In order to measure the electron cloud density, a microwave transmission measurement was set up over 30 m in the SPS beam pipe. The use of TE waveguide modes eliminates direct interaction with the highly relativistic beam. An electron cloud present in the beam pipe should lead to a small phase shift of the CW microwave signal. Since this phase shift is modulated by the bunch revolution frequency, it translates to a phase modulation which can be measured. However, defying all expectations, a

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... beam-induced signal attenuation was found which cannot be understood by current electron cloud models. This attenuation showed erratic memory effects in the order of microseconds after the passage of a batch or single bunch. Both effects are present for a wide range of microwave signal frequencies and beam parameters. Models to possibly explain this effect are discussed.