Spiral Coil Calculations

[Silver Dollar]

I've spent the last few days removing the shielding off some old
cable TV cable to use the inner wire for a spiral coil for a PI detector.

So I have a 10 7/8" coil housing. My wire has an outside diameter of
4.6mm. So I need to figure out how many windings I can make with
a single layer or a double layer and how much inductance I might
expect, Can anyone point me to some spiral calculators?

[Qiaozhi]

Can anyone point me to some spiral calculators?

Try this -> http://www.deepfriedneon.com/tesla_f_calcspiral.html

[Silver Dollar]

Thanks for the URL. That calculator told me I needed more wire than one layer would allow
so I tried winding 2 layers. It was a bit tricky as the diameter of the wire with covering is a
bit large so when you transition from one layer to the next it messes things up a bit.

Here's the stuff ready to wind;


Started;






Finished (300 uh);


The covering material is like tyvek so I hope it is fast!

It took about 70 feet of wire to get to 300 uh...

[Silver Dollar]

Hmm this coil doesn't work! It reads 330uh and 2.5 ohms but I cannot adjust the dampening.
This is the waveform I get from it (notice the 100us before it comes out of saturation);


Here is a 10" coil of AWG30 wire wrap wire (notice the 5 us until it comes out of saturation);


It is mounted on a 1/2 shell that has graphite paint on it. I tried grounding that side but the same result!
Any thoughts on what is going on here?

[Davor]

Looks like noise pickup.

[Silver Dollar]

I tested this coil back to back several times with the other one and it always returns the same results.

I did try measuring the capacitance from the coil to shield and got 150pf. Could that be the problem?

I did recoat the graphite paint so maybe the resistance got too low?

If I cannot find out what's wrong I'll have to junk it and start over again, I just would like to know
what I did wrong on this one...

[Silver Dollar]

I removed the shield and it still acts the same. There must be too much inter-winding capacitance.

[bbsailor]

I tested this coil back to back several times with the other one and it always returns the same results.

I did try measuring the capacitance from the coil to shield and got 150pf. Could that be the problem?

I did recoat the graphite paint so maybe the resistance got too low?

If I cannot find out what's wrong I'll have to junk it and start over again, I just would like to know
what I did wrong on this one...

Look up the technical details of the exact coax you used to answer the questions below.

1. What is the diameter of the copper center wire core?
2. Are you sure the center wire core is pure copper and not some ferrous allow?
3. Is the core a solid strand or is it many thinner strands of wire?
4. What delay are you set to sample using this coil wire?

Joseph Rogowski

[Silver Dollar]

Well there is no markings on the cable. It was for a cable TV install. It was double shielded.
The center conductor measures 0.04" or 1.02 mm and looks like solid copper. The cover on
the center wire looks like Tyvek...

[bbsailor]

Well there is no markings on the cable. It was for a cable TV install. It was double shielded.
The center conductor measures 0.04" or 1.02 mm and looks like solid copper. The cover on
the center wire looks like Tyvek...

I think you are picking up residual eddy currents in the solid wire coil mass. To test this,(1) use a PI machine with a known fast coil, capable of detecting very small link gold chains: (2) then cut off a foot of the coil wire and wave it under this fast coil. If you can detect the waved wire at the lowest delay increase the delay until it drops out. Try to estimate at what delay the waved wire droped out.

If you use tin plated AWG 18 (about .04" dia) stranded wire, you will have less eddy currents being generated in the wire mass itself as the tin plating on the wires is less conductive than silver plated wire or plain copper strands that are more conductive and allow eddy currents to form. I believe the best stranding size is for a single individual strand to be near AWG 30 for a 5.5 ohm coil and AWG 32 for a multi-strand wire equal to AWG 18 for a coil near 0.5 ohms.

Here is another point to remember. The optimum damping resistor value is only valid for a limited range of changes in flyback pulsed current due to different coil resistances, TX Pulse Rate, TX Pulse Width settings and, Delay setting.

If your coax core coil works at a longer delay, then reduce the delay until it stops working. Then readjust the value of the damping resistor and you may be able to lower the delay a bit more but you are now getting closer to detecting the decay of the eddy currents in the coil wire itself.

I hope this helps.

Joseph Rogowski

[green]

Hmm this coil doesn't work! It reads 330uh and 2.5 ohms but I cannot adjust the dampening.
This is the waveform I get from it (notice the 100us before it comes out of saturation);
[ATTACH]31669[/ATTACH]

Here is a 10" coil of AWG30 wire wrap wire (notice the 5 us until it comes out of saturation);
[ATTACH]31670[/ATTACH]

It is mounted on a 1/2 shell that has graphite paint on it. I tried grounding that side but the same result!
Any thoughts on what is going on here?

Interesting problem. Did you find why it took so long to come out saturation? What does the signal at the amplifier input diodes look like?

[Silver Dollar]

Well I think like BBSailor says the thickness of the solid center wire allowed the coil to see itself.
A longer TX might make it work but it would be for bigger deeper targets so I gave up on that idea
and have started a basket coil to use in that housing.




I did peel the coax center coil off the disc and it is still intact so I may return to it at a later time...