## 6 Imaging Radar
### Ka-Band Transmitter  
The transmission of microwave power, particularly in the higher frequency bands, is constrained by the availability of sufficiently powerful microwave amplifiers. To achieve a greater transmitted power than that permitted by commercially available high-power amplifiers, the output power of two electron vacuum tubes will be combined. In addition to the RF high-power amplifier, an appropriate antenna front end is required that can handle and transmit the high power and bandwidth to the antenna. At the same time, it implements a duplexer that separates the sensitive receiver channel from the high-power transmission channel. Several radar systems have demonstrated that quasi-optical antenna front ends can handle the highest RF power and minimize the microwave losses compared to traditional waveguide-based systems [34; 35; 36]. In essence, a quasi-optical front end is a quasi-optical transmission line combined with a quasi-optical duplexer. The quasi-optical transmission line is a configuration of focusing mirrors that periodically refocus a diverging Gaussian microwave beam [37]. The quasi-optical duplexer uses the polarization of the Gaussian beam to separate the TX and RX signals. Key component of the duplexer is a quasi-optical Faraday rotator, which is a nonreciprocal polarization rotation device that can be used together with polarizing grids to construct the duplexer [38; 39]. For the TIRA system upgrade, a quasi-optical antenna front end suitable for fully polarimetric radar operation (see Section 3.2) with a broad bandwidth and high transmission power will be implemented. A block diagram of the antenna front-end concept is shown in Figure 25.  
The radar signal is individually amplified by the two high-power vacuum tube amplifiers and launched by two horn antennas as fundamental Gaussian beams (TEM\({}_{0,0}\) mode) into the quasi-optical antenna front end which guides the RF to the antenna. In the transmission channel, the linearly polarized Gaussian beams are transmitted through wire grids following the horn antennas. The subsequent Faraday rotators are designed to rotate the polarization plane by 45\({}^{\circ}\). In combination with the polarization-splitting wire grids, the Faraday rotators function as an efficient quasi-optical duplexer (see [34; 36; 39] for details). At the third wire grid, the power signals from both high-power amplifiers are combined. The existing antenna geometry prevents direct quasi-optical coupling into the antenna as implemented, e.g., in [34]. Therefore, the power-combined signal is coupled by two adjustable mirrors into an oversized corrugated circular waveguide, which is used as the antenna feed due to limited space. However, oversized corrugated circular waveguides permit low-loss signal transmission similar to a quasi-optical beam line and support comparable high power levels [40]. The presented quasi-optical duplexer approach, which utilizes Faraday rotators and polarization-splitting wire grids, enables the simultaneous transmission and reception of RF signals with arbitrary polarization. The polarization of the transmitted signal is determined by the relative phase between the two high-power signals when they are combined at the third wire grid. This feature allows for the polarization of the transmit signal to be freely selected and switched from pulse to pulse or even within a pulse. The ability to freely select polarization with inter-pulse switching is a unique feature of the new TIRA system that will enable fully polarimetric measurements in the future (see Section 3.2). As previously mentioned (see Section 6.3), target tracking for antenna guidance is required simultaneously to the imaging for the new Ka-band system. Therefore, in addition  
Figure 25: Principle block diagram of the high-power polarimetric quasi-optical monopulse front end. to its imaging and polarimetric radar capabilities, the quasi-optical antenna front end is designed to support simultaneous multimode monopulse tracking [34; 41].