# An In-Line Coaxial-to-Waveguide Transition for Q-Band Single-Feed-Per-Beam Antenna Systems
## 1 Introduction  
High frequency systems require transmission devices with low losses and dispersion [1]. Moreover, the performance of microwave (300 MHz-30 GHz) and millimetre-wave (30-300 GHz) devices as antennas [2] or waveguides [3] strongly depends on geometrical parameters whose dimensions are smaller than the operative wavelength [4], so that as the variation scale of the electromagnetic waves involved decreases (e.g., at the frequency of 50 GHz, the free space wavelength is about 6 mm), thus calling for a higher precision and tight mechanical tolerances in the fabrication, to avoid long and complicated post-realization tuning procedures [1]. A higher precision is required in their fabrication. In this framework, microwave engineers tend to prefer devices whose geometry allows an easier manufacturing process when the operating frequency rises [1]. These issues make horn antennas the preferred solution in high frequency transmission systems with respect to planar or wire antennas: horn antennas are generally all metallic and are characterized by a simple geometry, easy construction and excitation [5]. Such features make the horn one of the typical feeds at millimeter-wave (mm-wave) frequency range, as a stand-alone antenna, array element [6] or as feed for a reflector system [7]. At mm-wave, the channel is characterized by a high signal attenuation which limits the long distance transmission capability of a stand alone antenna and requires the applications of large reflector dishes to obtain an high directivity [8]. Since a high-directive radiation limits the beamwidth [9], the need toguarantee the communication over a wide angular range requires a multiple narrow-beam radiation, which translates in a single-feed-per-beam (SFB) antenna configuration [10]. Satellite communications exploit multiple-beam reflector antennas (or, in particular cases, phased arrays [11]) to achieve a continuous coverage over a desired field-of-view on terrestrial surface [10]. Since a different feed is required per each beam, the need to cover all the directions in a determinate angular range is satisfied with a dense cluster of horns [12; 13]. A similar multiple horn-fed reflector antenna system is employed in radio-astronomy too [14], where the low power of the typical signals involved requires a large parabolic dish to collect the incoming radiation and focus it in a feed.The feed has a composite geometry, and a certain numbers of horn antennas are placed close each other in the focal point, providing a multi-beam radiation where the beams system covers a wide span of the sky [15]. Since not the whole radio frequency (RF) system in charge of the signal processing consists of waveguide (WG) components, a suitable transition is required to connect the horn to the RF front-end without degrading the signal. The traditional E-probe transition requires the orthogonal alignment of the second device with respect to the axis of the horn [16], this increases employment in the transversal area and limits the possibility of creating a cluster with a dense concentration of horn feeds, especially in the mm-wave range. In addition, the relative position between the reflector and the horn-feed is a design parameter which can be limited by the E-probe feed configuration. To overcome such limitations, the TE\({}_{10}\) mode of the horn waveguide can be coupled to the field configuration in the downstream devices through an in-line transition which keeps the same direction of the field and does not increase the transversal size and does not concur to the blockage effect [10]. Microstrip and coaxial cable can be employed for such a purpose, and different solution have been studied, by shaping the end-launcher [17] and studying configurations which ease the connection between the devices [18], or involving gap-waveguides to improve the matching [19]. However, usually high-performance systems have coaxial connections [14]. For this reason, coaxial-to-waveguide in-line transition is a common solution to feed horn antennas employed in reflectors.