Friday, April 3, 2015

4/3/15 Report - Concentric and DD Metal Detector Search Coil Test Comparisons. Ace 250 & ATX. Easter Surf.

Written by the TreasureGuide for the exclusive use of

The few days ago I talked about determining the shape and size of the area of sensitivity of detector coils.  Someone asked me about DD coils.

DD search coils are said to provide better performance in highly mineralized soil, provide better target ID separation, better accuracy or pinpointing and greater depth in hot soil.

As an example, Garrett's DD coil design has a broad area of target sensitivity - what they call "detection field." 

In two recent posts I used diagrams showing a detection field like what you would expect from a concentric coil.  The detection field for a concentric search coil would have a shape something like that shown in the left diagram in yellow, extending from the edge of the coil and narrowing as you get farther away from the coil.

Illustration of detection fields for concentric and DD coil.
Yellow represents the detection field for a
concentric search coil and green, that of a DD search coil.
I found this illustration showing the detection field for a Garrett DD coil as compared to the detection field for a concentric search coil.  The Garrett DD is said to produce a narrow field in the center of the coil that runs from the front of the coil to the back of the coil.  That is represented by the green area in the diagram on the left.

Looking from the side (illustration on the right), the detection field of the DD extends almost straight down from the edge of the coil and then has a more rounded bottom.  It therefore would be expected to detect targets deeper near the edge of the coil.  Using the term I introduced a few days ago, it has more peripheral sensitivity.

Notice the shape of the detection field for the concentric coil and how it is more narrow.

The DD field as shown in this illustration is more narrow when viewed from the front, but broader than the field for the concentric coil when viewed from the side.

I already had some general impressions of the DD as compared to a concentric coil but did a few tests before writing this. 

I used the Ace 250 with the standard concentric coil (7 by 9 inch) and the standard ATX  DD search coil (10 by 12 inch) for these tests.

I often use these two detectors for tests of various types simply because they are so different.  Ideally to test coils, everything else would be the same except for the coil, but even that would require many tests to compare the two coils while using various settings and different conditions.

First I used the Ace on four different coins, including a penny, nickel, and two foreign coins, one of which was magnetic.  I put them in a line with each just a couple of inches from each other.  The Ace didn't respond with four distinct separate IDs when the coins were that close.  I then moved the coins farther apart.  At about six to eight inches apart and using a relatively slow sweep speed, the Ace then could give a correct ID for each of the coins separately on a single slow sweep.  The magnetic coin was identified as iron and the others were correctly identified too.

The ATX in motion mode (with the 10 inch DD) coil did not provide separate signals for the different coins even at eight inches or more apart.  I did not get the improved target separation that would be expected from the narrower front to back detection field illustrated in the above diagram.   In fact it was much worse. 

As I pointed out above, I can not blame the coil exclusively since many other factors were different in this test, yet the results were not just different from what I would expect from seeing the diagram, but remarkably different.  I wondered if the DD coil diagram shown above might have been for a different type of DD coil than what I had.  I guess that is possible.

I also have to acknowledge the hugely different types of detectors being used.

In non-motion mode the same ATX did not give a good auditory distinction between the four different coins even when they were a foot apart, however I could see that there were four different peaks in target strength indicating four different targets by watching the visual target strength display.  If I was depending upon the auditory signal alone, I would not have noticed any indication of target separation.   Going on auditory signal alone, I could not identify and indication of target separation with the DD coil even when the coins were a good distance apart.  Maybe I'm not good at auditory discrimination or maybe I should lower the threshold or something.

I did another test using a piece of iron shipwreck spike and a nickel (see picture at left).

The Ace showed more target separation on those  objects too.

The ATX with the DD coil that I used in these tests, has a large detection field left to right.  That large detection field is even larger when the target is an iron object.  In fact if you watch the ATX coil as it moves towards, over and away from the targets, the signal begins before the edge of the coil is over the target and doesn't end until the trailing edge is from under the coil.

That is something that you might actually like.  The detection field is bigger than the coil.  However, it makes pinpointing more difficult.  And it does not provide what I would call good target separation.  You can, however, go into pinpoint mode to do your pinpointing.

These simple tests confirmed what I previously noticed in actual field use.  The ATX standard DD coil has a broad area of peripheral sensitivity.  That can be a good thing because you cover more ground, but it does not make for easy pinpointing. 

There are times when you might want good target separation and times when that is not so important.

When working in non-motion mode, there were times when I could not sense an auditory indication of multiple targets but could get an indication of multiple targets from the visual display.  The signal strength would show a peak over each of the four coins on the visual display.  That visual indication can be improved for shallow targets by lifting the coil to diminish overall signal strength.  That is a trick that I just learned that I will adopt in the field when I want to see if a large area giving off a signal might be a cluster of smaller targets.


There will be a little bump in the Treasure Coast surf on Easter - up to three to five feet, and an additional foot or so on Monday.

The weather couldn't be more beautiful.

Happy hunting,