Transmitting and receiving antennae in the form of slot antennae are positioned at the height of the liquid in the bottle (1). The antenna slots are on the same side of the bottle side by side or one above the other. The slots are pref. inclined at an angle of 45 deg. to each other. A h.f. alternating voltage of 1.5 to 4 GHz, pref. 2.2 GHz is fed by the transmitting oscillator (7) through a low pass filter (8) to a waveguide (9). - The microwaves from the latter are directed onto the liquid surface by adjusting a sliding support plate (6). The microwaves reflected by the liquid surface enter a receiver waveguide (13) and are fed to an analyser.
