# Design and Implementation of C-Band Large-Power Planar Butler Matrix in SRS
###### Abstract  
In satellite remote sensing (SRS), there is a demand for large-power microwave components. A Butler matrix is essential to a transmitting antenna array in SRS. This article illustrates the electrical and mechanical design, simulation, and test results of a large-power planar beamforming network for SRS at C-band. It is a 4 \(\times\) 4 Butler matrix based on square coaxial lines. Short-ended stubs are used in the Butler matrix to broaden its bandwidth by 10%, support inner conductors, and enhance heat transfer in vacuum environments. The simulation results are consistent with the measured results. The reflection coefficient is less than \(-18\) dB, and the isolation is more than 23 dB from 3.8 GHz to 4.2 GHz. The insertion losses are less than 0.6 dB, and the phase errors are better than \(\pm 6^{\circ}\). The measured peak microwave power of the proposed Butler matrix is 9 kW. Its size is \(440\times 400\times 40\) mm\({}^{3}\). The proposed Butler matrix beamforming network can be applied to SRS systems. B 202424213224213224213224213224213224213224213224213224213224213224213242132242132242132242132242132421324213242132  
contexts, it is preferred to microstrip technology for its higher immunity to interference, no air radiation, and lower dispersion. However, realizing coaxial RF components with variable line impedances is generally inconvenient with traditional manufacturing technologies [9]. The multipactor mechanism of introducing dielectrics between metal plates as dielectric-filled waveguides is analyzed. However, breakdown and thermal heating are potential challenges of using dielectric materials in a high-power radio frequency (RF) environment [18]. There is no size advantage at all over highly integrated transmission lines such as microstrip lines and substrate-integrated waveguides, but coaxials and waveguides sacrifice size to achieve kW levels of power, while other transmission lines reach only mW to W levels of power. In [12], a miniaturized large-power beamforming network using an 8 \(\times\) 8 Butler matrix based on waveguides was successfully designed and tested for an X-band synthetic aperture (SAR) system. The insertion loss and isolation of the unit are better than 0.4 dB and 25 dB, respectively, over 600 MHz bandwidth at 9.6 GHz. The measured average and peak microwave power are 500 W and 6 kW, respectively. The whole unit was machined with a modified non-planar structure with a mechanical size of 478 \(\times\) 362 \(\times\) 264 mm\({}^{3}\). In [16], a PCB-based beamforming network was designed for Ka-band applications. Due to the low power capacity of the PCB structure, the measured power value was only 0.01 kW. Finally, by combining multiple phase shifters through a new design approach, a millimeter-wave beamforming network is introduced to achieve beam-steering antenna arrays. The beam steering is performed with a single microfluidic actuator and a circuit model is introduced to facilitate the design of the beamforming network. The design example presented is for a four-element antenna array operating at 28 GHz, exhibiting continuous beam steering capability within \(\pm\)30\({}^{\circ}\) when its SMP is actuated within its \(-\)100 to 100 um displacement range. Unlike beam-steering antennas using beamforming ICs, they do not require active amplification to compensate for high loss and have a reduced hardware complexity associated with control bits and bias lines. Compared to previously investigated microfluids-actuated beam-steering antennas, it offers the advantage of not relying on switched beam techniques and bulky lenses. In [17], a square coaxial line beam former was designed. Simulation results demonstrated that the square coaxial line could reach a power capacity of 8 kW in C-band. Impedance matching was achieved by creating a groove discontinuity in the inner conductor. A cone was provided for smooth impedance transformation for required broad banding.