concentric coil inductance?

[Gwil]

Just so I've got this right, when we're talking about 1 inch spacing Tx to Rx, that means a total of two inches between the upper and lower Rx coils?

[green]

Just so I've got this right, when we're talking about 1 inch spacing Tx to Rx, that means a total of two inches between the upper and lower Rx coils?

Yes

[Gwil]

OK. Thanks.

[green]

http://www.geotech1.com/forums/showt...361#post207361

Used above calculator to make chart. Don't know if it makes sense to use chart to determine best spacing. 2 inch spacing is a little better than 1 inch on the chart, don't know if it would be better when making the coil.

[Chet]

Hi Green


From your chart an 8 inch (200 mm) PI coaxial coil that has 2” (5 cm) or more separation between the transmit coil and each receiver coil would be a good starting point. Basket weave should not be needed for the receiver coils.


The loss from the extra transmit distance may be negated by the cancelled EMI noise and the greater gain allowed by not saturating the first op-amp stage.


I plan on building a 4” VLF coil similar to the attached photos for my MXT detector. The main reason is to reduce EMI noise.


Have a good day,
Chet

[green]

http://www.geotech1.com/forums/showt...361#post207361

Used above calculator to make another chart. Compared three coil sizes. 200mm/1.5(133mm), (200mm) and 200mm*1.5(300mm). Doubled the distance scale before charting, coil to target and target back to coil. If I made the three coils with the same inductance(300uH) would they chart close to the same as calculated(same target) or should I be adding a correction? Looking for what effect coil size has on detecting small nuggets.

[Chet]

Hi Green


The coils for comparison should all be wound to equal 300 uH. Fewer turns of wire are required as the coil diameter increases to achieve 300 uH. Larger coils will have a lower ampere turns power product unless the input pulse width or voltage is increased. With the same input pulse the magnetic field per each square CM of earth will be weaker with larger coils. Much lower eddy currents will be generated in small nuggets with the larger coils.


As for another correction or comparison factor; you might consider using a field density in Milli Joules / sq CM as another measurement standard for each size coil.


Have a good day,
Chet

[green]

http://www.geotech1.com/forums/showt...361#post207361

Used above calculator to make another chart. Compared three coil sizes. 200mm/1.5(133mm), (200mm) and 200mm*1.5(300mm). Doubled the distance scale before charting, coil to target and target back to coil. If I made the three coils with the same inductance(300uH) would they chart close to the same as calculated(same target) or should I be adding a correction? Looking for what effect coil size has on detecting small nuggets.

Made the three coils, 133,200,300mm(mean diameter) spider web(flat basket). Used Prof Coyle to calculate number of turns. Charted amplitude vs target distance. Same coil current profile for all tests. Multiplied coil volts by 300uH/coil uH to correct for inductance difference before charting. The slopes don't match the calculated slopes with above calculator. Calculated chart from reply #21, tried again but don't get the right answer. Think I tried before and it made sense but this time I can't get it to make sense. Forgot to add coil specs. (133mm) 3 ohms, 34 turns, 305uH, 2.65MHz SRF. (200mm) 3.6 ohms, 26 turns, 329uH, 2.25MHz SRF. (300mm) 4.24 ohms, 21 turns, 364uH, 1.9MHz SRF.

[nickel_n]

what is the Wire Thickness?

[green]

what is the Wire Thickness?

AWG28 solid magnet wire. .0126 inches, .32mm

[Chet]

Hi Green


I plotted the magnetic transmit field of the coils. I don’t know how to evaluate the receive path. The target shape, angle and thickness will greatly vary the amount of eddy currents sensed by the coil.


Have a good day,
Chet

[green]

Hi Green


I plotted the magnetic transmit field of the coils. I don’t know how to evaluate the receive path. The target shape, angle and thickness will greatly vary the amount of eddy currents sensed by the coil.


Have a good day,
Chet

Hi Chet
Missed your reply. Thanks, Still don't see how to make the calculated data match the measured. Maybe the way I'm measuring. What I call zero would make some difference. Maybe others. Any thought on which coil would be better for nugget hunting?

[Chet]

Hi Green


I have attached a chart approximating your coils and magnetic fields generated on targets from your previous charts.

It is a daunting problem! One way to think of it is; a 300mm coil is pushing out 21 ampere-turns of magnetic field spread over 707 square centimeters. The magnetic field is spreading and diminishing inversely by a power of 3. A very weak field of 0.009 gauss is energizing a 1” x 1” piece of coke can at a distance of 375mm.

Eddy currents generated in the metal target are radiating outward unevenly in all directions. The receiving coil is somewhere at a spherical distance of 375mm from the target. The 707 sq cm coil surface is only receiving 4% of the full 1761 sq cm surface area of the sphere around the target.
So the 500v, 1 ampere transmit pulse results in a few microvolts from the receive coil excited by the weak eddy current return.

Have a great day,
Chet