please some advice to do the shielding

[Henrikras]

Hi Guys.
I got a VLF Detector 8 khz (Blisstool) are I did succes to make my first 13" DD searchcoil with a Shell from Georgi. BUT!
It was working very Nice in the lab, but when I try it on a regular field, I had some problems with to much EMI or electrostatic. My Stock searchcoils 11" and 15", didn't had the same problems on the same field, the same day.

I was using 2 component polyester to close the search coil.

The shielding I did, was with the spray contact 33 graphite. I sprayed inside the Shell, ( 1/2 bottle ), to I got
approximately 1-2 kohm was measured pr.inch. I did test that the ground wire (plug) was connected to the shielding.
I could had made a mistake when I finish the coil, because it was difficult to spray the graphite on the polyester and get connection to the graphite I sprayed inside the Shell, so the Sheilding down to the soil properly was bad.

I had read all lot of thread about shielding, but I am a little confused. Thats vry I was hopeing that you could help me with some good advise, how to do this Sheilding the best way. ( stupid to do same mistake on my second coil try).

Some questions I got !
1. What will recommend of resistance to a VLF 8 khz. ( 10k ohm / inch - 1K ohm / inch - or less / more????? )
2. How is the best way to finish the searchcoil, to get proper Sheilding, top / bottom and all sides.?
3. Could my EMI problems be because I did a bad finish with the shielding down to the ground?

In advance thank you very much.
Henrik

[WM6]

Hi Guys.

3. Could my EMI problems be because I did a bad finish with the shielding down to the ground?

Sure, this part of shielding is most important, cause it "remove" (take to circuit ground instead to signal line) stray capacitance between coils and soil. You need to add conductive ground contact point to bottom side of coil, spray bottom (ground contact inclusive) with graphite and paint it over again using epoxy or polyester.

Your "EMI" is caused by (by sweeping changeable) parasitic capacitance between soil and (down) unshielded coils.

[Henrikras]

Thanks WM6....
I must find a way to do this... My new plan is try to take the molding out of the Shell, and then spray it... Then put it back to the Shell, and then finish the bottom with a 2-3 mm polyester. The difficult part is to have 1-2-3 mm distance from the coil wires to graphite, by the thickness of the polyester.
Ps. Do you recommed that I go fore a low resistance like 1k or Maybee lower... ? 8 khz isn't so high !
Henrik.

[WM6]

Ps. Do you recommed that I go fore a low resistance like 1k or Maybee lower... ? 8 khz isn't so high !
Henrik.

For me graphite shielding resistance of about 500 - 600 E (measured between two shielding points at distance of 1cm) is working fine. 1kOhm should be good also. You need to rub a little your testing probe to graphite surface to establish good contact during measurement.

[Henrikras]

Coil developt

Its going okay with my second coil try... 500-600 ohm / 1-2 cm by the graphite 33.
I Notice that the small part of the Shell, needed more spray to get down under 1K ohm. ( had to spray, let it Dry, spray again).
I am useing a Tornado coil cover to keep the search coil together, and after it is finish, working and finetuned, I will use some part of the Georgi shell, as the upper part. I can show you later in the proces, but I think its a good way to make it. By this way to do it, I really can do the Sheilding as I want, before the coils go in the Shell. I can allso really finetune the nul, and do the connection before the upper part will Be glued.
I did put some Very small cobber wires from the graphite spray in the bottom, to get proper connection to graphite spray at the top of the polyester. ( this job is not made at the picture yet)
I Remembered to make a gape by a small tape at the shielding, to avoid shortblocking..

If you notice something I could hade forgotten, or something good adwise, please tell me.

Thanks.
Henrik

[Henrikras]

Succes with the coil

Thanks to WM6, My new coil is working. Been out today and enjoyed the 630 grams 13" DD coil.
I even found 2 rare finds.
The difficult part was to keep the Nuul when the epoxy/polyester was drying up. Had to use a small ferrite to find the balance again.
What a feeling useing a coil made by your self....
Henrik.