Depending on how trashy the ground is, I'll either use the 8inch or the 4inch coil on my F2. I've never been a fan of big coils, I just don't trust that they can separate the treasure from the junk.
I have always used a 11" Double D. I hunted last season with a Fisher F4. I upgraded to the F75 LTD SR and use the 11" Double D. My current back-up detector is a whites classic ID, it has the stock 8 inch concentric coil. I do have a smaller coil for the F75 for trashy sites. Been pretty lucky and haven't hunted a trashy site yet.
Use these coils on my E-Trac mostly but the FBS800 seems to really be killer with the Safari in particular in trashed areas.
In trashy areas I use the Minelab FBS800
in a bit less trashed areas the CoilTek Platypus
for fields that aren't too trashy I use the CoilTek WOT 15 inch
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