Ground capacitance of PI coils

[Teleno]

A lot PI coils are spiral designs or low capacitance spider webs and variants. They all have in common that every turn is at equal distance to the ground, thereby maximizing ground capacitance.

I believe a vertically wound coil will minimize ground capacitance effects because the surface it presents to the ground is much smaller.

Any comments/experiences on this?

[geoscash1]

I have been working on a form to build a wind for this very type of coil. I had been reading a bunch on all the different coils and wondered why this had not been researched more myself. Mine is a piece of 8 in. pvc in which the height will be .65"/.75", one at 6" and one at 8" in diameter.

So, as soon as one of mine is wound I'll be testing with my mirage, MPP and a crossbow to see how it works.

[jladre]

I've seen many people using coils like that style before in thier bfo machine and one person using for her sufp pi.

[Carl-NC]

Ground capacitance effects are minimized with shielding. However, a vertical winding is more magnetically efficient than a flat spiral.

[kt315]

I've seen many people using coils like that style before in thier bfo machine and one person using for her sufp pi.

this is a nice idea to put PI pcb in a BFO and make a bet with someone on depth ))

[Teleno]

Ground capacitance effects are minimized with shielding. However, a vertical winding is more magnetically efficient than a flat spiral.

Let's take a fast coil with very low capacitance. I'm talking PI.

By placing it close to the ground it's no longer a fast coil due to ground capacitance.
By shielding it's no longer a fast coil because the shield adds a large capacitance of its own.

It would seem shielding just keeps the (large) capacitance constant to avoid variations in the the response that could be falsely interpreted as a target.

The question then is whether it's worth going to the lengths of basket-weaving fast coils only to lose all the speed in the end, whether to ground or to shielding. Any answers?

[homefire]

Keeping a eye.

[green]

Let's take a fast coil with very low capacitance. I'm talking PI.

By placing it close to the ground it's no longer a fast coil due to ground capacitance.
By shielding it's no longer a fast coil because the shield adds a large capacitance of its own.

It would seem shielding just keeps the (large) capacitance constant to avoid variations in the the response that could be falsely interpreted as a target.

The question then is whether it's worth going to the lengths of basket-weaving fast coils only to lose all the speed in the end, whether to ground or to shielding. Any answers?

The shielded spider web coils I've made have been faster than the not shielded bundle wound coils I've made. I would think the vertical wound coil would need shielding also. A 300uH coil with 84p total capacitance has a 1MHz resonance. What circuit resonance(Rd and Rin disconnected) would you consider fast? baum7154 doesn't need shielding with his 3DSS coil. Don't know what the circuit resonance is.

[Teleno]

A 300uH coil with 84p total capacitance has a 1MHz resonance. What circuit resonance(Rd and Rin disconnected) would you consider fast?

1us decay time, which is 1MHz resonance and above.

baum7154 doesn't need shielding with his 3DSS coil.

I believe the 3DSS combines the best of both worlds by kepping a larger average distance relative to ground (which limits the grond capacitance) while allowing a horizontal winding for better target induction.

This page provides a formula to calculate the capacitance per length unit of a wire above a ground plane: http://members3.jcom.home.ne.jp/zaki...ripline_z0.htm While directly applicable to flat coils I don't see how it can be easily extended to describe a vertical winding.

My calculation for a 20cm diameter flat coil with 10 turns of 0.5mm wire 1cm above ground is 186pF.

A 3DSS would increase the average distance by 0.5cm, giving 107pF. That's a lot. Now if you shield it you'll get over 300pF which is catastrophic for detecting fast time constants.

[baum7154]

1us decay time, which is 1MHz resonance and above.



I believe the 3DSS combines the best of both worlds by kepping a larger average distance relative to ground (which limits the grond capacitance) while allowing a horizontal winding for better target induction.

This page provides a formula to calculate the capacitance per length unit of a wire above a ground plane: http://members3.jcom.home.ne.jp/zaki...ripline_z0.htm While directly applicable to flat coils I don't see how it can be easily extended to describe a vertical winding.

My calculation for a 20cm diameter flat coil with 10 turns of 0.5mm wire 1cm above ground is 186pF.

A 3DSS would increase the average distance by 0.5cm, giving 107pF. That's a lot. Now if you shield it you'll get over 300pF which is catastrophic for detecting fast time constants.

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Interesting discussion here Green and Teleno. The beauty of the 3DSS coil wind is the three dimensional concentric nature of the coil construction with ground placed on the wire end of the last/outermost windings. Using a continuation of the coil wire as a twisted pair feed line also keeps capacitance low and eliminates solder joints in the process. For a real world 335uh 8" round 3DSS coil the self resonant frequency with 34" feed is about 1.050 mHz. (This measurement was made using the technique BB Sailor suggests in his FAST MONOCOIL paper.) Once placed in circuit with a series HER208 diode between the mosfet and the coil I think the resonance drops into the 980kHz range.

Colorado this time of year is a very dry and static prone area and I have noticed effects of it on the detector especially in 'AIR TESTS" and since shortening my sample delay to about 7us on the Chance PI. Anti static spray tames it down and actually it is much less of an influence outdoors in normal use. It is my understanding from reading the forums here and elsewhere that even coils with a separate traditional shield suffer from static influence and many use static spray to control it. Some even go to the lengths of dragging a small chain conductor from the boom of their detectors to discharge it. It seems that any plastic dielectric coating or coil shell protecting the coil with its traditional shield inside will gather a static charge. I have heard of some that are using graphite much like anti static spray to help control it. I wonder if a resistive plastic material like that used in static protective bags could be fabricated into coil shells and use the protective shell itself as the coil shield with its own connection to system ground. This would eliminate the fabrication chore of a traditional shield and help with static charge dissipation too.

Regards,

Dan

[Teleno]

Ground capacitance effects are minimized with shielding. However, a vertical winding is more magnetically efficient than a flat spiral.

What would be the point of making a fast, low capacitance coil just to have it shielded and turned into a slow coil?