DD Coil vs Concentric Search Coils

[marcus118]

Hey guys! I wrote a post today on Hollands Brook that details the differences between concentric coils and the DD that I thought I'd share here on the forum. Would love to hear everyone's feedback!

http://www.hollandsbrook.com/blog/dd...vs-concentric/

HH, Marc

[demyansk1942]

Nice article and I liked the picture

[marcus118]

Thanks! The bottom picture is from garrett's website but the top one is a result of my 'superb' photoshop skills ;-)

[woodstock]

Great artical and photo. It very important information that some us don't know or forget about. HH, Woodstock

[Devildog2345]

Great article. Very easy to follow and understand even for a beginner. I look forward to reading more of your articles.

[Liberator]

I disagree with the Garrett image and the ‘supposed’ detection field of DD and concentric is debatable. This ‘urban detecting legend’ just like the ‘halo effect’ has been around so long the majority of knowledgeable detectorists seem to accept it as gospel. ML started it and Garrett jumped on the marketing bandwagon. David Johnson is the only metal detector design engineer I know of that has made a public statement regarding the ‘detection field’ emitted from different coil designs. I tend to believe what the detector and coil design engineer says, not what a picture from a manufacturer’s marketing departments made to sell with no facts to support it says.

Everyone should read some of the essays and books written by David Johnson on the Fisher website this guy knows his stuff….
Here is the link;

http://www.fisherlab.com/hobby/dave-johnson-essays.htm

If I could link another forum here it would be nice….? First Texas Products Chief Design Engineer-David Johnson debunks the ‘detection field’ myth along with dispelling the ‘halo effect’ theory?
It’s very enlightening…

Liberator
Liberation relics and coins since 1981

[dogpound]

i wouldn't call that "debunking" the halo theory, he even said its his opinion and "It is plausible that the electrochemical corrosion process of nonferrous metals influences the geochemistry of the iron minerals in the immediate vicinity of the target"

if a halo had no effect then why is it that fresh coins in a test garden don't register as good as coins that have sat in a test garden for years

[marcus118]

Thanks for the feedback Liberator and for sharing that resource - I'll check it out!

-Marc

[gman]

I will say in general concentics tend to
ID better than DDs.
But in heavy iron give me the DD

[demyansk1942]

I think so many variables are in effect when detecting. I think the weather, ground condition, swing speed, and everything else comes into play. I think it's important to go over places you have detected and you might find further treasure. I know I probably miss a lot of older items due tot he fact that I am using a Fisher F2 and its not always the deepest detector.

[Liberator]

Originally Posted by dogpound

i wouldn't call that "debunking" the halo theory, he even said its his opinion and "It is plausible that the electrochemical corrosion process of nonferrous metals influences the geochemistry of the iron minerals in the immediate vicinity of the target"

if a halo had no effect then why is it that fresh coins in a test garden don't register as good as coins that have sat in a test garden for years

David Johnson doesn’t debunk it in the link I sent-he debunked it on another forum. Iron can form a detectable halo-gold silver, copper and others not made of iron will not. David Johnson says;
The digging any target out of its matrix can disturb the 'lay of the land' and therefore the response of the metal detector. This can give someone an impression there was a "halo" that was disturbed when in most cases it was the uniformity of the matrix that was disturbed by the digging.
In rocky or already disturbed soil, the matrix is already non-uniformed and digging it will change its non-uniformity to something different altering metal detectors response. So even under these conditions the act of digging the target can give the impression of having disturbed a "halo" when in fact there was none.

For iron he says;
there is no question that metallic iron, as it rusts in the ground, can cause a "halo" of spinel (gamma) iron oxides which can alter the way the machine detects it either favorably or unfavorably depending on the response characteristics of the machine. This can be replicated in bench testing.

He says gold does not corrode and silver can corrode in some soils but not enough to ever alter the signal.
About copper Dave says;
Copper does corrode in many soils often leaving a physical halo which is visible to the eye. However this halo is not (as some have supposed) of sufficient electrical conductivity that it might have an impact on detection; and the lower electrochemical activity of copper with respect to iron makes it unlikely that copper corrosion has much impact on nearby iron mineralogy.
Metallic particles are not transferred to the surrounding soil in metallic form other than the obvious, that remnants of a disintegrating metal object may be moved by bioturbation, vertisol churning, freeze-thaw, gravity driven slope creep, and the soil movement mechanisms. The result is of coarse far less detectable than the original metal object.
One can take samples of these metallic element containing compounds, and in isolation verify that their electromagnetic characteristics are different from other compounds. However other than in the case of iron being altered to maghemite or rarely lepicocrocite (which have high magnetic susceptibility), the properties of the altered metal compounds are too weak to be detected in situ by a metal detector because of their small mass and their distance, relative to the soil bulk and its electrical and magnetic properties.
I have never seen a credible physical science based explanation for detectable “halo” around a corroded copper coin. And when it comes to dirt, I know quite a bit. But….there are things that dirt knows that I don’t.

--Dave J.
----------------------------------------------------------

There is a bunch more from his explanation but like I said-I believe the guy that has designed and brought to market more metal detectors in the past 30 years.
His explanation for ‘supposed’ coil detection fields and those diagrams made by some marketing departments from some manufacturers is even more enlightening…..

Liberator
Liberating coins, relics and gold nuggs since 1981

[John in NC]

With experimentation you would be amazed at what is possible. DD's are a different animal, but if constructed correctly, will give as good or better VDI reading than a stock coil. JMHO
John in NC