Shielding or No???

[Geo]

Hi.
I want to make a larger coil for my TDI than 12" that it has.
Maybe a 18" will be OK. I will work with 25uS delay (coins and bigger objects, no nuggets). So does i need the coil shielding at this delay????
It is not problem to shield a coil but without shield the coil is more sensitive.

Regards

[Carl-NC]

I built a 24" TDI loop (pictured somewhere on this forum) and did not shield it. I was having a lot of noise problems with it. I still have not shielded it, so I can't say this is really the problem.

- Carl

[Geo]

So, a trial will so.
I prepare a table with the detected distances on air at some objects. I can't find a good point with GB on. I"ll attach it here..

Regards

[mikebg]

Hi Geo,
I can give 5 answers:
1. Small coins.There are ancient Greek coins with unusually small diameter, which are very valuable. If you increase the diameter of the sensors, the machine will lose its sensitivity to them. Do you looking for large objects in places where there may be such small coins?
2. Coil diameter. The diameter of the TX coil sets the ground timeconstant. If you increase the diameter, the machine will receive a stronger and slower decaying response from conductive ground. Do you use large sensors on the sea beach?
3. Timeconstant of shield signal. Conducting shield creates a signal like a metallic object located too close to the coil. The timeconstant of the shield needs to be done other than timeconstant of the desired object.
4. Gain of preamp. In addition, the created from shield parasitic signal requires to reduce the gain of the preamp. The depth of detection depends mainly on the gain of preamp.Let us reflect on how it should work an ideal metal detector having real preamp. When no target, at the input of the preamp there is no signal. Therefore, we can increase its gain until output voltage begins to saturate input of synchronous demodulators. The output voltage is only large chaotic noise, but it does not interfere with the work because synchronous demodulator acts as a lockin amplifier.
This can not be obtained in a real metal detector, but designers should strive to be close to the ideal case. In real metal detectors the gain of preamp is not limited by termal noise, but by parasitic signals in input. These are two parasitic signals: a direct transfer of energy from TX coil to RX coil and the ground signal. They reduce the modulation index of target signal.
5. Coil configuration. The worst sensor for metal detection is monocoil, because with it, the mentioned two signals are strongest. With monocoil, the target signal attenuates by 6th power of depth.
If you want to discover larger objects at greater depth, my advice is to make sensor type "perpendicular coils" (Depth Multiplier). U.S. traders and unkle Garrett incorrectly call this sensors TWO BOX, but this term means quite different sensor system where the mutual inductance between the TX and RX coils inevitably vary during operation. In Depth Multiplier two coils are fixed in two opposite end of carrier rod and their mutual inductance is set at a minimum. This allows us to increase the gain of the preamp.

[bbsailor]

Hi.
I want to make a larger coil for my TDI than 12" that it has.
Maybe a 18" will be OK. I will work with 25uS delay (coins and bigger objects, no nuggets). So does i need the coil shielding at this delay????
It is not problem to shield a coil but without shield the coil is more sensitive.

Regards

Geo,

If you try to make a larger coil for the TDI, try to make it near the same resistance and inductance as the TDI coil. This will keep the damping resistor value near what is used on the TDI. If you want to try to operate it lower delays, do the following. Measure the self resonant frequency at the end of your TDI coil coax and use this frequency as a target for making your new coil. Balance the following to obtain the same self resonant frequency for your larger coil. I am assuming that the damping resistor in on the circuit board. If it is built in the coil, this self resonant technique will not work. Sorry!

1. Wire type, gauge and number of turns, try Teflon insulated wire
2. Shield type and spacing from coil, try Scotch 24 wire mesh
3. Coax wire length, try a shorter coil coax wire length to finally tune new, larger coil self resonant frequency to initial TDI coil self resonant frequency.

The shielding helps minimize noise pickup as well as keeps the coil capacitance constant relative to the ground to make searching more accurate as there will be less false signals.

Maybe Carl knows the TDI coax cable capacitance and TDI coil details?

From what I have read about the TDI, it can accept a wide range of coils. Keeping the coil capacitance of your new, larger coil near the TDI coil capacitance will allow the TDI to operate at the lower delay range. At the worse case, you may need to operate the larger TDI coil at about 15us.

