Coils - Elliptical Vs Round

For the discussion on equivalent depth of an Elliptical coil to a Round coil than if we were to compare a Commander 15"x12"DD and lets say a Coiltek 24"x12"DD then both “generally” should be equal in depth to one another if using the “elliptical coil is generally only as good as its width” theory.

Now for both these DDs I would expect both transmit and receive loops to be approx 8” in width including an overlap of 2”. Therefore if using the elliptical coil’s width theory both of these DD coils should then be equivalent only in depth to an 8” round mono coil which is depressing for swinging such a big coil.

However my depth comparison using a 24" elliptical DD coil I’ve mentioned above against an 8” round mono proved not be the case as I found the round coil had the edge on smaller targets while the elliptical had the edge on larger targets. Also if the targets increased in size then the greater the edge, as in depth, to the elliptical.

I would have to assume a similar result if comparing 15"x12" or 24"x12" elliptical monos with a 12" round mono although maybe not to the same extent on smaller targets but an ever increasing edge still with the elliptical as a target increases in size.


Brian

G’day Brian,

Here’s how you can work out the rough loop sizes, rough in that we can’t tell for sure the actual dimensions of the coil unless we dissect it. So going by the shell sizes for all comparisons should equalise this.

An elliptical calculator here; http://www.csgnetwork.com/circumellipse.html will give you the circumference of the housing. eg a 15x12 elliptical mono has a circumference of ~42.67 inches, divide this by pi (3.14159….) and we get an equivalent circular diameter of 13.6”

The 24x12 mono has an equivalent circular diameter of 19 inches, a lot wider than the proposition to use the short axis width (i.e.12”) as a gauge.

With DDs divide the circumference by 2 and add the diameter (with ellipticals add the maximum diameter) then divide by pi eg. 24x12 elliptical DD has a circumference of 59.6. Divide by two, and add 24 (long axis). Now we have the approximate circumference of one of the loops which is 53.8”. Now divide by pi and we have the equivalent circular diameter of 17” Thus a 24x12 elliptical DD has an approximate field density on the same detector to a 17” round mono.

In actuality of course there are other factors at work here, equality between coils relies on the RLC (resistance, inductance and capacitance), critical dampening, shielding and winding medium and method of the coils being the same, which will most likely not be the case with different makes and models.

I guess what this all boils down to, is that your observations Brian are correct, using the short axis of an elliptical to compare it to a round is not valid.

Cheers
Grey.

Whilst we caught up with depth on coils, the benefit of having an elliptical is getting around obstacles such as tuffs of grass, fallen tree limbs etc.

Over the past weekend, I use a 17" NF Elliptical and whilst it is sensitive to both small and large nuggets, the people who found most of the nuggets where those with 8 inch elliptical monos. A lot of the ground had been thrashed to death but they found alot of small ones by going around the grass in gullies etc. In one case they coil was several inches above the ground, when a 2.8 grammer sounded beside a tree under a fallen limb.

So it horses for courses. In a lot of situations, we get too lazy to change the coils, but a round coil on open ground is more efficient than an elliptical, but the amount of difference maybe not be worth changing.

Jeff