Abstract: A microwave sensor (3) for measuring the relative proportions of fluids is provided. The sensor (3) includes a tube (4) having a length (9) for accommodating the fluids flowing through said tube. The length (9) of the tube (4) makes up a part of the sensor (3). Also included are probes (6, 7) for transmitting electromagnetic energy into the sensor (3) and receiving electromagnetic energy from the sensor (3). The sensor (3) uses resonance at a resonant frequency (fm) below the cut-off frequency (fm) of the tube (4). The tube (4) includes at least one radially extending, conducting, internal fin (5) extending along the axis (10) of the tube (4). The fin (5) may be of regular or irregular shape and may be fixedly or detachedly arranged, e.g., through a slot (13).