Concentric coil for TGSL.

[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?

[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]

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

[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.

[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]

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..

[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

[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]

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

[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

[Thomas]

Progress


Just a small birdnest

[Thomas]

OK, got it, I think this is going to be a good one.

[dfbowers]

Don, any guess how deep a channel I should cut in the foam with a 4.5mm thickness?, I want to make sure I don't end up with spill over, or a deep recess. Or, what is the cross section diameter of your bundle of wire

Thomas,
It really doesn't matter how your coil ends up. There are three important things though that you need to pay attention to:

1. The mechanical stability of the assembly when you are done - It must be stiff and your wires should be fused together - I use krazy glue, but be careful not melt your foam.
2. The final inductances of the Tx and Rx coils - otherwise your phasing will be off and GB and disc will not be within the adjustment range of your pots.
3. The final null - Get the deepest null possible. If the residual signal is too high, your first op amp may end up in saturation.

Your coil could end up looking really ugly and still work just fine.. If you get it to work, I will send you something in the mail to finish your coil as a reward


Don

[Thomas]

Also I found the graphite spray, It was in the spray lube section, the guy at the door and lock section had no idea what I was talking about. If I get results close to yours in the video I'll be happy.

[Thomas]

Don, any guess how deep a channel I should cut in the foam with a 4.5mm thickness?, I want to make sure I don't end up with spill over, or a deep recess. Or, what is the cross section diameter of your bundle of wire