PI coil shielding.

[DanZ]

Would the currents still be present after the ground is re-applied?
Sorry to be asking these questions as it's a learning experience for me. Appreciate your replies.

[Silver Dollar]

I think your shield would become a giant capacitor, just the thing you don't want.

I'm a big fan of a center tapped RX and no shields. If you go a bit slower the standard
PI doesn't need a shield but a fast mono coil with low noise....Might be tricky..

[sinclairuser]

i agree with you silver dollar, for most of the beach pi detectors here shielding is not needed, untill you start dropping the delay below 20 us, the fastest coil i managed to produce has paper coated with graphite, also there is no extra metal in the coil, not even solder the three coil wire connections exit the coil, +,- are continuations of the coil wires, the shield connection is embedded in the graphite paint, these 3 lines terminate in female coil connector on the shaft just before the telescopic union.
each wire is heat shrunk, then another over the whole lot, like home made coax.

[Silver Dollar]

I'm building a basket coil (lucky I took Basket Weaving 101 in college) and will use the no coax
to connect idea. I'll just run the two ends twisted up to the control box. I never thought of
wireless connection to the graphite. I'd defiantly use graphite for shielding you have to make
sure your connections to the shield will never break somehow though.

[DanZ]

Wow, nice looking coil! If I try to make one and shoot for 380uh or close, does Qiaozh's coil calculator still work?

[Silver Dollar]

I've never gotten those calculators to work without some fudging of the numbers. I think some of the
wire sold on EBAY is a bit different diameter than the AWG calls for.

This was originally a simple round coil for the Hammerhead. I figured I needed 60 feet of wire for the
correct number of turns (aiming for 300 uh). I used the same 60 feet of wire here. A couple more hours
winding and I can read the inductance. The problem with this design is all the remaining wire has to
be pulled through each hole so it takes a while to wind it...

For shielding I might try the "fake shield" method where you make sure to use the outer windings on the
ground side of the connector. This makes the outermost windings a sort of leaky shield...

[Qiaozhi]

Wow, nice looking coil! If I try to make one and shoot for 380uh or close, does Qiaozh's coil calculator still work?

The coil calculator assumes an air-cored construction with a square cross-section. It is not suitable for the basket-weave type.

[Silver Dollar]

I finished that coil today. It's 7.5" in diameter and I used 60' of 24 awg silver stranded Teflon coated wire.
The final inductance is 290 uh (was shooting for 300). I'm going to try attaching the outer winding end of
the wires to ground for some shielding. I still need to figure out how to package it..

[sinclairuser]

hi silver dollar, i have one of those meters, if you short the probes on the scale you have there, what does it read?
i deduct this from the total to give a truer reading, i checked this meter along side my fluke ind meter and its near enough for coil building.
if you short the probes and the reading jumps around its the battery voltage, these meters seem to like quality alkaline batts when the voltage drops below 8v they can become unstable, not all but some looks like yours is ok as it would have been jumping around when you took the pic.

[Silver Dollar]

Thanks for the tip on the batteries. It's at the limit of it's range for PI coils but
so far it's done a good job helping me wind some coils!

[baum7154]

I finished that coil today. It's 7.5" in diameter and I used 60' of 24 awg silver stranded Teflon coated wire.
The final inductance is 290 uh (was shooting for 300). I'm going to try attaching the outer winding end of
the wires to ground for some shielding. I still need to figure out how to package it..
[ATTACH]28430[/ATTACH]

If you look at my spider coil posts and pictures in the CHANCE PI COIL thread of this forum you will see one way to enclose your coil in fiberglass using Backer Rod polyethylene foam to fill out the profile. I did not add any additional shielding than that provided by the geometry of the coil itself. Putting system ground on the outermost windings is the key to getting this coil to shield itself and the coil in the pictures does not see a hand wrapped around it with the detector set wide open.

Regards,

Dan

[Silver Dollar]

Well I'd that but I didn't cut out the center of the circle on mine so I'll have to go another way...

[baum7154]

You could still cut the center out of it with a hole saw for ceiling light cans. They are readily available. Or you could mount a router upside down with a rotozip bit in it and find center in your form to put a pivot screw through. Then drill a hole for the rotozip to enter at the desired radius and hand rotate the form past the rotozip 360 degrees and you should have a nice open form to fiberglass. Lots of fun!

Dan