Method and apparatus for measuring the distance to an object

A method for measuring the distance to an object, such as the surface of a liquid or another flowable medium in a container or similar environment, wherein a microwave signal with a predetermined frequency is generated, a part of the the signal is used as a reference signal while a second part is fed to an antenna which is directed against said surface whereby the surface and other present objects influence the input impedance of the antenna and the corresponding unit reflection coefficient through their distant dependent contributions to said reflection coefficient, characterized in making a measurement function of the input reflection coefficient of the antennaa at a combination of values of the reflection coefficient at at least two frequencies in an interval around a nominal measuring frequency in a manner to emphasize contributions to said function from objects in certain distances relatively to contributions from objects in other distances, calculating by fourier transformation a set of values of this function at the predetermined set of measuring frequencies, the different object's contribution to the fourier transformed function are separated according to their different distance from the antenna while contributions from fixed objects are eliminated since their contributions to the fourier transformed function are known, and the contributions of the surface is divided out due to its strength and/or position to enable the determination of the distance to the surface.