ELLIPTICAL COIL QUESTION

[baum7154]

I want to build a fast elliptical mono-coil to search for small gold nuggets in the range of 1/4 to 1/2 gram(3.8 to 7.6 grains). I would like to have input on the best elliptical coil sizes and proportions in terms of length to width. Right now I have a model laid out in MATLAB that is 80% longer than it is wide, i.e. 9" X 5".

1. Would a 7.5" X 5" be a better choice for this application?

2. Recognizing that everything in design is a trade off is there a proven optimal proportion for practical depth and sensitivity? I know that the circular coil most likely gives the greatest depth, but I like the idea of the elliptical coil fitting in tighter spaces and having a wider and more 'blade like' search pattern.

3. Does the short width dominate the detection depth of the coil?

Thanks for your input!

Dan

[Altra]

Hi Dan,

From my own work with PI coils I concur with your points 2 and 3. A rectangle or
elliptical coil will have about the same depth as a round coil when DIA = narrow width.
On the old PI forum Eric Foster shared some good thoughts on rectangle coils. He had
even worked out an optimum L x W ratio. I can't remember exactly but, something like
W = L/3 ?

I am not an experienced prospector so not sure about point 1

See attached photos of some rectangle coils I made for shipwreck recovery. Wide search pattern
yet still sensitive to silver coins.

Regards

[Henrikras]

Hello Dan..
If you had ask a DD coil small eliptical I would recommend you the DB Shell 5.5" x 7" (see picture).
ps. First I didn't read the thread correctly, then I saw it was a mono coil you were asking, to late to Delete this. So I had edit some.
forgive me.

Henrik.

[baum7154]

Hi Dan,

From my own work with PI coils I concur with your points 2 and 3. A rectangle or
elliptical coil will have about the same depth as a round coil when DIA = narrow width.
On the old PI forum Eric Foster shared some good thoughts on rectangle coils. He had
even worked out an optimum L x W ratio. I can't remember exactly but, something like
W = L/3 ?

I am not an experienced prospector so not sure about point 1

See attached photos of some rectangle coils I made for shipwreck recovery. Wide search pattern
yet still sensitive to silver coins.

Regards

Thanks for your reply and pictures. It looks like the coil you show on the left is the L/3 coil and the one on the right is a L/4 coil. Does the L/3 coil have greater depth?

Dan

[baum7154]

Hello Dan..
If you had ask a DD coil small eliptical I would recommend you the DB Shell 5.5" x 7" (see picture).
ps. First I didn't read the thread correctly, then I saw it was a mono coil you were asking, to late to Delete this. So I had edit some.
forgive me.

Henrik.

Thanks for the input Henrik. Yes my mono coil will be used with a PI machine and I would like it if the elliptical coil would be able see a 5 grain gold nugget at 10 cm in ground. Something to shoot for!

Thanks again,

Dan

[baum7154]

Did some more research in the forum and it looks like rectangular coils in the 3:1 profile as recommended by Eric Foster years ago look promising. The posts I saw were about 6 years old though and I wonder if these coils have proven out in the field. If so why are there not more people building them as opposed to the round coils? It would seem that the rectangular coils would be just about as easy to fabricate and according to some of the posts the performance is better than the larger round coils. Does anyone know of any downside of the rectangular coil?

Dan

[6666]

according to some of the posts the performance is better than the larger round coils.

Would it be possible to see the post that claims that ?
thanks

[baum7154]

Would it be possible to see the post that claims that ?
thanks

From the 3 to 1 ratio coil thread:

Reg
Guru

Join DateApr 2005LocationColoradoPosts559

Hi Boatow,

Thanks for the info but the 3 to 1 ratio idea did come from Eric. What I can't find is any specific info about just how effective it should be and I have searched the PI forum.

My initial tests using a rectangular coil were pretty crude since I don't have any way of minimizing local noise and that can have a dramatic effect on performance. To make matters worse, I introduced a problem into my test PI a couple of weeks ago that affected all results. Fortunately, I found out what was causing the problem and corrected it tonight, so hopefully, I will be able to do a little more accurate testing.

