The study was conducted to determine the motion of the structure and foundation of the Navy X-band antenna, at Waldorf, Maryland, during antenna operation. Tests were run to determine the magnitude and direction of displacement, natural frequencies, phase of movement, and attenuation characteristics of the structure and foundation, and the acceleration, velocity, and deceleration of the antenna assembly. Velocity-type transducers were positioned on the antenna tower to measure displacements in the vertical, radial, and transverse planes. Natural frequencies observed for the rocking and torsional modes of motion were from 5.0 to 9.2 cps and 6.1 to 11.0 cps, respectively. The maximum rotation observed near the top of the tower in the torsional mode was 28.6 microradians, whereas at ground level the maximum motion observed was 1.84 microradians. The maximum rotation observed during the rocking mode was 5.50 microradians at the top of the tower and 2.36 microradians at ground level. There was practically no phase shift between transducer locations. Attenuation measurements indicated damping ratios were 0.122 at the top of the structure and 0.065 at ground level during rocking operation and 0.065 at the top during torsional operation. The predicted frequencies and displacements, computed in accordance with the procedures described in EC 1110-345-310, for the torsional mode for the foundation were in very close agreement with the prototype measurements. The prototype measurements for the foundation during the rocking mode were about one-half the values predicted. (Author).