You can get a cheap dipole FM antenna at Radio Shack (or Amazon if RS folds). You want the antenna as high as possible, and away from metal.

DIY FM dipole antenna or half wave antenna are the most used because is easy to build and cost a few bucks but has some drawbacks like little directionality, small gain. The rf dipole antenna is build ...

He claims it will noticeably improve both transmit and receive by making a quarter-wave monopole into a makeshift dipole antenna system.

If you use an external antenna, like this fm dipole antenna connect it with a good coaxial cable because the power is low and you want to use most of it. If you don’t want to loose the rf power ...