You will need a antenna that is resonant on the frequency you're transmitting on for any hope of the signal leaving the immediate proximity of the Pi. It would also be a very good idea to filter the signal coming out of the Pi to reduce the strength of any frequency spurs accompanying your Pi generated signal and to keep your transmission legal.

This is a pretty good write up of using the Pi as a wspr transmitter: https://github.com/JamesP6000/WsprryPi