The EMF Pattern of Coils

[Ronk]

Is it really true that the target under the coil affects the Pattern of the field of the coil? I recently read somewhere that a Concentric Pattern is really not a Cone shape but something else.Also is this true for DD coils and would the Shape of the target change the field?I'm just curious.Would the Elliptical Concentric be different from the Round one?...and would the Elliptical DD pattern be different from the round one?
HH...
RonK

 

[sgoss66]

Ronk ---

As I understand it, (though I'm not an engineer), the traditional "round coils send a cone-shaped field, and DD coils send a narrow, beam-shaped field" are not really accurate, but are at best approximations. There is alot of discussion that crops up about this issue, but as I understand it, the pattern of the transmitted electromagnetic field beneath the coil is not as simple as we like to think.

As for a target altering the transmitted electromagnetic field? Absolutely -- alterations in the electromagnetic field, caused by a metallic object under the coil, are -- as I understand it -- precisely what the detector "reads" to "ID" the object.

Yes, the pattern for each type of coil is different, but understanding exactly what the electromagnetic fields look like is much more difficult...

Maybe Dave J. can chime in here...

Steve

 

[amberjack]

the shape of a coil has a lot to do with the field it puts out and its complicated stuff

mono's would be deeper in good ground but not so good in junk or as sensitive to small items as a DD coil, this is why the Gold Bug Pro runs DD coils for sensitivity to small gold and discrimination in iron junk to pick out gold. i think Dave has said on here this detector is made to pick up the gold the PI machines can not find either in loads of iron junk or super small gold like in Steve's tests.

to do this a DD coil is needed.

a mono coil would be useless on a VLF machine for gold hunting as where it would find gold would also be PI territory so it would be gone with PI's superior depth.

so there is no blanket answer to coils.

a more specific question may make it easier to answer

here is a pic i found hope the person who made it understands its being used for a educational purpose? both are DD coils.

so the moral to this story is?? if you are going gold hunting with the GBP use the little 5in coil

 

[Dave J.]

A month or two there was a good thread on this subject that I posted in, but don't remember which forum or the name of the thread, maybe someone else remembers and can link it (if it was on Findmall).

What matters from a user's point of view is not the EMF pattern of the searchcoil, but the response pattern to a particular target in a particular orientation moving in a particular direction at a particular distance at a particular speed (relative to the searchcoil, normally it's actually the searchcoil that's in motion) , with the machine at particular settings, in a particular environment which usually includes iron minerals and a lot more often than you'd like to think, also includes other targets. Then there's electrical interference.

In general it can be said that compared to a concentric of comparable size, a DD will exhibit a narrower zone of positive response near the searchcol, and up close will exhibit more prominent zones of anomalous response.

Sometimes you'll see drawings illustrating a concentric searchcoil with a conical "field shape" and a DD with a narrow knife-like "field shape". Those are worse than oversimplifications, they're just plain wrong. However there is no "right" way to draw the "field shape" so I won't try.


--Dave J.

 

[ron_c]

Dave J., is this the post you were talking about?

http://www.findmall.com/read.php?58,1392476,page=1

 

[amberjack]

Thanks Dave

glad i dont have to worry about pretty pics any more but concentrate on listening and diggiin

AJ

 

[Dave J.]

That's the thread, thanks ron_c.

--Dave J.

 

[amberjack]

ok i have heard the pro's talk about bench testing to understand what different targets sound and react like so this is as good as it gets so if we want the good oil ask the target it knows more than anyone else??

i know what detection fields look like, like the one i will be walking on tomorrow

 

[Ronk]

understanding now even though it does get a little confusing.I always like to know the Technical side of things.I guess most others are not too concerned about the EMF of the coil just as long as it works.I wanted to know some info because that helps me a little to know a little about which coil I should use for a particular type of target or soil condition or location/trash/iron...etc...even though most really don't give a hoot! As far a putting the small DD coil vertical on edge to help identify a bottlecap or other iffy signal,I have not tried that yet with a DD but I did with a concentric but a concentric will reject the bottlecap most of the time anyway.At least it does on 2 of my detectors. I've also noticed that some coils will detect farther out from the side than others but then you couldn't get too close to metal structures without it sounding off.I can get fairly close to a chain length fence with my 5.5 inch coil before it beeps.I don't know how close I can get with a DD the same size but I'll have to test it.
I know once in a while the subject pops up about the Concentric vs DD but I think I got enough info to better understand.
Thanks for all the Tech info everyone and the Nifty Drawings! I really appreciate it!
HH...
RonK