## 1 Introduction
In these single antenna systems, the monopulse performance can only be achieved with dual (or multi-) mode networks by means of two main topologies: multimode segmented (generally four) waveguides with different phase sector combinations to generate the sum \(\Sigma\) and the difference \(\Delta\) patterns [32; 33; 34; 35], or non-segmented waveguides with several (at least two) mode excitation circuits (mode converters) to generate the monopulse pattern [36; 37; 38]. Both feeding topologies commonly are bulky structures, especially if other extra capabilities are implemented (such as cross-polarization cancelation [39] or filtering properties [40]) by multimode combination networks. For simplicity, the commonly selected horn antennas for the radiating part are pyramidal [32; 33; 34] or conical [36; 37; 38]. Nevertheless, coaxial horn antennas are also an interesting choice for multiband operation [41], as well as corrugated designs for radiation pattern angular symmetry [42]. In this paper, an on-board satellite linearly polarized Conical Horn Antenna (CHA) is presented for millimeter band 5G communications. In addition, identification of the user direction of signal arrival is provided by means of a monopulse feeding network. As cellular communications are massive user systems, the cost is an important design factor, which is directly related to the system complexity. For this reason, a compact and simple non-segmented dual mode topology has been implemented for the monopulse feeding part. Thus, a circular waveguide will be its out port connecting with the CHA and involving ideally only two modes: the \(TE_{11}{}^{cir}\) and the \(TE_{01}{}^{cir}\). Nominal rectangular waveguides will be used as antenna system input ports. Therefore, rectangular-to-circular mode converters will be required. In this work, the design aims to produce the \(\Sigma\) and \(\Delta\) patterns in the following way. The sum \(\Sigma\) pattern will be generated when the fundamental mode of the circular waveguide (\(TE_{11}{}^{cir}\)), working in the vertical polarization, excites the horn input. The difference \(\Delta\) pattern will be generated when the \(TE_{01}{}^{cir}\) excites the horn input. Please note that the \(TE_{01}{}^{cir}\) is a high-order mode of the circular waveguide, and the excitation levels of other waveguide modes with a cutoff frequency between the \(TE_{11}{}^{cir}\) and the \(TE_{01}{}^{cir}\) (and those close above) will have to be very low to not degrade the system performance. This will be achieved in the presented design using a proper control of the symmetries of the structure and the feeder networks. The presented design is a suitable candidate for the future generation of 5G millimeter-wave applications requiring high data rates. Future 5G high throughput satellites can use the suggested antenna as a component of an on-board multi-spot array [43], in which the monopulse tracking method [4] can be used to estimate the signal angle at each satellite spot. The setup creates a beam with a high gain and low side lobes as well as a very deep null in the boresight. The paper is organized as follows. The design process is divided into two sections (Sections 2 and 3). In Section 2, the mode converters (\(TE_{10}{}^{rec}\) to \(TE_{11}{}^{cir}\) and \(TE_{10}{}^{rec}\) to \(TE_{01}{}^{cir}\)) are designed individually. In Section 3, the integration of the two mode converters to provide the DMC is described along with the connection to the CHA. The fabrication and experimental results are described in Section 4, validating the presented approach.