Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems is generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
Many products, in particular liquids, are at least to some extent transparent to the electromagnetic signals in the frequency range typically used in radar level gauge systems, which may possibly cause electromagnetic signals reflected from the bottom of the tank to interfere with the electromagnetic signals reflected from the surface of the product when measuring low filling levels, i.e. the distance resolution may be insufficient to distinguish the electromagnetic signals reflected from the bottom of the tank from those reflected from the surface of the liquid.
EP 1 242 794 discloses a radar level measuring device having an absorber for microwave signals positioned at the base of the tank. Through this absorber, the reflection at the bottom of the tank can be considerably reduced, which allows measurement of low filling levels. In the radar level measurement device according to EP 1 242 794, the absorber is provided in the form of a connected plate of spikes where the tips of the spikes point in the direction of the incident microwaves. The spikes are made of a material which has a good absorption capacity for microwaves. Although the absorber according to EP 1 242 794 allows measurement of lower filling levels than was previously possible, the extension from the bottom of the tank to the top of the absorber limits the lowest filling level that can be measured to in the order of at least one wavelength of the microwave signals. As the frequency range of radar level gauging systems is typically in the order of tenths of gigahertz, the wavelength is in the order of a few centimeters.