There are popular antenna upgrade options such as the Diamond RH77CA for VHF/UHF, and the Remtronix 800 for scanning in the 800Mhz range.
However - there's a lot of factors to consider. It's a loaded question. On a handheld, unless you're standing outside on a hill with nothing in the way... there can lots of things blocking reception and limiting factors.
Want to go 50 miles away? You'll want a discone or antenna up on the roof or mounted somewhere high up - and even then, it all depends where you are, and 50 miles is pretty far to expect "good" reception that far away.