  The active reflector is one of the three main innovations of the
Five-hundred-meter Aperture Spherical radio Telescope (FAST). The deformation
of such a huge spherically shaped reflector into different transient parabolic
shapes is achieved by using 2225 hydraulic actuators which change the position
of the 2225 nodes through the connected down tied cables. For each different
tracking process of the telescope, more than 1/3 of these 2225 actuators must
be in operation to tune the parabolic aperture accurately to meet the surface
error restriction. It means that some of these actuators are inevitably located
within the main beam of the receiver, and the Electromagnetic Interference
(EMI) from the actuators must be mitigated to ensure the scientific output of
the telescope. Based on the threshold level of interference detrimental to
radio astronomy presented in ITU-R Recommendation RA.769 and EMI measurements,
the shielding efficiency (SE) requirement of each actuator is set to be 80dB in
the frequency range from 70MHz to 3GHz. Therefore, Electromagnetic
Compatibility (EMC) was taken into account in the actuator design by measures
such as power line filters, optical fibers, shielding enclosures and other
structural measures. In 2015, all the actuators had been installed at the FAST
site. Till now, no apparent EMI from the actuators has been detected by the
receiver, which proves the effectiveness of these EMC measures.
