What is the most powerful gain anntenna available

Who makes the most powerful (db) antenna's for cellular use?
Biggest I see is around 14 db. Do they come bigger in gain output?
Looking for a friend who spends time in Winsted Ct, (06098). where
T-mobile virtually doesn't exist. where as Torrington (06790) is fully
covered by T-mobile. He lives in his MotorHome.
If he included a linear amplifier (booster) would that even be
a help or would the antenna be enough?
Sorry if I sound stupid but I really don't quite understand
all of this db stuff. How does gain help with transmitting power?
Someone said that 1.5db is equal to 1x boost of ERP. Ok so a 14db
would be equal to... 9.33 times therefore a 1watt GSM phone (maximum
output of my Nokia 6340i) would make it equal to a 9.33W Phone?




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A good dual band direct connect cellular amplifier should solve his problems as long he does not have a line of site or terrain issue . In that case not much anyone can do for them