ELLIPTICAL COIL QUESTION

[baum7154]

Thanks OZDIGGER for the info on the commercial coils.

SVEN1 good luck with your 3:1 coil build and let us know how it does!

Best of luck,

Dan

[SVEN1]

As a quick experiment, had a wound 7" round coil sitting in the drawer.
I crudly formed it into a rectangle at 3:1 with it and gave it a test.

Bench Test Results:
Narrow search pattern, tighter target signal, loss in depth on a gold wedding band by 2-3" over the same round coil.
Nails no longer give a double blip. Small gold nugget 1/16 x 1/4" target response somewhat better but, no depth increase.
Pulse delay 10-15us, Pulse Width 10us and 30us.

When I get around to it will make an actual jig in the proper shape of a 3:1 rectangle in a larger size and see what happens.

The biggest thing I noticed when making ellipticals (oval) mono coils, the smaller they get, tighter the signal, nails double blip (using a round coil) turns into a single blip on these types,
and it's harder to distinguish trash from good targets.

Is it possible that the width of a mono coil has something to do with the nail signal response when it comes to making ellipticals and rectangle coils?
Maybe these coils have to be larger in size to still get a double blip on nails?

[baum7154]

SVEN1,

Thanks for the quick info on the rectangular coil test. Do you know the inductance of the new coil? I would guess the dimension of your coil was about 2.5" X 8.5" and that is a pretty narrow width. It is not surprising to me that the detection depth suffers a bit as a result. At what distance was the gold ring detected with the round coil vs the rectangle? Same question for the gold nugget. Your gold nugget I guess is about 1/2 gram or 7 - 8 grains is that correct?

When you make your jig for the 3:1 coil I think a good size that is sold commercially is the 4" X 12". It would be very interesting to know how that performs vs a 10" round coil that has about the same circumference. Most folks comment on the blade like search pattern of the rectangular coil and I can see where that is a benefit providing there is acceptable detection depth, but the detection depth is a concern if the larger coil won't see the ring at 6" or a bit more.


Thanks again for the valuable info!

Dan

[SVEN1]

SVEN1,

Thanks for the quick info on the rectangular coil test. Do you know the inductance of the new coil? I would guess the dimension of your coil was about 2.5" X 8.5" and that is a pretty narrow width. It is not surprising to me that the detection depth suffers a bit as a result. At what distance was the gold ring detected with the round coil vs the rectangle? Same question for the gold nugget. Your gold nugget I guess is about 1/2 gram or 7 - 8 grains is that correct?

When you make your jig for the 3:1 coil I think a good size that is sold commercially is the 4" X 12". It would be very interesting to know how that performs vs a 10" round coil that has about the same circumference. Most folks comment on the blade like search pattern of the rectangular coil and I can see where that is a benefit providing there is acceptable detection depth, but the detection depth is a concern if the larger coil won't see the ring at 6" or a bit more.


Thanks again for the valuable info!

Dan

342uh in round shape
282uh when reshaped into a rectangle
2 1/2 x7 1/2"

7" round coil picked up my wedding band at 9", with the made up rectangle coil about 6". The gold nugget on both coils 3 1/2"


4x12" shouldn't have a problem seeing my wedding band at a greater depth than 6"

[baum7154]

Excellent results and very encouraging with your real world tests! I'm thinking of building the 4 X 12 but no larger as my purpose is small gold nuggets and the small coil needs to go into tight places. Right now I have a 3 X 9 on paper and it has the same perimeter as a 7.6" round but I am concerned about depth with just a 3" width. Another thought is to build a 3.5" X 10.5" and that would have the same perimeter as an 8.9" round coil and we are beginning to get too big for 1/3 to 1/2 gram gold detection. This is the kind of issue a coil analysis by AZIZ would help to resolve. I am going to make the mono coil with a 300 uh inductance and about 2 ohms resistance what ever the profile.

Do you know the weight of your nugget?

Dan

[SVEN1]

I'll have to weigh the nugget, right now it's in a 1" long x 1/4" wide test tube with the cork broken off inside.

[baum7154]

I did a little analysis of 3 various sizes of oval coils, not ellipses, but coils with two half circular ends with straight and parallel mid sections in a 3:1 profile looking a lot like an oval racetrack. The sizes I chose were 3 X 9, 3.5 X 10.5, and 4 X 12 inches.

