A DIELECTRIC LENS ANTENNA WITH ENHANCED APERTURE EFFICIENCY FOR INDUSTRIAL RADAR APPLICATIONS N. Pohl Ruhr-University Bochum, Institute for Integrated Systems, Universitätsstr. 150, 44801 Bochum, Germany, Email:
[email protected] For contactless monitoring and measuring of a filling level of an industrial tank, radar systems are widely used since more than one decade. Compared to optical and ultrasonic-based measurement solutions an advantage is especially their robustness e.g. against steam, foam and alignment. Depending on the application, frequencies at 5, 10 or 25 GHz are used with an identifiable long-term trend towards higher frequencies for better antenna directivity. Additionally, the industrial environment results in high demands on mechanical and chemical robustness: The function of the system has to be ensured in a wide temperature and pressure range as well as the compatibility with aggressive chemical substance and fouling is necessary. These harsh boundary conditions demand robust materials and forbid especially the use of large hollow spaces inside the antenna. flange with limited diameter
radar unit
TLPR
industrial tank
TLPR
stirrer disturbing assembly
waves
liquid medium
(a) (b) Figure 1. Setup of an industrial tank level probing radar (TLPR) under ideal conditions (a) and with disturbing assembly, stirrer, and waves (b).
A lot of industrial measurement environments with a plane liquid medium (as in Fig. 1a) are welldefined measurement setups and are covered by existing radar systems. However, in some applications (see Fig. 1b) additional disturbing targets, as assemblies and stirrers inside the tank, complicate the measurement. These disturbing targets can only be separated from the wanted target with a sufficient high directivity of the antenna, but this demands (at a given operation frequency) a large antenna aperture area, which is mostly limited by the standardized flange diameter (e.g. 80 mm). For benchmarks the aperture efficiency, which is the quotient of the antenna effective area and the physical aperture area [1], is useful. The goal of this work is a 25 GHz antenna with high aperture efficiency for industrial radar applications. winding for fixing and gasket
dielelectric ellipsoid (lens)
metal
directivity improvement due to guided waves
74 mm
feed into the antenna
feeding waveguide
plane phase front
stepped impedance transformer
dielectric (PTFE)
(a) (b) Figure 2. Cross section of the concept drawing of the antenna (a) and its realization (b).
The presented antenna concept [2] is based on a dielectric lens antenna using the robust material teflon (PTFE, er=2.08). Due to the feeding directly into the lens body, a geometrical-optical approach delivers an ellipsoid lens (cp. [3]), which transforms a punctiform radiating element into a plane phase front (see Fig. 2a). For realization, the antenna is fed by a circular waveguide. A stepped impedance transformer is used to guide the wave into the lens body. The mechanical fixing and gasket can be done by a winding at the outside of the waveguide, which does not influence the electrical properties of the antenna.
Figure 3. Transient E-field simulation at different time steps.
The transient electro-magnetic field simulation (excited with a modulated gaussian pulse using CST Microwave Studio 2009, Fig. 3) shows the operation of the antenna. The feeding of the waveguide excites spherical waves inside the antenna body (Fig. 3a). On the sides of the structure, this leads to guided waves outside the antenna body (Fig. 3b). Afterwards, a plane wave in the aperture, with larger than the physical antenna dimensions is excited (Fig. 3c).
Figure 4. Simulation and measurement of the far field antenna gain (E-Plane) at center frequency (25 GHz).
The high aperture efficiency is confirmed by simulations and measurements of the antenna far field radiation pattern (cp. Fig. 4). The simulated and measured antenna gain (at 25 GHz) is 26.1 dBi and 25.9 dBi, respectively. This corresponds to an aperture efficiency of 110% and 104%, respectively - a quite high value especially compared to the theoretical value of a TE11 horn antenna (83%, cp. [1]). The tight main lobe is nearly round with a 3 dB angular width of 8.4° and 8.6°, respectively (E- and Hplane). The surrounding side lobe is as low as -17.0 dB and -17.6 dB. Furthermore, the geometricaloptical approach results in a very broadband behaviour. In a sufficient wide frequency range (>>2 GHz) the input related reflection is below -20 dB, which is less than the reflection of one plain PTFE-air transition. Furthermore the reflection shows excellent transient characteristic, dominated by the reflection at the antenna body surface. This enables robust monostatic radar measurements even close to the antenna. [1] G. Armbrecht, E. Denicke, N. Pohl, T. Musch, and I. Rolfes, “Compact directional uwb antenna with dielectric insert for radar distance measurements,” IEEE International Conference on Ultra-Wideband (ICUWB) 2008., vol. 1, pp. 229–232, Sept. 2008. [2] N. Pohl, “Dielektrische Antenne,” German Patent Application Publication DE102008008715A1, Aug. 13, 2009. [3] A. Olver and B. Philips, “Integrated lens with dielectric horn antenna,” Electronics Letters, vol. 29, no. 13, pp. 1150–1152, June 1993.
