What causes spin fading of satellite signals?

Many amateur satellites are designed to spin in space. This spinning motion helps keep the satellite gyroscopically stabilized and prevents one side from getting too hot from constant exposure to the sun. As the satellite spins, its attached antennas rotate as well. This constant rotation causes the antenna's radiation pattern and polarization to continuously change from the perspective of a receiving station on Earth. The result is a rhythmic rise and fall in the received signal strength, known as spin fading. While Doppler shift affects the signal's frequency, spin fading affects its amplitude (strength). Interference from the sun or other stations might degrade the signal overall, but they do not cause the rapid, rhythmic fading associated with a spinning spacecraft.