Enjoy your new PI machine.

bbsailor

[Geo]

Hi Geo,
I can give 5 answers:
1. Small coins.There are ancient Greek coins with unusually small diameter, which are very valuable. If you increase the diameter of the sensors, the machine will lose its sensitivity to them. Do you looking for large objects in places where there may be such small coins?
2. Coil diameter. The diameter of the TX coil sets the ground timeconstant. If you increase the diameter, the machine will receive a stronger and slower decaying response from conductive ground. Do you use large sensors on the sea beach?
3. Timeconstant of shield signal. Conducting shield creates a signal like a metallic object located too close to the coil. The timeconstant of the shield needs to be done other than timeconstant of the desired object.
4. Gain of preamp. In addition, the created from shield parasitic signal requires to reduce the gain of the preamp. The depth of detection depends mainly on the gain of preamp.Let us reflect on how it should work an ideal metal detector having real preamp. When no target, at the input of the preamp there is no signal. Therefore, we can increase its gain until output voltage begins to saturate input of synchronous demodulators. The output voltage is only large chaotic noise, but it does not interfere with the work because synchronous demodulator acts as a lockin amplifier.
This can not be obtained in a real metal detector, but designers should strive to be close to the ideal case. In real metal detectors the gain of preamp is not limited by termal noise, but by parasitic signals in input. These are two parasitic signals: a direct transfer of energy from TX coil to RX coil and the ground signal. They reduce the modulation index of target signal.
5. Coil configuration. The worst sensor for metal detection is monocoil, because with it, the mentioned two signals are strongest. With monocoil, the target signal attenuates by 6th power of depth.
If you want to discover larger objects at greater depth, my advice is to make sensor type "perpendicular coils" (Depth Multiplier). U.S. traders and unkle Garrett incorrectly call this sensors TWO BOX, but this term means quite different sensor system where the mutual inductance between the TX and RX coils inevitably vary during operation. In Depth Multiplier two coils are fixed in two opposite end of carrier rod and their mutual inductance is set at a minimum. This allows us to increase the gain of the preamp.

Hi Mike.
I want to detect for medium to large coins (>20mm) and for big treasure (for example as a soda can or bigger).
I will stay at your fifth answer.What about 2Box?? I think that you speak about 2 horizontal coils and not for one horizontal and one vertical as the real 2box detectors has.
Please inform me about it.
Regards

[Geo]

Geo,

If you try to make a larger coil for the TDI, try to make it near the same resistance and inductance as the TDI coil. This will keep the damping resistor value near what is used on the TDI. If you want to try to operate it lower delays, do the following. Measure the self resonant frequency at the end of your TDI coil coax and use this frequency as a target for making your new coil. Balance the following to obtain the same self resonant frequency for your larger coil. I am assuming that the damping resistor in on the circuit board. If it is built in the coil, this self resonant technique will not work. Sorry!

1. Wire type, gauge and number of turns, try Teflon insulated wire
2. Shield type and spacing from coil, try Scotch 24 wire mesh
3. Coax wire length, try a shorter coil coax wire length to finally tune new, larger coil self resonant frequency to initial TDI coil self resonant frequency.

The shielding helps minimize noise pickup as well as keeps the coil capacitance constant relative to the ground to make searching more accurate as there will be less false signals.

Maybe Carl knows the TDI coax cable capacitance and TDI coil details?

From what I have read about the TDI, it can accept a wide range of coils. Keeping the coil capacitance of your new, larger coil near the TDI coil capacitance will allow the TDI to operate at the lower delay range. At the worse case, you may need to operate the larger TDI coil at about 15us.

Enjoy your new PI machine.

bbsailor

Hi bbsailor.
Thanks for your reply. But i have one wonder about big coil on this detector.
As we know big coils for detect big objects at large depth, they need big pulse width. For example Superscan has 1ms pulse width, and TDI only 100us. How will work the big coil with a so thin pulse??? Energy will be very small

Regards

[bbsailor]

Hi bbsailor.
Thanks for your reply. But i have one wonder about big coil on this detector.
As we know big coils for detect big objects at large depth, they need big pulse width. For example Superscan has 1ms pulse width, and TDI only 100us. How will work the big coil with a so thin pulse??? Energy will be very small

Regards

Geo,

The high PPS frequency of the TDI, makes it search better with smaller coils 12" diameter. However, the faster PPS rate also integrates more signals so even with low pulse width, you should be able to obtain good results even with 100uS PW. Note that when you use a larger diameter coil, you are integrating more signals because the larger coil is being swept relativly slower due it's larger diameter.

