Concentric coil for TGSL.

[multieagle]

It is twined stranded (.48mm approx. 24AWG) speaker wire. Negative wire is tinned, or silvered. Excuse my ignorance if wrong, but if there is too much graphite (I did give it a second layer, as had missed some spots on first try), would it not prevent all detection, even when the wire ends are joined? Strange there is a few inch aura, with strong continuous sound-off only when the wires are connected. With ends loose (loop is open) or with negative/graphite shield wire connected, there is limited (sparse, spaced, slow) pulse-like detection. On a hunch I suspended the coil in the air (had previously held it in my hand) with same results as when hand-held. Wish I had received my oscilloscope to demonstrate. I've added an mp3 file which may better example my question. First part of sound clip is with wires loose, second part with neg/pos wires connected. You may have to turn your volume up to hear. In my inexperienced opinion, this is strange. Can anyone explain in layman terms? Thanks.

[Davor]

Maybe too much of a good thing?

Unless your new coil is wound with litz wire or tinned braid that effectively reduces eddy currents in wire, it could be that you have a wee bit too much of graphite there

[multieagle]

After some testing with two other coils I constructed, it is apparent that the Garrett pro-pointer, in the two unshielded coils, is detecting the insulated wire. In these two, when the circuit is complete by joining the negative/positive terminals, there is an increase in pointer response. In the shielded coil, there is no detection of the coiled wire UNTIL the wires are joined. Then the whole coil becomes active. I assume the pro-pointer (or any such pointer) can be used, sans a multimeter, to prove the effectiveness of a shielded coil.

In the unshielded coils with the wire ends not connected, the pro-pointer must be almost against the coil. When the wires are connected, the pro-pointer detects the coil several inches above the coils. In the shielded coil, there is no detection close to the coil. When the shielded coil positive/negative end wires are connected, the field can be detected a few inches above the coil. With the shielded coil, when the shield ground wire is added to the positive/negative wires, the response drops back toward the coil.

Interesting.
It is difficult learning basic electronics as a senior citizen!!!

[multieagle]

Well, I've completed the coil, graphite shield, coated with resin, but cannot test as several items are on a slow boat from China. One observation. Using a Garrett pro-pointer I cannot get a signal from the coil or bare wires until I complete the loop (positive/negative). If I use a separate piece of the same wire (coiled or uncoiled) it signals okay without the ends joined. Does this mean that the shielding is working? Guess the question is, why does the wire not produce a signal when the ends are not joined together?

[golfnut]

Shell ready for paint and coils.

pcb thru lugs pinned using M5 nylon stud



S

[multieagle]

Thanks. Wow, that is real cool! I picked up a couple of items this afternoon at a second hand store for $5, a heavy duty exercise thingy which I took apart 10" diameter, and some manner of weaving contraption with removable pegs, OD around 11" I figure the pegged item will be good for some experimental coil weaving.

[Davor]

IMHO this is too neat

[golfnut]

[QUOTE=multieagle;170122]Thank you golfnut. I was looking at coating with epoxy, but rather expensive. I constructed a 9" coil with doubled (-+ wires), applied a coat of glue, wrapped in two layers of spiral wrap, and then coated with graphite dry lube. What I planned on doing is to place the coil in a trimmed flowerpot holder and then wrap with resin and fiberglass mat, which could be sanded and molded to look half decent.




Worth a try!! If is a mono for a PI sometimes you dont benefit much from a screen.
You may need to get the graphite across the flats more to give a good area with a low series ressitance in all directions. Most commercial outlets paint the shell with the graphited paint and have a fine wire bonded to that - which goes to cable ground.




This is my tesoro 8" scuff plate with the Lugs designed and placed in - made earlier today.. Going to try making a slimline affair using a single cover. If good I will make a spare..



S

[multieagle]

Thank you golfnut. I was looking at coating with epoxy, but rather expensive. I constructed a 9" coil with doubled (-+ wires), applied a coat of glue, wrapped in two layers of spiral wrap, and then coated with graphite dry lube. What I planned on doing is to place the coil in a trimmed flowerpot holder and then wrap with resin and fiberglass mat, which could be sanded and molded to look half decent. See images.

Gord

[golfnut]

Glassed old one

Bit heavy and ugly, Al foil wraps.



S

[golfnut]

What Don does is best.

Ive glassed a coil and it comes out heavy and not pretty. The glass is tough to wrap neatly around the curve of the coil.

Carbon fibre sheet would be lighter but pricey and difficult to work. - dust is irritant.


Im experimenting with a Tesoro 8" scratch cover - they are rigid, good quality covers - attaching the lugs is then the chore - Im going to try glass fibre pcb panel.

You could have 2 covers back to back like this.... Or a single one..





S

[multieagle]

Great thread. I do not have access to a coil case and was wondering if the completed coil could be fiberglassed, or epoxied?

One other question. I stripped some 24 AWG flat FM wire out of its rubber casing. I do not think it is varnished. Can bare, unvarnished wire be used for coil construction, or would act as a solid plate?

[kabier]

hi no complete circle (Faraday shield) and i have a good null 3mv.and detecting a copper 2cm diameter 24cm in air.

[dfbowers]

HI Don, I cant hear the sound on your clips as no cans..

What sort of depth were you getting on the those 0.5" shot??

I would be interested to see a head to head with your 10" DD vs concentric


Steve

Hi Steve. Yea, the sound problem must be with your PC as I can hear the audio OK with my PC browser and on my phone brower.

Not much difference between the 10" DD and concentric but I will have to give a slight advantage to the concentric.. Just trading sweep area and weight for a little bit of depth.
But. cost is the big thing. I can put a concentric together for around $20.

Don

[golfnut]

Time for some ground testing!! Let's compare the 8" to the 10" home built concentrics. Well the 10" performs a bit better than I thought it would. I buried some .577 lead Minie balls in the back yard a few years ago after clearing away anything iron and are buried at 1" increments. Turns out, the 10" detects about an inch deeper than the 7.5" coil (for this test anyway).. Both are ground balanced to ferrite so doing an actual ground balance may improve depth slightly.

8" and 10" ground tests:

http://www.youtube.com/watch?v=4T8pjKr1Weg

http://www.youtube.com/watch?v=TWrJwB3Pmu4

HI Don, I cant hear the sound on your clips as no cans..

What sort of depth were you getting on the those 0.5" shot??

I would be interested to see a head to head with your 10" DD vs concentric


Steve