3" X 9" - circumference - 21.425" - equal to a 6.8" diameter round coil
- circumscribed area - 25.06 square inches - equal to a 5.65" diameter round coil
- ratio of equivalent round coil circumference/circumscribed area - 1.2 : 1


3.5" X 10.5" - circumference - 24.99" - equal to a 7.95" diameter round coil
- circumscribed area - 34.12 square inches - equal to a 6.59" diameter round coil
- ratio of equivalent round coil circumference/circumscribed area - 1.2 : 1

4" X 12" - circumference - 28.566" - equal to a 9.09" diameter round coil
- circumscribed area - 44.566 square inches - equal to a 7.53" diameter round coil
- ratio of equivalent round coil circumference/circumscribed area - 1.2 : 1

To me the circumscribed area of the oval 3:1 coils look like they have greater energy density than the equivalent round coils. In other words the available TX signal power is more concentrated than the round coils even though there is a 'blade like' search pattern instead of the round coil cone/focal point.

Any thoughts or comments?

Dan

[green]

I've been thinking about trying the same as reply #22 starting with a 10 inch round, 2/1 (6 1/8 x 12 1/4), 3/1 (4.4 x 13 1/4). And a 3/1 rectangle with rounded corners. Any thoughts if the corners need to be rounded and if so how much? I remember WAIKIKI_SWEEP showing some rectangle coils with about a 2/1 ratio. I'm guessing about every coil shape has been tried. Probably just a learning exercise.

[baum7154]

This is where a coil analysis from AZIZ would be a great help to visualize the difference between a square cornered rectangle, a rounded rectangle coil but with 4 rounded corners, and an oval coil with 1/2 circles on the ends instead of the square ends. Intuitively I think that the 1/2 circle ends would not have a detrimental impact on the coil performance but at this point only building/testing it will tell. The key thing is that Eric years ago specifically stated that the 3:1 to 4:1 ratios provided the benefits. This is also reinforced by several manufacturers offering rectangular coils in the 3:1 profile. My main concern is loss of depth or rather what is the tradeoff in depth loss for a much better blade type field pattern?

Dan

[baum7154]

If you look at the pictures of coils Wakiki Sweep posted in the thread below, post 11:
Looking for a solution to improve slow coil electronically to let it decay fast and keep R-dump high


you will see that most of his coils look like 3:1 ratios with 1/2 circle ends. Perhaps he can shed some light on the depth and field performance.

Dan

[baum7154]

I've been thinking about trying the same as reply #22 starting with a 10 inch round, 2/1 (6 1/8 x 12 1/4), 3/1 (4.4 x 13 1/4). And a 3/1 rectangle with rounded corners. Any thoughts if the corners need to be rounded and if so how much? I remember WAIKIKI_SWEEP showing some rectangle coils with about a 2/1 ratio. I'm guessing about every coil shape has been tried. Probably just a learning exercise.

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If you can, try both the rounded corners and a semi circle on a 3:1 shape of your 10" coil. I look forward to the results of your tests of the 2/1, 3/1, and maybe even a 4/1 shape factor.

Good Luck!

Dan

[SVEN1]

I doubt you will be able to make the 4 corners anything but rounded

[baum7154]

I doubt you will be able to make the 4 corners anything but rounded

[ATTACH]30167[/ATTACH]

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Agreed…even with unnecessarily crimping the corners there is always some minimum attainable bending radius. From a practical point of view the question seems to be 'what effect does a 1cm radius corner have on the coil performance'? Another question is 'what is the effect just having a semi-circle or large radius on each end of the coil'? Hopefully we can get answers to these questions.

Thanks Sven,

Dan

[green]

Since the thread is about detecting small gold I've been thinking about using a 1/4 x 1/4 inch aluminum can, 1 x 1 inch aluminum can and a nickel for targets when comparing coils. The 1 x 1 inch for a larger signal and the nickel because I have data with other coils. Open to suggestions to why different targets might be better.

[baum7154]

Since the thread is about detecting small gold I've been thinking about using a 1/4 x 1/4 inch aluminum can, 1 x 1 inch aluminum can and a nickel for targets when comparing coils. The 1 x 1 inch for a larger signal and the nickel because I have data with other coils. Open to suggestions to why different targets might be better.

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Excellent idea to use the standardized targets. I was thinking about this last night too and the 1/4 X 1/4 piece of aluminum can is an especially good small target to simulate small gold. The 1 X 1 and the nickle will give a good idea of the depth loss, if any, on larger targets.

As a side note: I was testing with the .090", .180", and .270" long X 1/8"dia. aluminum TIG welding rod targets last night as they are very compact and represent 5, 10, & 15 grain gold nuggets by volume and have less surface area than can or foil material. I noticed that lowering my supply voltage from 12.5 V to 11 volts increased detection distance a bit. Understandable because it helps speed system decay time.

Thanks and good luck on the tests,

Dan