We should not use a coil sweep speed, such as 1 meter per second, we should state the coil sweep speed in how may coil diameters are being swept in a period of time. This provided a better understanding about the number of samples being inegrated.

Normally you do not need to shield coils over about 30us delay as the ground signal will have died down before the sampling. At 25us delay, the maximum of the TDI, you may still need a shield to keep the noise low.

Eric Foster mentioned that there are two ways to improve PI sensitivity.

1. Brute force. Low resistance coils, wide PW and high battery voltage.
2. Faster PPS rate. Higher resistance coils, narrower PW, integrate 1K to 2K pulses, use faster sampling to detect higher on the target discharge curve.

At least you can try the larger TDI coil and compare it with a Delta Pulse (low PPS, high power type) and let us know how it compares with a few different type targets ranging from gold rings, coins and larger relics.

bbsailor

[Geo]

Geo,

The high PPS frequency of the TDI, makes it search better with smaller coils 12" diameter. However, the faster PPS rate also integrates more signals so even with low pulse width, you should be able to obtain good results even with 100uS PW. Note that when you use a larger diameter coil, you are integrating more signals because the larger coil is being swept relativly slower due it's larger diameter.

We should not use a coil sweep speed, such as 1 meter per second, we should state the coil sweep speed in how may coil diameters are being swept in a period of time. This provided a better understanding about the number of samples being inegrated.

Normally you do not need to shield coils over about 30us delay as the ground signal will have died down before the sampling. At 25us delay, the maximum of the TDI, you may still need a shield to keep the noise low.

Eric Foster mentioned that there are two ways to improve PI sensitivity.

1. Brute force. Low resistance coils, wide PW and high battery voltage.
2. Faster PPS rate. Higher resistance coils, narrower PW, integrate 1K to 2K pulses, use faster sampling to detect higher on the target discharge curve.

At least you can try the larger TDI coil and compare it with a Delta Pulse (low PPS, high power type) and let us know how it compares with a few different type targets ranging from gold rings, coins and larger relics.

bbsailor

bbsailor,

i will inform you when i make bigger coil.
Now with small coil and GB=off the difference betwen TDI and DP is big...
but with GB=On .... read a tread who i will post after some minutes

Regards

[Geo]

Hi.
Can Carl or any other to tell me what size of wire i can use so to construct a 1x1m coil for the Whites TDI.

Regards

[Carl-NC]

I would use 20-22 awg stranded. You'll have to convert to swg or mm.

[Agraz]

Hello Geo:
If you want build a coil for detector whites, you need build a TX (transmitter) and RX (receiver) loops, similar to "Sierra Madre" model.
I think for whites is better loops 3 feet in diameter and if is PI (pulse induction) you need only a single loop, and any size. 25cmx25cm, 50cmx50cm, 1mx1m, 2mx2m, 2.50x2.50m, and I dont know if more big,...

Carl what is you opinion?

[Geo]

I would use 20-22 awg stranded. You'll have to convert to swg or mm.

Thank you Carl.
I will try to find and to order teflon wire for a 1x1m loop. I found an easy way to change the delay, but finally it was a bad idea.... because i will lose the "discrimination".

Regards

[Geo]

Hello Geo:
If you want build a coil for detector whites, you need build a TX (transmitter) and RX (receiver) loops, similar to "Sierra Madre" model.
I think for whites is better loops 3 feet in diameter and if is PI (pulse induction) you need only a single loop, and any size. 25cmx25cm, 50cmx50cm, 1mx1m, 2mx2m, 2.50x2.50m, and I dont know if more big,...

Carl what is you opinion?

Hi Agraz.
I wanted the diameter of the wire for the loop, not the dimentions.
TDI is a PI detector, so i will try to construct a "fast" mono loop.
As Carl said, No 20... 22 is ok.

Regards

[msysta]

I tried shielding my 16" circ mono coil and the result was absolute silence. The shielding was just out of curiosity so it is not mandatory to have it as coil works well without, but it would be nice to find out what went wrong.

I used aluminium tape throughout the coil and attached the ends to coaxial outer lead.

Should it have gaps, should only the other end be connected to ground, or what...?

Thanks for any ideas,
Markus