Coil shielding

[wyndham]

I have a very basic question about coil shielding. From what I have read here and other forums, the shielding is sometimes a spiral wrap with gaps left open in the winding. Does the gap have anything to do with the response from the target. It almost seems as if it might be a filter to only allow the weak target signal to get thru.
If this is anywhere near correct, If this is so, shouldn't the shielding gap be designed for a certain pulse response? How about a shielding tape that is perferated with a certain dia holes along the tape and the coil wrapped solid. It seems from by lack of tech training POV that the coils shielding could be the make or break of a coil design. It seems you want the most signal response but not the ground chatter that might come in with the target signal. Or is it that the coil is so overpowered by the rx signal that the shield works as a brake or governor for the coil.
If anyone has a thought on this, thanks for the time Wyndham

[bbsailor]

wyndham,

The shield on a PI coil only needs to have a gap so as to prevent a complete electrical loop around the circumference of the coil. In fact, if electrical tape were put over one end of the shield, the shield could overlap the taped region with no ill effects as there is no electrical connection.

The choice of shield material depends on the minimum delay that you are using. House hold aluminum foil and the foil used in chewing gum wrappers have a different amount of eddy currents being generated in them bases on their thickness.

Try this experiment on the Hammerhead. Cut household aluminum foil in a 1" square and a gum wrapper also in a 1" square. Wave these pieces under the PI coil and adjust the delay to the point where the response is minimum. You will find that the thinner foil will be detected between 10 to about 15us delay while the thicker foil will be detected between 10us all the way out to much longer delays. Try it and let us know what you find?

You want to use a shield that will have a minimal/no response at the lowest delay that you plan to use. Scotch24 is a wire mesh shield that can be used down to about 7.5 us (maybe even lower?). As a mesh it has less surface area than a solid shield and as such will have less capacitance than a solid shield material.

The coil self resonance without a shield will be higher than with the shield. The shield adds distributed capacitance and will lower the self resonant frequency a little, which is normal. The total capacitance measured between the coil lead and the shield will not be the basis for which the self resonance is lowered. Only about 20% of the coil-to-shield capacitance is imposed on lowering the coil's self resonance based on some of my measurements.

Faster coils have lower capacitance and a higher self resonance which results in higher damping resistor values.

bbsailor

[wyndham]

thanks Wyndham

[Tinkerer]

Coil shielding

Wyndham,

some time a go I wound a fast coil, then shielded it with a graphite coating.
I forgot to pull off the tape that was supposed to produce the gap.
When I connected the coil to the circuit, I found that I had a 1Mhz sine wave superposed on the signal.
Looking for the cause of the 1Mhz noise, I found the fault, no gap in the shielding.
After clearing the gap, the coil responded perfectly.

The 1Mhz frequency was caused by capacitive coupling between the shield and the coil. The coil self resonant frequency being 1Mhz.

Tinkerer