japanese car and european cars

i have used european car were it have long thin antenna it recivied FM and AM and now im using japanies car where it has come directly to my country from japan the antenna is short and thick and it recived a range of FM and never recive AM why how does the lenght and thickness effect on RF


some one told me if i change the radio i will fix the problem coz this radio is according to japaness standerd ...
what do u think friends
what is the solution

Try placing your radio in other(someone else's ) car & test.
or try as suggested to you change the radio.

try ur radio in somewhere else