This is the biggest catch 22 when it comes to prospecting. Have a look at the Detech 15" DD Ultimate. Still very sensitive on small gold.
Another good patch coil on the 2200 would be a Commander 15x12" DD. The old 16" Nugget Finder DD fibreglass coils would probably match up great to a 2200, but they are rare to find these days. They occasionally pop up, but are pretty beat up - very rare to find one in good condition. I wouldn't go any bigger than that.
G'day everyone so i did this really cool mod to my sd2200d.
By utlising everything velcro on the side of the machine it balances the weight of a heavier coil and means everything is now on the machine, no more harness and cords.
I've also got a set of external speakers that i use too sometimes.
Anyways what i did was got a 4.5AH 6V battery and modified the terminals
Took the coil for a spin today out bush and had no dramas, picked up 0.5g at 30cm deep and while it was a very weak signal, hard to spot i got it in the end.