What I have learned so far is with a mono coil, I don't lose that much depth when going to a narrow rectangular coil but do pick up some sensitivity to really smaller stuff because of the narrow concentrated field. Depth capability appears to be better in rectangular form than if the windings were to be reshaped into a round coil. As an example, my test windings in my 3 1/2" by 12" are about equivalent to a 8 1/2" round coil. However, the depth capabilities of the rectangular coilseem to be a little better than a round coil. In fact, the rectangular coil about matches my 9" by 12" elliptical for depth capabilities when testing a nickel as a target.

Overall, I am impressed with a rectangular design and certainly will be using one in the future. It is quite possible that it will be my main type of coil I will use, especially when using a mono coil.

Reg

[6666]

baum7154

Thanks for posting the info, quite interesting
cheers

[baum7154]

Another one of REGs post on the Rectangular coil:

09-26-2005, 08:39 PM

#2

Reg
Guru

Join DateApr 2005LocationColoradoPosts559

Works Great!!!

Hi All,

Thanks to Bill Hays, I had the opportunity to try a couple of different rectangular coils. One particular size, a 3 1/2" by 12" really impressed me. Building up a mono coil for my PI, I tested it against another coil that is about 9" by 12" and found the narrow coil to be about as sensitive. I did try a 3 1/2" by 16" coil but it didn't show any obvious improvement over the smaller one, so the smaller designed was selected.

The nice thing about this new coil design is it fits in places where other coils can't fit so it works great without losing any obvious depth. This is going to be a killer nugget hunting coil.

As a result of this test, I will be trying a big brother to this initial design if a few weeks. The new size will be 5 1/2" by 18". This is large enough that it will be easy to build up in either a DD or a mono coil. I am anticipating this new size will be very sensitive and be competitive to a much larger coil, maybe a somewhere between a 12" and 14" round mono or close to that size. However, the narrow footprint will allow its use in tight places, thus making it another ideal nugget hunting coil.

Actually, both sizes should work very well in the nugget hunting field.

After thinking about it, I can understand part of why it is very sensitve to smaller objects as well as deeper ones. Anyway, the design clearly has merit for those wanting to add a wide range of coils for all applications. As of now, I am hoping Bill Hays will be building these two size forms in the future. I am sure they will be a couple of my primary coils I will be using.

Reg

[baum7154]

Another of REGs interesting comments from 2005

09-27-2005, 07:45 AM

#4

Reg
Guru

Join DateApr 2005LocationColoradoPosts559

Hi nncunew,

The 20 awg should be fine for the coils. I use a smaller wire, generally about a 26 awg since I don't pulse the coils for an extremely long time.

Two good sizes of coils are the 8" diameter and the 11" diameter. These two sizes are very common.

Now, I will use the 3 1/2" by 12" rectangular coil instead of an 8" coil in the future. This rectangular coil seems to have as good of depth as an 8" round coil, or very close to it. The narrow width appears to "sharpen" the signal from objects, thus making them easier to hear. At least, it appears that way.

According to Eric Foster, a rectangular coil with a 3 to 1 or a 4 to 1 ratio will go deeper than a round coil and give more of a blade type field.

Reg

[6666]

Thanks for posting the extra info

[baum7154]

No problem, I'm just trying to do proper research before going to the trouble of building another coil. I want to make sure it is the best profile for the purpose. Unfortunately there doesn't appear to be anyone out there currently building rectangular coils these days and I still wonder why even after Eric recommended them years ago. Makes me wonder what the down side of them is if any.

Regards,

Dan

[OZDigger]

Some commercial rectangular coils here;

http://www.pmcoils.com/apps/webstore/

[SVEN1]

I have made several oval elliptical mono coils, some worked good, some do not, some so-so.
I still prefer the round over the oval.
Now mine were not made to the 3 to 1 ratio. But at approx 2:1 ratio.

No I am curious, will try making a rectangular coil and with the 3:1 ratio