Question

[BarberBill]

I've been detecting long enough and reading articles, forum threads etc. that I feel I'm pretty knowledgeable on most aspects of the hobby. However, I do have a question that I'd like to see some explanation for. Generally it's printed in user's manuals, and suggested right along that one should use a smaller coil in order to have more sensitivity to small targets. If this is true, why do we see the super Minelabs and other gold detectors being sold and used with large to quite large coils? I realize that a prospector wants to reach some pretty solid depths, but supposedly large coils are less sensitive to small targets and especially at depth.

The two explanations that come to my mind that may or not be involved with the answer(s) are the high frequencies generally used and/or the fact that many of these machines are PI machines. If anyone can respond with how the physics of this works, I'd be grateful and just a bit more knowledgeable.
Thanks in advance,
BB

 

[Elton]

Years ago, it was advertised that bigger coils..say a 10" Concentric over an 8" gave you about 20% more depth. The downside was they lost a little sensitivity to small items..

Then they come out with the DD, and it was supposed to be better in Highly materialized ground..with little to no loss on sensitivity to small items...... I also remember that the DD coils were not well received at first.........on anything but the Minelabs.

I hope someone has much more information on this subject too.

 

[fowlercharles]

Hi Bill, As I have said before I am pretty much stump dumb about all matters tech wise, and that shouldn't be for as long as I have been detecting, but I will venture out there a little on this and see how it goes. Pretty common knowledge to use a smaller coil in the more trashy (iron/alum/whatever) areas because the larger coils don't handle a heap of signals very well all at once,and "masking" takes over, defeating depth and in general defeating the whole process. So, the smaler coil not having so much coverage means less targets together, less confusion, more concentration of signal, allowing more depth,more success of job at hand(as intended). Never hunted nuggets, but I think there is (hopefully) much less trash involved in that type hunting and the larger coil will have the opportunity with less targets to go deeper.Best done in All Metal, and no descrimination, which really opens up to mineralization if not constant GB. Could it be there is not a lot of sensitivity to smaller targets (larger coil) lost until the coil is increased to a "particular" larger size in the open, less trashy areas.Also, the nugget hunter is not so much in hopes for the smaller nuggets anyway, as his real concern is covering more area and depth (more dirt) where the larger nuggets will "speak" to the coil for themselves, Probably will involve Freq also, and maybe even a simple answer, if that's ever possible with detector tech. As I said, stump dumb and have to show it now and then. Certainly nothing like what we would hear from Monte or JB and others, but a dang good question Bill, to give us something to think on. Thanks and HH Charlie

 

[Ism]

Bill, One would think that a large antenna would be more sensitive to small signals. The opposite is true for a detector. The metal detector works like a hybrid between a transceiver and transformer. Smaller objects can't "couple" well with larger coils. In addition, the magnetic lines of force part away from the center of the coil field at depth, and wrap around like iron filings wrap around a bar magnet. You can do an air test to observe for yourself. Take a large coil, dime, and perform an air test. With the coin at the most extreme detection range, move it back and forth across the center of the coil. You should observe a "double blip". This is because the coin is passing through the dead zone where the lines of force are parting out from the center of the coil field. With the larger coil, this area is bigger and the electromagnetic lines of force are weaker.

Another thing that will help you understand how a detector couples like a transformer. Take a 3" length of solid copper wire, straighten it and try to detect it. You won't be able to detect it with most detectors. Now make a loop out of the wire and twist the ends together. Now what do you observe? Three turns and it changes the conductivity of the Target from low conductor to high conductor (an observation I made with the Whites TDI PI machine). Now you know why rings hit so hard when laying flat and have a poor response when detected on end or the ring is broken.

Hope my crude explaination helps

 

[TerraDigger]

My opinion.....no detector....Minelabs or otherwise can circumvent coil size disparities regarding sensitivity. Bigger coil=more depth/loss of sensitivity to smaller targets.
Smaller coil=reverse of above
There is no getting around it.
I think Minelabs are geared to larger coils due to their australian background in their goldfields, which have a tendency to have had larger than average size nuggets, thus depth is more important than finding smaller/closer to the surface nuggets. Especially since these fields have been hit hard by VLF detectors in the past.

 

[BarberBill]

Terradigger - I believe you may have the most pertinent answer to the question. Anyway it would seem to fit what I was wondering.
Thanks everyone for you posts.
BB

 

[Ism]

BB, my comprehension of your question was severly lacking when I supplied an answer (gotta stop reading these when I first wake up).
Minelabs use a DD coil on their detectors. As you already know, DD receive coil is 1/2 the size of an IB coil.
So the physical coil has to be larger to obtain the same depth performance as an IB or mono coil.

For the PI units, the large mono coils can obtain some pretty good results on very small nuggets.
But no matter what, the depth will be lacking on the larger coils for small targets.