Large Mono Coil Shapes

[MartinB]

Round is most efficient, anything else reduces efficiency but can have other benefits. Ferinstance, a 24"x12" elliptical coil will have raw depth close to a 15" round coil, but it will have better coverage and the tighter radii at the toe & heel will improve sensitivity to smaller targets.

Interesting. Thanks Carl. Is there a formula to calculate efficiencies of different shaped coils?

[bbsailor]

Round is most efficient, anything else reduces efficiency but can have other benefits. Ferinstance, a 24"x12" elliptical coil will have raw depth close to a 15" round coil, but it will have better coverage and the tighter radii at the toe & heel will improve sensitivity to smaller targets.

It sounds like you are building a sled coil to pull over the water and swamp. Depending on what you are hunting, I'd probably go with a 2:1 or 3:1 sideways rectangle for coverage. Coil size depends on the intended target; if you are looking for Atocha bars @ 2m deep then 1x3 meters might be good. Tons of variables.

Carl, and other forum members,

While tinkering with coils that I reshaped from round to oval, I made this interesting discovery when compressing a round coil into an oval with half the initial coil diameter. The area of the new oval with round ends and a center rectangular area is exactly 75 percent of the initial circle area. The oval coil inductance drops to approximately 75 percent of the initial round coil inductance.

Once you do the math you will see how this area reduction percentage works with any diameter circular coil when squeezed to exactly half its initial diameter. Having good mental models makes tinkering with coils more productive.

1. The coil circumference is pi times diameter
2. The round coil area is pi times radius squared.
3. The area of the oval coil is the two round ends half the initial coil diameter plus the area of the center rectangle is 75 percent of the initial coil area.
4. The circumference of the initial round coil is the same as the total perimeter of the oval coil.

Once you do the numbers, it will leave you with something very easy to remember. I hope some Geotech members try this and post their validation of my discovery.

Joseph J. Rogowski

[Carl-NC]

Here is my data point:

Round 10" coil: 940uH
Same coil reformed to a square: 882uH
Same coil reformed to a 2:1 rectangle: 860uH

[KingJL]

Here is my data point:

Round 10" coil: 940uH
Same coil reformed to a square: 882uH
Same coil reformed to a 2:1 rectangle: 872uH

Nice data point! Real world measurements, I assume.

Assuming direct area relationship the inductances would be as follows:
10" diam circle (circumference:31.416") area=78.54"sq 940uH (measured)
7.854" sqare (perimeter: 31.416") area=61.685"sq (78.54%) 738 uH (calculated by area relationship)
5.236"x10.472" rect (perimeter: 3.416") area=54.831"sq (69.81%) 656 uH (calculated by area relationship)

I used to just blindly assume that the change in inductance (assuming the same coil perimeter) was directly related to area, but...
I have noticed similar real world results to yours, while playing around with a TX coil for a superD.

[Carl-NC]

If the round coil area is A0, the square coil is A1, and the rectangular coil is A2 then

R1 = A1/A0 = 0.785
R2 = A2/A0 = 0.698





Based on only 2 data points, but it doesn't fit Joe's numbers at all.

Please note I've edited my previous post because I typed the wrong number for the rectangular coil.

[Tinkerer]

The inductance of the coil is not the only important factor relating to the depth capability of the coil.


The magnetic field lines in a round coil are evenly distributed. This is because the same charge electrons repel each other.


A uniform magnetic field gives the best result for transmit as well as receive.


When we pinch a coil into a rectangular shape, the field lines get pinched in the corners.
The increased electron proximity accelerates the speed of the electrons that try to move to an area where the distance between them is uniform. Increasing the speed means higher kinetic energy.


So the pinched part of the coil is not equal to the other part of the coil. This means that the total magnetic field is not uniform.


The field lines are of course imaginary, but they provide a good way to form a mental image of how the real magnetic field behaves.


There are numerous software for calculating and visualizing magnetic fields available.