Why Shield Coil ?

[ronbyrne]

Hi Guys, I have built my surf pi and wound a simple 8" coil measuring 1.8 ohms, It works great and i am delighted with its performance. BUT, when i try to shield it, (cooking foil wrap, 2 cm gap,) i wrap a few turns of copper wire round one side of foil ad connect to same part of coax as one end of coil, this then goes to the "ground" side of coil (right in picture) connector. Then I get nothing ! no signal, Nada !



What am i doing wrong?
and what are the advantages of shield ?


Thanks for all the help so far. My Garrett Ace 250 is for sale in local paper and i will only build from now on !

[WM6]

If your detector is working well without shielding coil then you do not need to shield it. By shielding coil you always lost some of sensitivity.

But if your detector is going to unstable work in some circumstances then you probably need to shield your coil. In this case look at this:

http://www.geotech1.com/forums/showp...9&postcount=17

[Rumcajs]

Hi,

how many layer is the kitchen (cooking) foil?
Max two layer is good in the BFO, IB and VLF detectors. The shielding is not easy: if over dimensioning it (too many foil layer or the shield is an metal tube), the sensitivity is worser. It not enough the shield thickness, the ground effect is exist.

Surf PI working without shield: the properly configured delay time is eliminate the ground effect.
If you have making an faster PI, or modifying it (the delay time is shorten than 30 microsec.), must be shielding the coil.

The average PI detector (Surf, Bara, PI2000, etc.) coils therefore never shielded.

***
The 2nd side, if the your shielded coil is ,,working'' without signal, maybe your damping resistor is not correct.

What setting it properly? see this part of documentation the Carl Moreland: Hammerhead 2 , anywhere in the geotech forum...


Good hunting

Rumcajs

[Comp]

????? That seems odd,.... I can only suspect you are 'shorting' the coil out somewhere, either at the connection point, or through a break in the insulation on the coil, somewhere,....
I put a layer of electricians tape over my coil when built,... then a layer of 6mm spiral wrap plastic, then a layer of copper tape with a 30 cm gap, and it is fine,..... I did try it without the screen, and had some slight 'ground effects',.....

The photo doesn't show how you have wired up at the coil/screen end,... any chance of another photo?????

(good luck on the sale of the '150,.... but bear in mind the Surf has NO discrimination,.... you, like me, are going to have a HECK of a big scrap iron/ringpull tab bin!!!!!)

But, on the other hand,... you will develop muscles where you didn't know you had them, and you arn't likely to miss anything!!!!!!!


Good luck, good huntin', Fred

[Altra]

Never mind!