What your detector coil signal really looks like - a hair net on steroids.

[NM5K]

The only real way it should do that would be if the attenuation of the earth
were greater along the sides of the coil, than at the center. Which I suppose
could happen in some cases. But if the attenuation is equal through the patch
of earth, then the attenuation should be equal at all angles through the earth.
But depending on the sensitivity, etc of the machine, it could still seem like the
pattern is compressing more than usual.
Say if it took full sensitivity to get the full 8 inches on the original plot.
If I lowered the sensitivity to a point where I lost one inch at the deepest
point, I would expect to see something like this plot. The new plot is outlined
in red. Note that I'm plotting the new chart to show equal attenuation at all
angles, which would be the case from either reducing power out, or reducing
receiver sensitivity. So at "half depth" I've lost about 1/5 to 1/4 of the detection
width. But the pattern is still the same, being as all angles show an appx equal
amount of attenuation. The actual pattern into the ground is still the original
black plot being as the power out from the coil is fixed. But the reduced
sensitivity only allows one to the detect the red plot range. Which would make
it seem like the pattern has compressed, when actually it's still the same..
Hope this makes sense.. And this is just my take on it so far from what I've
seen, and knowing RF properties from a small loop.. I'm not saying it's
"etched in stone" as proven fact..
Oh yea.. The new "reduced sensitivity plot.. Note the loss at most all
angles is equal. You could further reduce sens, and draw the plot equally
even less deep. And the plot would seem to shrink further. But it's really
the same. It's just that the receiver has been stunted and is not detecting
the full pattern out to it's limits.

That's my $2.31 worth anyway..

 

[NM5K]

I was looking at that plot and noticed I got just a bit sloppy drawing parts
of the red plot lines.. I was doing that using "paint"..
Here is one a little better, where I try to make sure I'm staying along the
correct plots lines. Not much different, but a small bit fatter at the sides..
I was just going to replace the image in the last post, but don't seem to
see a way to edit posts. BTW, to use the lines as a guide, that one is
set to a bit less sensitivity. About 1.25 inches less depth than maximum.
I'm guessing that would be about equal to running the 250 at 5-6 vs 8
on the sens setting. ??

 

[Mike Hillis]

Bill,
Sorry for the delay in responding back. Work and home get busy.

What you are describing is just the ground signal masking the coin. Pretty common when you have any ground minerals. Hotter the ground, the more noticeable it is.

Ground minerals create a very powerful signal that can be hundreds of times stronger than a target signal in the ground. That large ground signal easily hides the tiny target signal. So our metal detectors use several electronic tricks to tease this weak signal out. One of those tricks is to look at the "rate of change" of the signal. The ground signal is a strong signal but it has a SLOW rate of change. The target signal is tiny but it has a FAST rate of change. The detector circuitry is designed to ignore the slow changing ground signal and pass the fast changing target signal through to be further processed. Another trick done with the ground signal is to ground cancel/ground balance, or in essence, discriminate the ground signal out.

When you dig a hole in the ground you have disturbed the ground matrix. The slow changing ground signal has now been changed into a fast changing ground signal for that particular spot. A fast changing ground signal that is much stronger than the target signal. And that fast changing ground signal is now being passed through and processed as a target signal and since is it much stronger, it masks the now tiny target signal just like oxidized iron does in wet ground. Since the ground signal is also being cancelled/balanced/discriminated out you don't hear anything.

This is why tests beds have to age awhile. Sometimes years. The disturbed ground has to integrate back into the surround ground matrix and return to a slow changing signal.

Remember...talking about ground that has iron mineralization. The less iron minerals a site has, the less ground masking that takes place. Thats why one guy can dig a 10" hole and drop a quarter in it and hit it with his detector and another guy in different ground can dig a 5" hole and drop a quarter in it and not hear anything.

This would have an affect on the test, as the iron mineral content of the site would determine how deep you can continue to bury the coin. However, it would still be enough to see results that would illustrate what I'm talking about. And some detectors handle this situation better than others.

Hope that helps.
HH
Mike

 

[Mike Hillis]

Hi NM5K,

Just do the experiment. Then we can talk about it and discuss the results. Until you have seen it for yourself, it is pretty much science fiction.

I think we may also have a disconnect. TX vs RX. I'm talking RX. The coil has a TX pattern or footprint. But it also has a RX pattern or footprint. I'm talking about the RX pattern or footprint.

HH
Mike