Minor polar flutter does not stop Boysan DU1SAN from running 'em in the EA contest. The path is directly through the North Magnetic Pole and the auroral region of the ionosphere. Polar flutter is a Doppler effect and is caused by rapid variation in the vertical location (or simply speed) of the ions that are "reflecting" the radio waves (absorbing and re-emitting actually). This speed of movement is not added or subtracted to the speed of the radio waves but in the world of quantum physics and relativity, what is actually does is make small changes to the energy of the emitted signal instead. Since the frequency is proportional to the energy, the frequency of the emitted photon (radio wave) is changed. The changing frequency of the tones is what causes the "fluttering" sound.