A device for measuring an electromagnetic field radiated from a high power microwave source, comprising: - a testing cage, in which are provided: • the microwave source; • an electromagnetic field measuring sensor; and • a variable attenuation device for variably attenuating a radiated microwave signal, which comprises a watertight member filled with an electromagnetic energy absorbing liquid, in which a piston can be displaced, and remote control means for controlling displacement of said piston, wherein said member is a cylinder formed by a cylinder wall closed on either side by a first and a second flange (12 and 13), and wherein the piston bounds with the first flange a variable volume filled with this liquid, the cylinder being pierced at a first end, located on the first flange side, with a liquid inlet/outlet port, the second flange being pierced with an air inlet/outlet port, a displacement sensor enabling the displacement of the piston within the cylinder to be known, the variable attenuation device being arranged axially with respect to the microwave source and the electromagnetic field measuring sensor, between the microwave source and the electromagnetic field measuring sensor; - a control cage, in which are provided: • a control member; and • a viewing member. - a testing cage, in which are provided: • the microwave source; • an electromagnetic field measuring sensor; and • a variable attenuation device for variably attenuating a radiated microwave signal, which comprises a watertight member filled with an electromagnetic energy absorbing liquid, in which a piston can be displaced, and remote control means for controlling displacement of said piston, wherein said member is a cylinder formed by a cylinder wall closed on either side by a first and a second flange (12 and 13), and wherein the piston bounds with the first flange a variable volume filled with this liquid, the cylinder being pierced at a first end, located on the first flange side, with a liquid inlet/outlet port, the second flange being pierced with an air inlet/outlet port, a displacement sensor enabling the displacement of the piston within the cylinder to be known, the variable attenuation device being arranged axially with respect to the microwave source and the electromagnetic field measuring sensor, between the microwave source and the electromagnetic field measuring sensor; • the microwave source; • an electromagnetic field measuring sensor; and • a variable attenuation device for variably attenuating a radiated microwave signal, which comprises a watertight member filled with an electromagnetic energy absorbing liquid, in which a piston can be displaced, and remote control means for controlling displacement of said piston, wherein said member is a cylinder formed by a cylinder wall closed on either side by a first and a second flange (12 and 13), and wherein the piston bounds with the first flange a variable volume filled with this liquid, the cylinder being pierced at a first end, located on the first flange side, with a liquid inlet/outlet port, the second flange being pierced with an air inlet/outlet port, a displacement sensor enabling the displacement of the piston within the cylinder to be known, the variable attenuation device being arranged axially with respect to the microwave source and the electromagnetic field measuring sensor, between the microwave source and the electromagnetic field measuring sensor; - a control cage, in which are provided: • a control member; and • a viewing member. • a control member; and • a viewing member.