One application of RADAR (RAdio Detection And Ranging) is to measure the distance to a moving or fixed object using an electromagnetic wave. The distance of the object is determined by the time difference between the transmitted and reflected wave. This example describes such a distance measurement using a RADAR system for measuring the tank filling level in an industrial storage tank. The so called Tank Level Probing Radar (TLPR) has been widely used in industrial tanks since it provides a robust and precise measurement. However, the remaining challenge for the TLPR is to design a proper antenna as it is limited by size of the flange. Furthermore, the presence of interfering objects inside the tank (for example the disturbing assembly shown in Figure 1) requires a well focused antenna with a narrow beamwidth....
A focused antenna is characterized by narrow beamwidth, low side lobe level and high gain in the main beam direction. With the effective antenna area, the gain can be computed as Figure 2 (a). Based on the given flange diameter of e.g. 80mm, the antenna effective area can be calculated with Figure 2 (b). Finally, the aperture efficiency is given by Figure 2 (c), which is one of the important parameters to characterize the antenna focusing performance at a limited antenna size.
A horn antenna is one of the most commonly used antenna types for a focused antenna. However, the horn antenna is limited by its dimensions, which must have a certain size relative to the wavelength in order to work properly and efficiently. Since this will typically be too large to fit into the flange, a dielectric lens antenna with reduced dimensions is developed. The lens has the elliptical form which is able to enhance the gain by making use of guided surface waves. The antenna is fed by a circular waveguide and at the boundary of the dielectric half space. A stepped impedance transformer has been introduced in order to provide broadband input matching impedance.
The antenna has been constructed and optimized using the transient solver within CST MICROWAVE STUDIO®. A hexahedral mesh with the PBA Technique allows the generation of accurate results even along the curved object boundaries.
Figure 5 shows a good return loss and wideband behavior of the dielectric lens antenna and its corresponding transient input matching behavior. The transient input impedance shows a good input matching impedance as the multiple reflections of field inside the dielectric lens can be achieved at a level below -40dB.
A transient nearfield animation of the dielectric lens antenna is shown in Figure 6. Within the usable frequency range, the propagating field combines with the guided surface wave to focus the radiated field and achieve a higher gain. After reaching the surface, a small portion of signal is reflected. This portion will be absorbed at the port or by dielectric losses inside the lens.
The resulting farfield can be seen in Figure 7. A low side lobe level of -15dB and a high gain of 25dBi at the main beam can be achieved with this dielectric lens antenna configuration. The important parameter of antenna aperture efficiency is seen to be around 104%, which indicates that the effective aperture area is larger than the flange size. And therefore a high precision measurement of the tank filling level can be reached.
Conclusion: A dielectric lens antenna for the industrial tank application has been introduced. A well focused antenna can be designed to have a high aperture efficiency. This dielectric lens antenna can also be easily manufactured and proved to be robust in terms of mechanical and chemical realization.
References Nils Pohl, A Dielectric Lens Antenna with Enhanced Aperture Efficiency for Industrial Radar Applications