STUDY OF HIGH-EFFICIENCY CORRUGATED HARD-HORN ANTENNAS USING CLASSICAL APPROACHES: HARD CIRCULAR WAVEGUIDE MODE, PHASE FACTOR, AND APERTURE INTEGRATION Omid Sotoudeh, 1 Per-Simon Kildal, 1 and Zvonimir Sipus 2 1 Chalmers University of Technology, SE-412 96 Gothenburg, Sweden 2 University of Zagreb Unska 3, HR-10000 Zagreb, Croatia Received 31 August 2004 ABSTRACT: In this paper, a classical-type approach is used to ana- lyze the hard horn antenna. The quasi-TEM mode in the longitudinally corrugated cylindrical waveguide is found by using the homogenized asymptotic boundary conditions. The finite horn length is accounted for by including a phase factor in the waveguide-aperture fields, and the radiation patterns are finally obtained by aperture integration. The bandwidth is found to be much wider than previously anticipated with respect to low cross-polarization. Also, an advanced mode-matching code has been used for verification of the aperture field and the radia- tion patterns obtained with the much faster classical model. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 44: 516 –523, 2005; Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/mop.20684 Key words: hard horn antennas; cluster-fed multiple-beam antenna feeds 1. INTRODUCTION A horn antenna with high aperture efficiency and low cross- polarization has interest for several applications. For instance, it may be used in arrays in order to decrease the spacing between the elements and at the same time keep the desired gain, as described in [1] and also mentioned in [2], or in cluster-fed reflector anten- nas, as in [3– 6]. In the latter case, the high aperture efficiency makes it possible to place these elements close to each other without jeopardizing the requirements on beam isolation and smooth beam coverage. Theoretically, there are mainly two ways of achieving high efficiency in horns: (i) Using a smooth metal surface in the horn and adding several appropriate higher order modes in the aperture of the horn to get an almost uniform aperture distribution, that is, a multimode high efficiency horn, or (ii) exciting a single dominant quasi-TEM mode inside a horn with so-called hard surface walls. In the first case, the higher-order modes are generated by several steps inside the horn and, as a result, the solution becomes intrinsically narrow band, in particular for large horn apertures (see, for example, [7– 8]). In [7], a mul- timode horn with 4 diameter is designed to give 10% bandwidth, and in [8] a dual-band (8.6 –9.8 GHz and 10.75–11.15 GHz) multimode horn with smaller than 3 diameter is described. In the second case, the horn walls are constructed as hard surfaces, as introduced in [9, 10], in order to support the quasi-TEM mode. In this case, there are no fundamental limitations on the aperture size, and the bandwidth is instead controlled by the characteristics of the hard surface. The purpose of this paper is to study the available bandwidth of the hard horns. The easiest way to generate a dual-polarized hard wall is to use longitudinal corrugations filled with dielectric material (Fig. 1). Another way is to use longitudinal metal strips on a grounded dielectric substrate [11]. In the latter case, strip modes following the strips must be removed by ground- ing the strips with via holes. The development in satellite communications and the increas- ing demand for multimedia-via-satellite systems has lead to tougher requirements for onboard antenna systems. Onboard mul- tiple-beam antennas are normally planned to be cluster-fed reflec- tors. These systems may be built using either the one-beam-per- feed concept, or overlapping subarray concepts. They are planned for operation in the Ka-band, covering two separate bands 17.7– 20.2 GHz (downlink) and 27.5–30.0 GHz (uplink). In general, the technical requirements consist of values for the minimum directive gain and the beam isolation in the footprint. These may be met either by using two different sets of antenna systems containing appropriate numbers of reflectors and feed clusters (one for down- Figure 1 Illustration of a longitudinally corrugated hard horn antenna with the diameter 50 mm, wall thickness t = 2 mm, and horn length 250 mm Figure 2 Geometry of the longitudinally corrugated hard waveguide used for the classical-type model (corrugation period p  ). [Color figure can be viewed in the online issue, which is available at www. interscience.wiley.com.] Figure 3 Definition of the E-plane (or ridge) cone length L E and H-plane (or bottom wall) cone length L H of a longitudinally corrugated horn with constant wall thickness 516 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 44, No. 6, March 20 2005