DD's and bottle tops?

[Mick in Dubbo]

I've heard a lot of comments about DD coils having a fetish for bottle tops and was wondering what was meant by this? Is it suggested that a concentric coil can tell these apart better than a DD coil? I have noticed on my Ace, that about a third of the time you can distinguish a coin from a bottle top because the coin has a much tighter pinpoint. As the pinpoint on my X-terra has been non functional for most of the time I've had it, although recently repaired, I haven't gotten round to taking it out to experiment and find out (was also shutting itself down till a couple of days ago, when I restretched the springs). If somebody could throw some light on this subject, I'd be thankful.
Mick Evans.

 

[Monte]

Some makes & models seem to work a little better with a concentric coil than with a wide scan (Double-D) type coil, while others do fine with a DD and are even supplied with one.

Generally, although it is not a total fact, the wide-scan (DD) coils seem to have a little more problem with discrimination than to the concentric coils, .... but as I said, it all depends on a number of variables and some models have circuitry that was designed around the wide-scan coil and they do a fairly good job of discrimination.

As for pinpointing, the bottle caps will usually display a larger signature than a coin, because most coins are made up on N0n-ferrous material while the bottle caps is made of iron or some other magnetic property. Ferrous and non-ferrous metals have different effects on an electromagnetic field.

Personally, my preference is for a good concentric coil, but with some makes and models I like a Double-D coil.

Monte

 

[Mick in Dubbo]

Thanks Monty. That one has had me wondering for a quite while.
Mick Evans.

 

[Farmercal]

Mick,

I use the 5 x 10 DD and can't remember finding many of them using that coil. Perhaps I have been lucky where I search. I personally like the DD coils and am getting the 7.5 Khz for Christmas. Look out cellar holes, here I come.

 

[Mick in Dubbo]

Thanks. Good luck with the cellar holes.
Mick Evans.

 

[ericdj]

After I put the 10.5" 7.5Hz DD on my Xterra-70, the first 4 things I found were bottle tops. Sounds too close to a quarter, but the caps do have a slightly larger signature. I would just take it as areas other MDer's haven't hit yet...unless the caps are brand new.
After that, I had my best day in months. Even found a small gold I.D. chain. A first. (no name though)
It's not coming off, until I go prospecting, and maybe even not then.

 

[Mick in Dubbo]

Thanks Eric. I'll be in Sydney next week, so I'll duck into the Minelab shop and see if I can check the coil out in person. I'll make my decision to buy or not to buy at that point.
All the best for the season.
Mick Evans.