best type of coil for iron infested ground

[Koala]

Which type of coil is better to find target between a bed of nails


4B
coaxial coil
con-centric coil
con-centric coil with elliptical center
OO
DD

Most smaller snipper coils seem to be DD in the UK but see most people in America that hunt in old ghost towns seem to use a concentric.

I would have thought the coaxial followed by con-centric but this article suggests the DD

[Carl-NC]

I looked at the article and chuckled at the depiction of the DD field pattern. Many years ago some manufacturer put that out, and amazingly no one has ever checked to see what the field pattern of a DD coil really looks like. Then I got down to the depiction of single/dual/multifrequency and laughed out loud. Straight out of a bogus ad.

I generally prefer concentric over DD in iron because it gives a more consistent iron ID, especially with flat iron (bottlecaps, but also shards of tin roofing). Smaller is better, depending on the density of iron.

- Carl

[BillG]

I looked at the article and chuckled at the depiction of the DD field pattern. Many years ago some manufacturer put that out, and amazingly no one has ever checked to see what the field pattern of a DD coil really looks like.
...

I always wondered about the field pattern images. I've read countless articles which refer to a cone on a round/concentric and a wedge on a DD coil. My work with radar and sonar simulations told me there was something wrong, but I just figured there was some aspect of the emitters/frequency/ground interactions which caused some sort of masking/filtering. Now, I know that the pictures are wrong. Thanks!