Question on coils

[Anonymous]

On larger deeper conductive targets what are the most important factors in determining detection depth as far coils go? All target factors being equal is it the diameter of the transmit coil or receive coil or both. As an extreme example if one had a coil of say 20

 

[Anonymous]

The bigger the TX The deeper the field.
The bigger the RX the bigger the capture area.
How about 2 coils, 1 TX one RX wound anti phase.
Next to Zero coupling. TX X turns RX +30% more.
Try it before you buy it.

 

[Anonymous]

Coil/target optimisation can be quite complicated, but it can be broken down into a number of separate considerations. I will deal with just one, at the moment.
The diameter of the coil in relation to the target size is perhaps the most dominant characteristic. If we look at a mono coil, which is the simplest to understand, the optimum coil size is one that detects the target at a distance equal to the coil radius. e.g. if you have a 10in coil that just detects a target at 5in, then going either up or down in coil size will result in less range. If, however, your 10in coil detects a target at 10in, then the coil is not optimum, and going to a larger size will give more range <span style="background-color:#ffff00;">on that target</span>. In fact, the optimum coil for that target is 27.5in diameter, and it will detect the target at 13.7in. Say your target was big enough to detect at 15in with the 10in coil. The optimum coil size would then be 46in diameter and the target range 23in.
Coil sizes get bigger quite rapidly, and more cumbersome, but because the coil radius/range curve is very flat as you approach the optimum, you could reduce to a 30in coil and still get just under 23in.
All this assumes that everything else stays constant. i.e. TX current, inductance etc, and also pickup noise.
If I can find it, I will post a graph of coil radius/range curves.
Eric.

 

[Anonymous]

Here are the curves I have used for many years. The range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.
If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it won

 

[Anonymous]

Thank you very much Eric.I greatly appreciate your posts.

 

[Eric Foster]

n/t

 

[Anonymous]

Last weekend I wanted to test a SD2000GPM Ismael Jones 6 frequency unit with a 25" Nuggetfinder coil. I had some areas I wanted to go over in the Pennsylvania mountains looking for some lost gold bars. I use the 25' coil after I have a area pinpointed. It is not something you want to go over miles of forest with. I received a very strong signal at the base of a tree. It turned out to be a few pieces of bird shot burried 6" down in low minerized soil. I had to hold the coil at a angle at the base of the tree and the coil was 8" off the ground. I did not expect to hit on something that small with a 25" coil and receive such a stong signal. Junk targets in that area are few and far between as the area is remote.