Shielding, another topic in the series...

[Malospelos]

https://www.3m.com.es/3M/es_ES/p/d/b40022846/

Good afternoon, what do you think of this type of sponge? Can it be used to protect the electronics of the detector or coil?

or the work frequency is not valid?

[lamoiz]

Hello everyone, since the insulation for capacitive effects is important around the coils of our dear inductors, I have this question: has anyone here ever used plumber's Teflon to make this insulation? Or, a bit more expensive, Kapton in self-adhesive form? The dielectric strength of these two materials is actually quite good, with a dielectric constant of 2.1 for PTFE and 3.2 for Kapton. Of course, this layer will need to be covered with paper tape to receive a shielding paint, or with a Faraday tape and its drain, for proper operation and protection from electrostatic discharges. Thank you for your feedback on this ever-debated topic.

[Nightryder]

Hello everyone, since the insulation for capacitive effects is important around the coils of our dear inductors, I have this question: has anyone here ever used plumber's Teflon to make this insulation?

I thought about using pipes tape as a spacer a while ago and I've come to a conclusion. Which is this tape may work in theory, but practically applying it is going to tighten your coil windings against each others more than necessary, which you definitely don't want. Besides, a good spacer is about 4 to 5 mm in thickness meaning that you would need a lot of tapes and this shouldn't be a problem expense wise; however, the challenge here is that it's hard, at least for me, to know exactly the thickness after applying a couple of them. Cheers.

[coinfarmer]

It seems as though having the wires tighten together would be an advantage, perhaps I don't understand what you are saying.

The gap they are talking about, as I understand it is what stops the shielding from forming a complete loop, it sounds like you are suggesting to wind a 5mm thick layer of PTFE tape over the coil before the shield?

Years ago I built some coils with the PTFE tape between the coil and the shield. No comparisons or experiments were done so I will refrain from making statements on it's performance.

[lamoiz]

"Good evening, thank you for your responses. Indeed, the fact of wanting to tighten the turns with tape could alter the inductance, but as far as I am concerned, this cannot happen because I glue the turns already tightened before even placing the spacers. For information, and for those interested, this time I am building a coil for a quasar, at 9 kHz. I used Kapton adhesive tape, two layers, then a layer of masking tape, and finally two coats of graphite paint, like guitar cavity shielding. The drain is applied in a mesh pattern across the entire surface (back and forth). The result is the complete absence of background noise and interference, tested with a plastic ruler charged electrostatically by friction on a sweater, directly discharged onto the coil. So, I approve!"

[lamoiz]

"I forgot, the resistance value between the drain wire and any point on the surface of the coil's paint is at most 40 ohms."

[bbsailor]

Hello everyone, since the insulation for capacitive effects is important around the coils of our dear inductors, I have this question: has anyone here ever used plumber's Teflon to make this insulation? Or, a bit more expensive, Kapton in self-adhesive form? The dielectric strength of these two materials is actually quite good, with a dielectric constant of 2.1 for PTFE and 3.2 for Kapton. Of course, this layer will need to be covered with paper tape to receive a shielding paint, or with a Faraday tape and its drain, for proper operation and protection from electrostatic discharges. Thank you for your feedback on this ever-debated topic.

Geotech1 members building coils should take coil resonance measurements which will allow capacitance to be calculated. Here is a good way to implement this test.
1. Attach coil to signal generator through a 1M Ohm resistor to minimize circuit loading and allow the resonant frequency to be better seen.
2. Attach the coil and shield through a 1pF capacitor to the O-scope.

The thicker spacing between the coil and the shield will also lower the coil seen capacitance to the shield and create a higher resonant frequency.

The dielectric constant of your chosen coil spacer will affect the coil resonant frequency. If you want a low delay to find small gold targets you can get a lower delay by reducing the coil seen capacitance by using a spacer with a lower dielectric constant.

While building many coils, I discovered Scotch24, a fine mesh screen, to reduce the area of the shield and allow a higher value of the damping resistor which allows a faster transition from TX to RX in a PI mono coil.

I share this with Geotech1 members to suggest you keep this in the back of your mind when building coils and/or doing experiments.

Joseph J. Rogowski

[lamoiz]

Thanks BB SAILOR. Indeed, on PI coils, I measure the capacitance and use thicker spacers. But here, I’m working with a VLF coil. Do you think I should do the same, or is it less important than with a fast PI coil?"

[Carl-NC]

Teflon plumber's tape is very thin so you would need to wrap a lot of it to make an effective spacer. I use plastic spiral wrap which has a permittivity of 2.4, very close to teflon, and is much thicker. On aggressive coils, I've used two layers of spiral wrap. Three layers doesn't seem to offer any additional improvement. For the shield I typically use nickel fabric tape which is about 10 cents/foot. Scotch24 probably works about the same but is $1-2 per foot.

[Carl-NC]

Thanks BB SAILOR. Indeed, on PI coils, I measure the capacitance and use thicker spacers. But here, I’m working with a VLF coil. Do you think I should do the same, or is it less important than with a fast PI coil?"

Sinusoidal VLF? Shield capacitance is far less important. Main consideration is to keep the self-resonant frequency much higher than the operating frequency.

[lamoiz]

Thank you, Carl, yes indeed, on a sinusoidal coil.

[Carl-NC]

And SRF is only important on the RX coil. On the TX coil, parasitic C just adds to the overall tank C, no big deal.

[lamoiz]

So, since I isolated the turns with Kapton, and my results are good, at least on the workbench, can I assume that this coil will perform well in wet grass?

[Carl-NC]

I don't know what you mean by "my results are good"... did you test the shielding on the bench?

[lamoiz]

I don't know what you mean by "my results are good"... did you test the shielding on the bench?

When I say that the results are good, the coil reacts normally to various metals, and it passes both the hand test and the electrostatic discharge test."