# Conceptual Design of a Semi-Dual Polarized Monopulse Antenna by Computer Simulation
## 2 Feed Design  
The design of the proposed antenna consists of the following steps. (i) Review design requirements: frequency range, antenna gain, polarization requirements, monopulse comparator requirements, (ii) design a feed geometry including waveguide aperture dimensions, (iii) design a septum polarizer for the sum channel, (iv) design a coaxial probe feeding the sum and difference channel radiators, (v) simulate the monopulse feed, (vi) design the reflector geometry including feed placement, and (vii) simulate the whole antenna structure--the feed and reflector--as a single unit. The structure of the feed proposed in this work is depicted in Figure 1. In our feed design, we adopted a five-element geometry, which greatly simplifies the monopulse comparator network. At the center is a square waveguide radiator with a septum polarizer for dual-circularly polarized sum patterns. For the azimuth difference pattern, two rectangular waveguide open-ends are placed with the narrow wall in the horizontal direction so that the distance between them is minimized to match the sum and difference pattern beamwidths. The configuration for the elevation difference channel is obtained by a 90-degree rotation of the azimuth difference channel radiator. Sum and difference channel waveguides are excited by a coaxial probe. The waveguide broad wall width is made as small as possible, as far as it is possible to obtain good impedance matching for the rectangular waveguide and low axial ratio for the square waveguide. The novelty of the proposed feed configuration lies in the fact that a complicated monopulse comparator network is not required. The sum channel is physically separated from difference channels, and, thus, there is no need to connect it to a monopulse comparator. Coaxial probes in the difference channel waveguides are oriented in opposite directions, and, thus, adding two coaxial probe signals with a simple power combiner gives a difference pattern. Horizontal and vertical difference channels are physically separated, and, thus, there is no need to connect them to a monopulse comparator either. In essence, two power combiners are all that is required of a monopulse comparator network for the proposed feed. In order to suppress the back radiation from the feed, two choke grooves are placed close to the waveguide radiators. The thickness of the wall between two waveguides is made as small as can be fabricated to minimize the distance between the difference channel waveguides. A feed design is carried out for operation at 15.25 GHz with a bandwidth of 1.5 GHz. First, we present the design of a stepped septum polarizer in a square waveguide whose dimensional parameters are shown in Figure 2. The design parameters of the polarizer are the number of stepped sections, septum profile, step dimensions, and septum thickness. Our design of the polarizer is based on a classic design by [PERSON] and [PERSON] [19], with some modifications to the septum profile and septum thickness. The  
Figure 1: Structure of the proposed feed. (**a**) Front view. (**b**) Rear view. parametric analysis for the septum polarizer design is too involved and omitted for brevity. The polarizer is first designed using waveguide modes as shown in Figure 2a, where Ports 1 and 2 are excited with the rectangular waveguide TE\({}_{10}\) mode. Port 3 in the square waveguide has two modes; Mode 1 (TE\({}_{10}\) mode) for the horizontal polarization and Mode 2 (TE\({}_{01}\) mode) for the vertical polarization. With the TE\({}_{10}\) mode inputs at Ports 1 and 2, a four-section stepped septum polarizer makes Modes 1 and 2 in Port 3 have equal amplitude and a 90-degree phase difference. Linearly polarized input waves at Port 1 and 2 are converted into RHCP and LHCP waves, respectively, at the Port 3 output. After the septum design using waveguide modes, an SMA coaxial probe is designed to excite the input rectangular waveguides, as shown in Figure 2c.