Coil Selection for Omega

[TerraDigger]

This was created on a antennae modeling software program. The creator of this states that a concentric coil is really an antennae for all intents and purposes. Does the field portrayed look accurate to you?

 

[Ken in Georgia]

Thanks Bart !!!

 

[Dave J.]

There is no practical way to graph the response field of a searchcoil because the response pattern depends on the size, shape, orientation, electrical conductivity, and magnetic permeability of the target.

The graph that was posted looks like electric dipole modeling, which is the wrong model. An induction balance searchcoil does not exhibit a pattern like that on any kind of a target.

Monte is right about the "cone" (concentric) and "knife" (DD) explanations often given. They do contain an element of truth, but so little of it that one is probably better off without those explanations.

--Dave J.

 

[all iron]

So what does the force field look like coming form the coils concentric and DD
Can anyone explain

 

[Dave J.]

So what does the force field look like coming form the coils concentric and DD
Can anyone explain

The magnetic field intensity of the transmitter could be drawn, but that doesn't represent how the searchcoil responds to targets.

With standard size searchcoils (8 inch round to 11 inch elliptical), beyond about 5 inches the response pattern of a DD and a concentric are fairly similar. As you get closer in, the response field of a concentric will tend to concentrate near the center of the searchcoil, whereas the response field of a DD will tend to become long and narrow. These are crude generalities.

The way to understand how a searchcoil responds to particular targets in particular orientations at particular depths, with the machine in a particular operating mode and settings, is to do air testing. This is not the same as in-the-ground (which can't really be replicated in an air test) but it's very useful knowledge.

--Dave J.

 

[ron_c]

I've tried this very thing in an air test which isn't hard to do just time consuming. Make any changes and the configuration changes. Disc. mode was used and doing so, motion has to be used and trying to hear just where a clad dime starts to signal. The coil has to remain stationary. The test machine was a Tesoro Vaquero with the 5.75" and 9 x 8" coil. The day this was tested EMI's were very low and I was able to use Max Sensitivity and Max Threshold (Super Tuned) with no chatter. Your results will probably be different depending on which machine is used and settings used. I also tried to do a in ground test of a clad dime at 1" increments and it was very hard to do and I question the results. The soil was sandy type soil.

 

[Dave J.]

That's some nice work, ron_c. Thanks for posting it.

I hope everyone understands that this represents a dime target (held horizontally) under a specfic set of conditions, and that's all it represents.

--Dave J.

 

[Ken in Georgia]

This is why you can pick up a belt plate at 24 inches and a dime at 9 inches. I hope that's right. And I think given the swing speed of the coil this will change too.

 

[Jackpine Savage]

when used properly. There is one thing that stands out in Dave J's posts regarding DD vs concentrics,

With standard size searchcoils (8 inch round to 11 inch elliptical), beyond about 5 inches the response pattern of a DD and a concentric are fairly similar.

Forget what other say about the "blade shaped" pattern of DD's cutting thru trash. When using DD coils in heavy masking conditions overlapping sweeps just like you would with a concentric coil is absolutely neccessary for best results.

HH Tom Z

 

[Ken in Georgia]

Thanks Tom !!! I will, I'm going to be learning a new Detector and knowledge is power.

 

[Mike Hillis]

You question the in ground results cause they looked "FUNKY" ? Cause in-ground results do look funky. As the soil mineralization increases, the funkier the results look.

Congrats...most people talk about the detection field. Very few people go to the trouble to actually KNOW their detection field.

HH
Mike

 

[ron_c]

With standard size searchcoils (8 inch round to 11 inch elliptical), beyond about 5 inches the response pattern of a DD and a concentric are fairly similar. As you get closer in, the response field of a concentric will tend to concentrate near the center of the searchcoil, whereas the response field of a DD will tend to become long and narrow. These are crude generalities.--Dave J.

The reason I didn't include the Wide Scan configuration compared to the Concentric configuration is I thought the detection area would be a narrow edge to the maximum detection depth until I saw Dave's post about this. On the In Ground test it's harder to test as you don't know the exact location of the coin and probably not that accurate.