This paper investigates the geolocation of unknown HF transmitters for medium range distances in the order of 1000 km. In such conditions, the signal propagates through skywaves in the ionosphere-Earth waveguide, and single-hop transmission can be assumed. To evaluate the accuracy of geolocation techniques, we use the Quasi-Parabolic model of the ionosphere, providing analytical expressions of the ground distance and group path length. Classical trilateration generates small errors in the order of 25 km, but requires the assessment of the ionospheric reflection height. We propose a new blind method, independent of the ionospheric profile, and evaluate its efficiency under realistic propagation conditions.