500mhz antennas.So why do not them effected by skin effect.they are generally told to be working through elctromagnetic fields as PI,VLF.....some with sound .

Ground Penetrating Radar is something different to standard metal detecting and involves sending electromagnetic pulses into the ground and measuring variations in the received signal. This enables structures to be indentified underground, and is used to locate large objects. Skin effect is not an issue because GPR does not rely on the generation of eddy currents in a metal target. In fact, GPR is used mainly to identify subsurface anomalies such as underground cavities.

[okantex]

Hi Qiaozhi,
indeed, I want to build something like my father's friend's old detector which was stolen during earthquake in 1999.
I know you wont believe but my father and hiz friend found a gold coin in a small(fist size)closed cup which was at almost 4 meters dept.
I am trying to solve it's system.it was something from 1975's.
it head(where transmitter and reciever are) was soo small .something like 3cm*8cm*12cm
according to told me by owner, 3 or 4 coils inside.
first was on iron core(inside empty like pipe)
second was around it like ( look at carl's BFO theory file.first like figure 3 ,second like the figure figure 4.b
http://www.thunting.com/geotech/page...theory/bfo.pdf
these two in same plain(propagateing same direction.(down to ground)
third is on u core ,and winded perpendecular to first group.
do not know working frequency.
and while these two perpendecular groups too close (opposed to TR (two box)detectors) how can it detect from 4 meters
thanks

[Qiaozhi]

Hi Qiaozhi,
indeed, I want to build something like my father's friend's old detector which was stolen during earthquake in 1999.
I know you wont believe but my father and hiz friend found a gold coin in a small(fist size)closed cup which was at almost 4 meters dept.
I am trying to solve it's system.it was something from 1975's.
it head(where transmitter and reciever are) was soo small .something like 3cm*8cm*12cm
according to told me by owner, 3 or 4 coils inside.
first was on iron core(inside empty like pipe)
second was around it like ( look at carl's BFO theory file.first like figure 3 ,second like the figure figure 4.b
http://www.thunting.com/geotech/page...theory/bfo.pdf
these two in same plain(propagateing same direction.(down to ground)
third is on u core ,and winded perpendecular to first group.
do not know working frequency.
and while these two perpendecular groups too close (opposed to TR (two box)detectors) how can it detect from 4 meters
thanks

This does not look like a BFO design to me. Firstly there is more than one coil in the search head, so it is possibly some sort of induction balance design. Have a look at Carl's latest coil article, in particular figure 4.
http://www.thunting.com/geotech/page...info/coils.pdf
This uses orthogonal coils in a similar manner to a 2-box detector, but they are in close proximity to each other. Of course, finding a gold coin in a jar at 4 metres could just be a lucky coincidence. You can only really believe such a claim if it is repeatable.

[Unregistered]

Hi Qiaozhi,
I have some questions.
1. say you have BFO,PI ,VLF
all works at some frequency,have same coil size and somecurrent pass through transmitter coils that same induction will occur in target.
Q. Will dept penetration be same or not? why.

2.I reade that in vlf s dept is metered by current correlation.if it is high target iş near surface.
is this for same targets? will not big target in dept and small in shallow give same current follow in coil.

3.can not we increase dept penetration by increasing transmitter voltage.
I suspcious about the cores in my old detector.aare they reciever or power cores.
4.and once a time, while walking under high voltage line(that means more than 10 meters to user ) this machine took the user up from ground.the owner says. high voltage ,I think 60HZ and 10000 volt .
can this be a clue for it's working frequency or something else.

thanks

[Qiaozhi]

Hi Qiaozhi,
I have some questions.
1. say you have BFO,PI ,VLF
all works at some frequency,have same coil size and somecurrent pass through transmitter coils that same induction will occur in target.
Q. Will dept penetration be same or not? why.

2.I reade that in vlf s dept is metered by current correlation.if it is high target iş near surface.
is this for same targets? will not big target in dept and small in shallow give same current follow in coil.

3.can not we increase dept penetration by increasing transmitter voltage.
I suspcious about the cores in my old detector.aare they reciever or power cores.
4.and once a time, while walking under high voltage line(that means more than 10 meters to user ) this machine took the user up from ground.the owner says. high voltage ,I think 60HZ and 10000 volt .
can this be a clue for it's working frequency or something else.

thanks

Q1 - For the BFO and the IB (VLF) detectors, the same amount of power will be generated in the target, if these are both using the same coil size and current. The PI will most likely transmit more power to the target because it uses a high-power pulse rather than a sine wave. The BFO is looking for a change in inductance of the coil, whereas the IB is looking for a change in the amplitude of the signal at the RX coil. Depending on the complexity of the detector, it may also be measuring the phase shift. The PI is a different animal, and waits for the TX signal to subside before "listening" for an "echo" from the target. In a mono coil design, this "echo" causes the TX signal to decay more slowly, which indicates the presence of a metal target. The PI will probaby have a greater depth penetration, but the received "echo" is considerably weaker and may even be in the uV range.

Q2 - In IB (VLF) detectors the depth measurement assumes that the target is a coin shaped object. Of course, a large deep object can give the same reading as a small object on the surface. The detector has no way of knowing the actual depth of the target. A seasoned metal detectorist can usually tell one from the other by the actual size of received signal. A small close target may give a short blip, but a large deep object will be give a longer signal. It's all a guessing game, despite what the manufacturer's may imply.

Q3 - Increasing transmitter voltage (or even overall TX power) has much less effect than you would intuitively think. I believe that the TX (and the reflected RX) signal drops off by the sixth power. Which means that you need to generate one helluva lot more output before it has any real effect, and this also causes other problems in the RX circuitry.

Q4 - I'm not sure what you mean by "this machine took the user up from ground". Hopefully he didn't take off and start flying.
Many detectors can be affected by overhead power lines, especially PI type detectors. It also depends on internal filtering and the amount of amplifier gain.

I do not know that there are any clues here, unless you can find some more information.

Unfortunately I'm beginning to suspect that this is a homemade LRL. In which case I would give it up as a lost cause.

[Unregistered]

Hi Qiaozhi
if I can draw schematic ,could you understand it type .

[Qiaozhi]

Hi Qiaozhi
if I can draw schematic ,could you understand it type .

OK - please post the schematic. I will be more than happy to take a look.

[okantex]

hi

[okantex]

hi
I tried to show wire connections with yellow lines.and numbers that are written on board.
I thought you could understand from shapes ,so just written numbers of components.
I directly copied board .because of that some numbers are not used.
capasitor
c1 10uF
c3 68n
c5 68n
c6 47n
c7,c9,c10,c8 = measured value changes bwtween 2.8 -3 pF
c11 10uF
UT1=UT2=UT3
UT4=UT5=UT6=UT7
resistors
r3,r8 10k
r10,r9 1k5
r1,r2 3k
r7 (100P) = 1k
r5,r4,r11,r12 100k
r13 100 ohm