Hi all, if I had a penny for every post I have read over the years on different forums and from different people on the question of relative coil depths then I would be a wealthy man.
The very same question is also asked about machines, so I thought I would do a little diagram that will hopefully explain why, regardless of machine and coil combination, VLF motion machines have a limit that is governed not by price but by the laws of physics.
This law equates to the fact that all soil is mineralised to some extent, this is read by the machine as a target as it has it's own conductivity. This is why we have ground balance, either manual or automatic, to basically discriminate this target out before we begin detecting.
Unfortunately this conductivity is cumulative, the deeper the machine goes the more mineralisation it sees, unfortunately for us the items we search for, lose their strength the further away from the coil they get.
Eventually the weakening signal from the target equals the strengthening signal from the surrounding mineralisation, making the target totally camouflaged by it's surroundings, as shown (badly) in the picture below.
This "effect" is obviously dependant on both the size (conductivity) of the target and the amount of mineralisation present. If you watch "Test" videos, especially from Eastern Europe and Turkey you will notice that the vast majority are showing targets buried in sandy soil, sandy soil is well drained and has therefore had most if not all of it's minerals washed out of it. Similarly air testing involves no mineralisation at all and so the machine can use full power on the target.
So why are the more costly machines seen as "better"?
Well they are more finely tuned to identify signal that are on the margins of this target/mineralisation area, on lesser machines it's a little like trying to read Braille with leather gloves on. Plus some do have more power to find deeper larger conductors.
Why then do larger coils go deeper?
They don't, well at least on the normal hammered and Roman coins we seek but do punch a lot deeper on larger conductors, some are also more finely tuned to the working frequency of the machine which again helps in those marginal areas between target and mineralisation.
The above explanation is why Pi machines naturally detect deeper as they are not effected by mineralisation and are limited practically entirely by the target strength.
This situation I'm afraid to say will never change as it's how VLF motion machines work, so until a totally new system comes along, always question what the manufacturers claim