38 J. PIDANIČ, D. ČERMÁK, V. SCHEJBAL, GAIN ESTIMATION OF DOUBLY CURVED REFLECTOR ANTENNA Gain Estimation of Doubly Curved Reflector Antenna Jan PIDANIČ, Dušan ČERMÁK, Vladimír SCHEJBAL Jan Perner Faculty of Transport, University of Pardubice, Studentská 95, 532 10, Pardubice, Czech Republic jan.pidanic@upce.cz, dusan.cermak@upce.cz, vladimir.schejbal@upce.cz Abstract. A simple formula of approximate gain estima- tion is verified for the doubly curved reflector antenna. Numerical simulations using physical optics and experi- mental results of the shaped-beam doubly curved reflector antenna are compared with the simple approximation of gain. That approximation could be very valuable for sys- tem engineers to accurately estimate antenna gain and coverage pattern and perform EMC calculations (estima- tions of interferences and susceptibilities) even for the operation and out of operation frequency bands of shaped- beam antenna. Keywords Antenna gain, antenna beam width, doubly curved reflector, estimation of antenna gain, electromagnetic compatibility (EMC). 1. Introduction Radar coverage and electromagnetic compatibility (EMC) ask for accurate estimation of antenna gain, which is the most important performance parameter of an antenna. Sometimes, it is not possible to measure or calculate the gain of an antenna. Many simple formulas are available for gain estimating. The beam widths and directivity of a rela- tively large planar array or aperture antennas are related by various well-known approximate equations [1] - [5]. That approximation could be very valuable for system engineers to accurately estimate antenna gain and perform EMC calculations. As EMC calculations are very important for radar antennas (estimations of interferences and suscepti- bilities), numerical simulations and experimental results are compared with the simple approximation of gain for the operation and out of operation frequency bands of shaped- beam antenna. It is a common radar requirement to have a narrow beam in one plane and a shaped beam in the other. The shaped-beam doubly curved reflector antenna is a classical reflector type, which was described in 1940’s. The far-field radiation of antenna, shown in Fig. 1, can be calculated using aperture method or physical optics [3] - [7]. Radar antenna design can be done using suitable software. From viewpoint of radar antenna systems, it is necessary to ana- lyze not only radiation pattern of an antenna but various related problems. In the early 1960’s, the set of programs computing the shape and the radiation pattern of the doubly curved reflector has been developed [6] - [8] and plenty of Czech radar reflector antennas have been designed and produced [9], [10]. The radiation characteristics of radar RL-41 in the operation (2.7 to 2.9 GHz) and out of opera- tion (3 to 10 GHz) frequency bands have been measured and calculated using physical optics [11]. Fig. 1. Doubly curved reflector antenna. The doubly curved reflector of RL-41 antenna is shown in Fig. 2 with the secondary surveillance radar (SSR) antenna on the top. The primary feed employs two pyramidal horns, which create two independent beams. The polarization can be continuously changed (horizontal – elliptical – vertical). The lower and upper beams are shown in Fig. 3. The angle θ is an elevation angle measured in the vertical plane (symmetry plane) of the RL-41 antenna in Fig. 2. The lower beam is designed as a nearly “pencil beam” for lower elevations. The upper-beam is shaped and its cosecant squared (cosec^2) vertical pattern is modified for higher elevation angles. 2. Doubly Curved Reflector Antenna The programs, used for computations of antenna characteristics for doubly curved reflector antennas con- sisting of primary feed and reflector, have been coded, debugged and gradually improved [7]. It computes far-field rectangular components of primary feed electromagnetic fields and therefore, it allows the electromagnetic field computation at reflector surface. It is assumed that the