Very Large PI coil construction?

[Boattow]

my 2 cents

For building large coils, the professionals use a multi cable in one jacket. Usually 7, 8, or 9 wires in the jacket and unshielded cable. They will all be different colors. Take one color wire, make it the end to be connected to your feedline, typically coax, then the other end to another color wire, then the other end of that wire to the next color, so you are in effect creating several turns by doing this. That's why on the closeup of the video you saw, it was just one large cable in the PVC pipe.

The large cables are not shielded. Your large coil will not be able to be operated at a very short delay. Probably the shortest will be approximately 30us, therefore you won't have to worry about capacitance effects with the ground and won't need a shield.

Again, you don't want to pot it. Leave the wire loose inside the PVC just as you saw in the video. The coil will be heavy enough with the weight of the PVC pipe. It will be very heavy with potting and there is really no good reason to pot it since you are using it on dry land and not dragging it through water.

This is how the big boys do it

Good luck,
Boattow

[Roughwater]

Large completed mono coil for whites TDI

Boattow,

Appreciate the info, wish I had this info prior to building my large coil. I suspected a large coil could be built just as you previously described aside from the lack of shielding but never saw it in print so I went with what I know.

If I build another large TDI coil I'll definitely do as you suggested and find some premade multi-stranded cable of the appropriate # of conductors and go that route.
It would be so much quicker and neater than the method I used. But if I build a coil less than 1M but larger than 18" I probably would try to find a SHIELDED multistrand premade cable. If the shielding proved to be too much I could easily disconnect it from the ground.

For what it's worth I didn't attempt to pot my 1M coil but I did shield it. I cut an entire roll of scotch 24 down the middle and it took the entire half roll to shield it with one continous piece of shielding. I have never previously used such a large coil so I don't have anything to compare it to but I am very happy with it's air test depth and effective detection radius on large metal objects. It will air test 6' on a small roundish tea kettle but what impressed me more is that it will detect it more than 3' to either side of the coil giving it a 9' + field of detection.
I can only imagine what field of coverage 25 or 50' of of coil wire would give. Now I just need to find a place where I can best put the coil to use.


Terry