IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 49, NO. 12, DECEMBER 2001 1657 Hybrid PO-MoM Analysis of Large Axi-Symmetric Radomes M. A. Abdel Moneum, Z. Shen, J. L. Volakis, Fellow, IEEE, and O. Graham, Life Senior, IEEE Abstract—Over the last three decades, intensive work has been done to develop techniques aimed at accurate and efficient analysis of antenna radome systems. Some recent applications involve radars operating in the millimeter wave range and for those cases the radome size can be on the order of one hundred wavelengths or so in length. For practical simulations of such large radomes, a hybrid physical optics-method of moments (PO-MoM) technique is presented for accurate and efficient analysis of electrically large radomes. The procedure combines the Method Of Moments (MoM) for modeling the tip region of the dielectric radome and ray optics in conjunction with physical optics (PO) for treating the flatter smooth section of the radome. Calculated far-field patterns using the new technique agree well with measured data for a reflector antenna radiating in the presence of a large radome. The computational time for simulating the performance of a reflector in the presence of an long radome was a mere 4 h on a 233 MHz PC. Index Terms—Antenna radome, axi-symmetric radome, hybrid moment method, physical optics (PO). I. INTRODUCTION N OSE radomes are often used to house airborne scanning radar antennas. They are also used to protect antennas from a variety of environmental and aerodynamical effects. A typical nose radome configuration is shown in Fig. 1, consisting of an antenna (reflector, array, etc.) radiating a field that im- pinges upon the dielectric radome. The latter, in addition to serving as an aerodynamic structure, may also create undesir- able blockage due to interactions of the antenna fields with the radome. Needless to mention, the radome’s presence affects the antenna pattern (bore sight error and pattern distortion) leading to possible errors in the scanning and guidance of the nose an- tenna systems. A careful analysis of the antenna-radome system is thus appropriate. Such analysis should include effects asso- ciated with the radome’s tip whose treatment is more difficult due to its associated diffraction. The latter is rather challenging when dealing with large size structures (over long and wide) that may also be composite/nonmetallic. Several techniques have been developed for the analysis of antenna-radome structures aimed at providing accurate tools for radome design. These can be divided in two categories: 1) high-frequency (HF) methods based on ray tracing methods [1]–[4] or the so called plane wave surface integration technique Manuscript received January 20, 2001; revised April 3, 2001. M. A. Abdel Moneum, Z. Shen, and J. L. Volakis are with the Radiation Lab- oratory, Department of Electrical Engineering and Computer Science, The Uni- versity of Michigan, Ann Arbor, MI 48109-2122 USA. O. Graham is with the Science and Applied Technology, Inc., Woodland Hills, CA 91367 USA. Publisher Item Identifier S 0018-926X(01)10827-6. Fig. 1. Radome geometry. [5] and 2) low-frequency (LF) methods such as the method of moments (MoM) with resistive sheet modeling [6]–[8], finite element method (FEM) [9], finite element-boundary integral method (FE-BI) [10], and the method of analytical regularization (MAR) [11]. HF methods assume locally planar surfaces and have been used successfully for the analysis of smooth spherical and cylin- drical radomes. Being fast and easy to implement, HF methods permit the analysis of smooth tip radomes on the order of [4]. However, HF methods cannot be employed for the analysis of sharp radome tip as the local flat surface assumption is not satisfied at the tip. On the other hand, LF methods, although ro- bust, are associated with extensive computational requirements. Thus, previously reported results [6]–[11] have been limited to small radomes on the order of a few wavelengths in length. To take advantage of the robustness of LF methods and al- leviate their computational demands, in this paper we propose a hybridization of HF and LF techniques. Under the proposed hybrid technique, the tip sector of the radome is modeled using the MoMs to take into account diffraction and multiple inter- actions which may be significant. Away from the tip, ray tech- niques in conjunction with physical optics (PO) are used to com- pute equivalent currents which are then combined with the MoM currents at the dielectric boundaries for complete modeling of the radome. Near zone antenna fields are computed at the inner surface of the radome to serve as the excitation for the MoM 0018–926X/01$10.00 © 2001 IEEE