Why A satellite tumbling or spinning changes the apparent orientation of its antenna, and the ionosphere rotates the polarization plane of a signal passing through it (Faraday rotation), so a fixed linear antenna on the ground sees deep fades as the polarizations drift in and out of alignment. A circularly polarized antenna, such as a crossed Yagi or helix, has a constant response to any linear orientation, so those fades largely disappear. You pay about 3 dB in mismatch loss to a purely linear signal, but you trade deep nulls for a steady, predictable signal.