If you are talking about a FM radio receiver, then just about any piece of wire will do. Typical 1/4 wave whip antenna is about 30 inches long. However, most radios use a shorter whip.
For the radio included in your MP3 player, they often use one of the headphone wires as the antenna. That is why radio reception will vary as you move around or greatly diminish if you coil up the excess headphone wire.
Table top radios that plug into the wall, often will use the AC power cord as the antenna. To receive distant stations, you often have to stretch out the power cord in a particular direction. This is baffling to consumers, as the radio will sometimes appear to work much better when plugged into a long extension cord and using a remote outlet.
This is no right answer man.
Best solution for FM radio antena is open or closed dipol with asimetric part
(ferite transformer).
So you may build that dipol like in line or rounded.