Coil too big for dimes

Assuming very little trash and the ability of your detector to handle the ground, at what
size coil do you start losing sensitivity to dime size targets?

In other words, your detector might run great with your 15" coil and be good for hunting
bigger relics but the 11 or 13" actually will find dime size targets deeper.

Maybe this is a detector or coil specific but I thought there might be some law of physics or something
that gave a hard number on coil size for small targets.
Bryan

This is a good question, and tough to answer.
Physics does come into it, no doubt. There are published charts (attributed to Eric Foster, I recall, and specifically about mono PI loops) that explore this. The summary was: if a target is just detectable at half the coils diameter away from it, then that is the optimum coil diameter. If you can detect the target further away, then a bigger coil could get you more depth. And vice-versa.
However this is for a mono loop, how it translates to VLF concentric / DD coils, in all their round/elliptic variants is not clear. And it also applies to air performance, which may be similar to PI in-ground performance, but not so for VLF. With VLF, you have to take into account the fact that bigger coils pick up bigger ground signals, hence they need a correspondingly bigger target signal to make it detectable. I think this ground problem is quite significant.
It's hard to actually compare different 'size' coils, even just doing air-tests. Because they are all different shapes. Look at the Fisher F75: The stock coil is the 11" x 7" bi-axial. What's their big coil, is it the well-regarded bi-axial shape? No, it the 15" round shape. Ditto, the 5" coil is apparently 'round' internally, too. And aftermarket coils are inbetween shapes 'butterfly' for example.

What testing I've attempted does suggest that a coil slightly bigger than the Fisher 11 x 7 would be better for finding dimes, but I doubt the 15 inch one would show any benefits.

Something like the 10.5" Fisher concentric round coil (X-series/CZ's) is about the max size for dimes IMO. DD coils are a different kettle of fish.

I see on many different brand forums that the SEF 10x12 is a favored size on many machines, giving responses that are improved over stock coils and very favorable compared to say the 13" Ultimate. Now whether round shaped DD's would outperform an elliptical on coins I have no clue. In the minds eye elliptical's have better "see thru" but is that really true in actual use?

I have and like the G2 and G2+, but for coins I'm seriously considering the Omega 8500 with the 8" concentric and a couple DD's in the above size range.

As a note: DD's are all the rage but the aftermarket needs to take a look at concentrics as well. That Fisher/Tek has not developed a concentric in a round shape larger than the 8" for the Greek series is a failure IMO.



Edited 1 time(s). Last edit at 01/28/2016 11:50AM by Jackpine.

I would think your freq has a lot to do with that the deus can hunt at 4kHz that could really help on silver

LowBoy

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The deepest dimes I have ever found were located with a 15 inch DD coil on my Etrac.

Bayard Wrote:
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> The deepest dimes I have ever found were located
> with a 15 inch DD coil on my Etrac.

This is why I was thinking about coil size on small targets.
I run a CTX which is really coil limited.
My options to go deep are stock 11" and the 13x17 which doesn't seem to add depth
over the 11" in a park setting.
I often wonder if I could reach deeper with an Etrac and say a 13" ultimate or a 15" like yours.
My primary target when park hunting are old coins which are usually penny or dime size.
Bryan

Paraphrasing Monte "big coils for big objects". I'm not a big coil user since I could never stand the weight for long anyway.

TabWhisperer Wrote:
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> Paraphrasing Monte "big coils for big objects".
> I'm not a big coil user since I could never stand
> the weight for long anyway.

If you set up a good harness system it makes swinging the big coils almost effortless..
Mine works so good I even started using it with the 11" coil which I never thought I needed a bungee for.
Bryan