Shield spacer question

[GT Blocker]

Howdy,
I realize that not enough spacer between the coil shield and coil is a bad thing, but what is the upper limit?

Can one space the shield too far away? I mean, given the limits of the coil housing's volume, of course....though vacuum forming my own coil housing does give me some leeway.

Atm, I have two wraps of electrical tape covered by about .2" Teflon tape (300 feet of PTFE plumber's tape on a 9" x 3" rectangular coil).

I'm using 1/4" x .001" copper tape (used for stained glass window work) in a slightly spaced spiral wrap for my shield (Yes, I know there are other materials, but that's what I have handy).

I stripped the copper tape off of my rec coil (DD) to try a thicker spacer (hence the tef tape). Right now, I can add more tef tape if you folks think I need more....

But how far should I go? Or is this one of those "give it a try, you're on your own" kinda things?

I'd rather not re wrap the shield yet again, if you know what I mean.
Any input on this one?

Curious, GTB

[homefire]

Time to show My Ignorance again!


Q: Why do you want to shield the coil?

The PI does not use RF for detection.

Q: Would Not the Shield Impede some of the Eddy Current power from being Inducted within the Coil?

Just some thoughts.

Jim

[homefire]

Never Mind! I got schooled here.

http://thunting.com/geotech/forums/s...lding+PI+coils

[Tinkerer]

Shielding spacer

Howdy,
I realize that not enough spacer between the coil shield and coil is a bad thing, but what is the upper limit?

Can one space the shield too far away? I mean, given the limits of the coil housing's volume, of course....though vacuum forming my own coil housing does give me some leeway.

Atm, I have two wraps of electrical tape covered by about .2" Teflon tape (300 feet of PTFE plumber's tape on a 9" x 3" rectangular coil).

I'm using 1/4" x .001" copper tape (used for stained glass window work) in a slightly spaced spiral wrap for my shield (Yes, I know there are other materials, but that's what I have handy).

I stripped the copper tape off of my rec coil (DD) to try a thicker spacer (hence the tef tape). Right now, I can add more tef tape if you folks think I need more....

But how far should I go? Or is this one of those "give it a try, you're on your own" kinda things?

I'd rather not re wrap the shield yet again, if you know what I mean.
Any input on this one?

Curious, GTB

More space should reduce the capacitance.
Reducing the capacitance reduces the time needed for the TX flyback to decay.
shorter decay time increases the sensitivity.

I use 5 to 8mm polypropylene\air spacer or as much space as I have.

However, under water no shielding is needed.

Tinkerer

[GT Blocker]

Interesting

Hey homefire, Tinkerer,

I'm curious as to why a shield isn't needed for U/W use.

I figured (wrongly, it appears) that the ground noise issue would still exist U/W.

Not shielding would definitely be a plus for my U/W use. The U/W HH will be used in the freshwater rivers and streams up in N. California's gold country.

Nice to know, thanks.

Since the first unit will be tested here in Arizona's gold fields on dry land(Bradshaw Mtns.), I'll probably need to shield 'em for this one, though. Should be an interesting fall/winter (if it EVER cools down, 106F today!).

I'll double up the tef tape and give it a try. Guess I could add some spiral wrap as well, since it appears hard to 'over space' the shielding.

Any other tips/pointers beyond the obvious posts?

I've read practically every page I can so far on the subject and find myself wondering about things like this.

Thanks, GTB

[homefire]

GT:

I just don't see that much advantage of shielding the coil on a PI.

More loss of TX and less RX is all I see. As mentioned earlier, More coil Cap if not completed right.

In the Water your subject to less RFI and the coil is being subjected to grounding via the water it's self.

Out in the field, your not subject to RFI as your away from it all.

The Ground is not going to effect the coil like a detector that is using RF simply because it is not using RF! Magnetic Pulse Yes, Not RF.

I plan on Etching a coil and have no plans to Shield it.

I too am after Small Gold.

Oh, Yea! Don't sell the Board yet Please! Sorta short on Funds and will get it to ya!

[Tinkerer]

detecting small gold

Hey homefire, Tinkerer,

I'm curious as to why a shield isn't needed for U/W use.

I figured (wrongly, it appears) that the ground noise issue would still exist U/W.

Not shielding would definitely be a plus for my U/W use. The U/W HH will be used in the freshwater rivers and streams up in N. California's gold country.

Nice to know, thanks.

Since the first unit will be tested here in Arizona's gold fields on dry land(Bradshaw Mtns.), I'll probably need to shield 'em for this one, though. Should be an interesting fall/winter (if it EVER cools down, 106F today!).

I'll double up the tef tape and give it a try. Guess I could add some spiral wrap as well, since it appears hard to 'over space' the shielding.

Any other tips/pointers beyond the obvious posts?

I've read practically every page I can so far on the subject and find myself wondering about things like this.

Thanks, GTB

Tips?

Use the highest PPS rate that you can and reduce the power way down. 30us Tx time is plenty for small gold.

This will help reduce the delay to the first sample.

Pay lots of attention to the cable from coil to pre-amp. It is easy to loose 5us just because of the cable.

Tinkerer

[Ferric Toes]

GT:

I just don't see that much advantage of shielding the coil on a PI.

More loss of TX and less RX is all I see. As mentioned earlier, More coil Cap if not completed right.

In the Water your subject to less RFI and the coil is being subjected to grounding via the water it's self.

Out in the field, your not subject to RFI as your away from it all.

The Ground is not going to effect the coil like a detector that is using RF simply because it is not using RF! Magnetic Pulse Yes, Not RF.

I plan on Etching a coil and have no plans to Shield it.

I too am after Small Gold.

There are big advantages to shielding a coil in a PI, in fact I would not contemplate designing one for any application without a shield. Shielded correctly, and outside rf from transmitters etc is attenuated. Such transmitters may be outside the bandwidth of the detector, but the demodulated products can be well within it. How is it demodulated? There are usually protection diodes on the input that do this quite effectively and the RX is very non linear at rf which also demodulates or produces offsets.

Shielded correctly, there is no attenuation of either TX or RX signals as the shielding time constant will be much faster than the shortest target time constant.

Sea water certainly attenuates rf but for best performance and least false signals you need to keep the coil to water/ground capacitance constant. This is the case with a shielded coil, but not with an unshielded one. Particularly on the waters edge, each time you dip the coil in and out of the sea, or touch wet seaweed you will get a false signal.

If you are designing a coil for small gold, you will need a shield of some sort between the ground and the coil. The shorter the delay you use, the more critical the coil becomes regarding exterior capacitive effects. Coil to ground capacitance results in a signal that is in opposite polarity to the eddy currents from a metal target, which will result in loss of sensitivity and much ground noise. Having a shield prevents this as the coil winding always sees a constant capacitance.

Eric.

[homefire]

Ferric:


Thank you for the info!

"If you are designing a coil for small gold, you will need a shield of some sort between the ground and the coil. The shorter the delay you use, the more critical the coil becomes regarding exterior capacitive effects. Coil to ground capacitance results in a signal that is in opposite polarity to the eddy currents from a metal target, which will result in loss of sensitivity and much ground noise. Having a shield prevents this as the coil winding always sees a constant capacitance."

Jim