Shield Spacing

[Bill]

I just finished winding a new "fast" mono coil to go along with my new HH PI. I had a question concerning shield spacing. The coil specs: 28 turns of 26awg silver plated stranded copper wire with teflon insulation. 2 layers of 1/4" PE spiral wrap followed by a layer of 3M 1190 copper cloth shielding tape. L= 295uh

I used one strip of 3M 1190 tape but the coil is pretty fat so with one strip there is a .155-.20" gap around the inside of the coil. SEE PIC. Should I go ahead and cover this gap or will it have any noticable effect. I do remember reading somewhere that shielding layer consistancy is important (eg small portions of doubled up layers can cause problems)

[bbsailor]

Fast Mono Coil

Bill

Run a quick test before sealing up the coil in the coil housing by doing the following.

1. Attach a ground connection lead to the 3M 1190 tape by carefully soldering a piece of stranded wire to one end of the shield near one end of the 1190 tape. Do it by carefully lifting a short piece and slipping a small piece of flat metal under the solder point. This will act as a heat sink and help prevent burning through the tape. Practice on a 1" piece of scrap first.

2. Attach you coax, the same length you plan to use on the final coil, to the coil connections and the 3M 1190 ground lead.

3. Connect the coil and coax to the PI control box.

4. Adjust for the longest delay and wave a nickel under the coil to see that the coil is responding. Reduce the delay while waving the target under the coil and ensure that the coil continues to respond at the lowest delay. If not the coil and coax have too much capacitance or your damping resistor may need to be slightly adjusted. You may need to add a 10 to 20 ohm series resistance to reduce the current in the coil to sample at lower delays.

5. Suspend your coil in mid-air from a few strings and orient for the least amount of noise pickup.

6. At the lowest delay, move your hand near the coil and see if you hand is detected. If so, the shield needs to be improved for better coil coverage. move your hand near the gap to see if the gap is too wide. That gap only needs to be about 1/8 inch so as not to make a completely shorted ground ring on the coil.

7. On your final coil, spiral wrap the shield with electrical tape to hold the shield secure against the coil spacer. This will prevent false signals if the shield moves. You can always add another shield layer on the inside but you need to ensure that this piece also has a soldered ground lead also attached to the coax outer braid connection.

What is the inside diameter of the coil in the photo?

What is your detection distance on a nickel?

How did you measure/calculate the coil inductance?

These tips should be helpful to try before finalizing your coil.

bbsailor

[Bill]

Thank you BBsailor. The ID of the coil is 8". I used a bench model LC meter to measure the inductance. I am still waiting on my low capacitance 2 conductor coax to arrive in the mail. As soon as it does I can begin taking working measurements.

I have not yet done the free air resonance but I will tonight when I get home.

I know that at higher frequencies black electrical tape is a no-no, it will not effect coil performace? I planned on wraping the coil very tightly with a cloth tape and then saturate the tape with Varnish.

[bbsailor]

Bill,

According to my calculations a 24 turn, 8" diameter coil would have 319 uH inductance. 23 Turns willl give you 294 uH and with the shield added just over 300 uH. 28 turns appears a bit high, but give it a try. If you can't get down to 10 us delay, remove 4 or 5 turns.

To see if the vinal electrical tape makes a difference, measure the free air resonance without the tape and then again with the tape. If the tape is affecting the coil you will see a lower self-resonance with the tape. I doubt that that tape will have much more than a few hundred cycle difference which is hardly noticible in coil performance.

Why are you using two conductor coax... for a DD coil?

Mono coils only require a single conductor coax.

You can use any short piece of coax, RG8X (stranded center conductor) from marine stores for a quick test. Even 75 ohm TV coax will work for a test.

Keep us posted about your progress.

bbsailor

[Bill]

The dual conductor coax is for a DD but I also ordered single in the same shipment, so yes I will be using single for the mono. You are right with your calculations, I screwed up, the ID is actually just over 6.5". I did not measure very well last night. I will try a small length of coax tonight to see what kind of numbers I can get and also find out if the shielding is suffiecient. Thanks again for your help! I am hoping to make a PI with a good sensitivity to small gold.

Bill