Hi Turtleman,
One could build a smaller coil to make pinpointing easier, but there will be a depth loss as you mentioned. One other advantage of a smaller coil is target separation so a person could easier find a single object next to another.
Now, when I mentioned that it might be the best idea of trying to build a larger coil, that doesn't mean one shouldn't try just to see what happens. The nice thing is it doesn't cost a lot of money to build a coil. Also, if a coil is built but not put in a housing, then the cost is less. So, a person could build a larger coil, attach it to something like a thin piece of board for quick and simple testing just to see what will happen.
I suspect one could build a larger coil than the factory coil and gain depth. However, there will be a size where there will be diminishing returns. Right now, I am not sure just what that size is, so I don't want to discourage anyone from trying, but do want people to think about maybe increasing the size in increments rather than simply picking some very large value to begin with and find it doesn't do what one might expect.
The signal strength of a target that is amplified is the result of the coil field, the distance from the coil and a few other factors. Now, the target signal will increase or decrease with the field strength of the coil. The field strength as measured at any point will be influenced by the current in the coil and the coil diameter. Increase the diameter and the actual field strength affecting a target signal will reduce accordingly.
Some pictures of the field strength sort of depict something like arrows pointing out of the coil. Now, if we use that analogy and assume there are 100 arrows coming out of a coil, regardless of size, then logic says that any smaller coil has a better chance of more arrows intersecting a target than if the coil is large. On a large coil the arrows are displaced farther from each other, so the possibility of fewer arrows hitting the same size target is a fact that will occur.
This is why the general discussion about increasing coil size normally states, one can detect deeper but only for much larger objects.
Another way of sort of looking at this low power high power thing is to think of something as simple as a flashlight. Now, use a flashlight with a half dozen batteries and a penlight flashlight and shine both into the darkness. Obviously, the one with the greatest number of batteries will shine the farthest indicating that the more energy in the greater the distance out. This is an over simplification of what is happening but it does help to better understand why a higher powered unit will do better.
Getting back to building coils, the best way to really determine just what advantage or lack of advantage will occur is to simply build one and see.
My mentioning what I did in my earlier post is to sort of forewarn people that they may not be as happy as they might think with the results of a much larger coil.
Regardless, people with the ambition and desire should, by all means, try things to see what really happens. That is how we learn.
Reg
PS; One thing I forgot to mention is the fact that it is harder to make a larger coil that will operate at the lower delay settings. So, if a person does build a larger coil and it doesn't seem to work right, you might increase the delay a little at a time. If a longer delay is required, there will be a point where the coil will suddenly begin to work just fine.
