Concentric coil for TGSL.

[Thomas]

Progress


Just a small birdnest

[ODM]

Nice formers Thomas. Rigid foam and firm sheeting make for lightweight and solid structures, lots of that in the composites industry - extruded instead of expanded foam makes for more stiff coil formers, and air filled material has better dielectric properties than solid plastic insulators.

Would be fun to see Aziz analyse that lower "coil" in his field simulation program

[dfbowers]

Thomas,

Good idea with the foam board. I picked up a 1/2" sheet at the local craft store today. I think it will save some time not having to building a foam "sandwich" as it already includes the cardboard skins. It seems plenty stiff enough.
Don

[6666]

Here is a quick Video on what to expect. I'm using a gold ring for testing.

http://youtu.be/r-F5bu0m3N4
Don

Don have you tried that at the beach yet ?

[dfbowers]

Don have you tried that at the beach yet ?

No, just one field test with the 7.5" coil.

http://www.youtube.com/watch?v=a6kkK...IoCBxg&index=1


I will probably vacuum form a shell for the 10" coil this week.
Don

[dfbowers]

10" Concentric for the TGSL project.

I'm going to start making a case for smaller coils.. Making a pair of 10" concentric shells was twice as tough as making a pair of 8" shells! It's really tough to get the size just right so that the two halves fit together snug.. but after a handful of wasted shells I think I have it figured out.

Scaling up a 7.5" coil to 10" adds 5 ounces (12 oz total for the finished 10" coil). Not only that, but it only adds about 1" or so of detection range for coins over the 7.5" coil.

On the positive side, the 10" concentric seems to have slight edge on detection range over 10" DD coils. Are we reaching the point of diminishing returns on coils for this project??

Don..

[Mullihaka]

Another great experiment, thanks!

How about nulling, compared to the DD coils: is it any easier i.e. how microscopic movements does the "nulling" loop require?

I'm trying to figure out, if the relative positions of RX and TX/bucking coils need to be as well fixed as with the DD coils?

If not, the epoxy could be replaced with lighter material, like polyurethane foam.

[dfbowers]

Nulling is not that difficult. The problem is that adding turns to the bucking coil will increase frequency just a bit.. so you must add a turn or so to the tx to get the frequency right again.

[dfbowers]

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

[kabier]

hi dfbowers here am adding my shield to the cc and hope to be useful.i used bare copper wire and fix alu. foil to the foam,then wind the coil and fold the foil and again apply foam with white glue.

[dfbowers]

Might work as long as you do not create a complete circle with the aluminum and fold it tightly around your windings. The spray graphite is actualy easier and less likely to be detected by the coils. Also, you are likely to spoil your null if you use large areas of aluminum. I have actually used small bits of aluminum to fine tune nulling. . You can cover the coil with paper and spray the paper with graphite as an alternative. an Any pictures?

[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

[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

[kabier]

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

[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?