The aim of this study is to develop a mechanically simple monopulse tracking device suitable for "ground" antennas operating in circular polarization in the K/Ka band (i.e with Rx in K band and Tx in Ka band).
This device already exists and has been studied by ESA, with two « turnstile » back-to-back junctions for on-board antennas with the tracking function in Ka-band and has been the subject of a patent in 2013 filed by ESA under this reference : « Very compact TM01 mode extractor ». Nevertheless, the frequency bands being reversed between on-board applications and ground stations, technical problems appear, in particular resonances, when one tries to apply this monopulse tracking no longer in Ka band but in K band for ground stations, and these difficulties are accentuated if we seek to extend the K band below 17.7GHz up to 17.3GHz. The 2 modes TE11h and TE11v of the waveguides are extracted by these 2 junctions and are retransmitted thanks to 4 bridges in rectangular waveguide which bypass the central guide by which the tracking mode TM01 is extracted. The main difficulty to solve concerns the generation of the TM01 mode which must not be coupled with these 2 TE11 modes: in the absence of a good isolation (i.e. 40 to 50dB), resonances are likely to appear and lead to device failure. The proposed solution therefore consists in improving these two turnstiles junctions by exploiting the advantages of OMJ type junctions with the aim of eliminating the appearance of these higher modes and make the concept suitable for ground applications. It is essential to remember that these choices must be made taking into account mechanical manufacturing considerations. This is a critical point concerning all TM01 extractors with waveguides bridges. We must therefore apply an engineering methodology method as DFMA that focuses on reducing time-to-market and total production costs. Different alternative concept architectures will be analysed and compared during the study.
