Metal Detector Coils?

[PimentoUK]

That graph shows this not-too-intuitive phenomenon quite well.
It also applies to PI machines with mono coils. And it's possible to calculate mathematically the behaviour of mono coils fairly easily, so it can be shown in a theoretical way, as well as physical measurements on a real detector. There are some discussions over on Geotech1 forum where charts like the above were posted.
PI's do see the ground differently. From some discussions on Geotech1, I recall Eric Foster stating that PI's pick up the same level of ground signal regardless of coil size.
And a mono coil has quite different behaviour to DD's ( and CC's ) when the target gets close , ( below 1/2 of the coil diameter away ). Essentially, the signal strength plateau's at close distances, whereas for a DD, it continually increases ( in practice, until the target touches the coil housing).

[Geotech]

Nice plot, GB. Pimento, I was looking on Geotech for the very plot we're probably both thinking of, can't find it. Locator, that illustrates it quite nicely... a big coil will go deeper for big objects than a small coil. Ferinstance, if you want to detect a deep ammo box full of silver, use an 18" coil, not a 6" coil.

[PimentoUK]

Carl: The graph I was thinking of was prepared by Mr.Green, in a thread related to finding the optimum coil size for a given target/depth. We discussed Eric's semi-circular graphs, and then Green used the 'Hyperphysics' website to analyse a mono coil. When all the maths misunderstandings were sorted out, the result was a neat Excel plot.
This is the thread, post #39:
https://www.geotech1.com/forums/showthread.php?24020-detection-depth&p=237587#post237587

"I charted distance vs amplitude with Hyperphysics for a 200, 300, 400 and 500mm coil to see if that would help determine best coil size.
Her's an attempt at charting the coils. Example: a 200mm coil can detect a target at 400mm, what diameter coil would detect the target the greatest distance? The 500mm coil has the same signal amplitude at almost 600mm distance. A target just detectable at 100mm with the 200mm coil wouldn't be detected with the 400 or 500mm diameter coils at any distance."

[when I get on a newer computer, I'll edit this post with the graph, and sort out the hyperlinks. This ever-changing forum keeps outsmarting my browser]

[Geotech]

Yes, that's exactly the plot I was thinking about, thanks!

[PimentoUK]

And it also shows the plateau of signal strength at close distances that mono coils show. If that were reproduced for a DD, the blue trace, for example, would keep going up, to 100000 ( very approx ).

Simply put, when a target is in the middle of a 10" mono coil, it's still 5" away from the nearest coil winding. Whereas a target in the middle of a DD is just 1/2" or so from both TX and RX.