###### Abstract  
A very compact waveguide orthomode transducer (OMT) is described in this paper. The design is characterized with a twofold rotationally symmetric cross-section in the probing area, adapted from a side-coupling OMT design, simultaneously enabling low port-to-port coupling and high cross-polarization discrimination (XPD) over a fractional bandwidth of about 15-20%. Compared to previously reported compact waveguide OMTs, the proposed design is simpler, thus facilitating its manufacture at millimeter-wave frequencies. The concept is demonstrated with a design in the K-band for a broadband communication satellite downlink over the frequency band of 17.3-20.2 GHz. For test purposes, transitions to standard waveguide WR42 are included, and the OMT is assembled with a conical horn antenna. The measured reflection and coupling coefficients are below \(-19.5\) dB and \(-22.9\) dB, respectively, over the nominal bandwidth, and they are in good agreement with the simulation's results. The on-axis XPD, measured in an anechoic chamber, is better than 30 dB over the nominal bandwidth, which is also in line with simulations. The proposed waveguide OMT may be designed to fit in a lattice below one wavelength at the highest operating frequency, which is desirable for dual-polarized closely spaced array antennas in low and medium Earth orbit communication satellite systems. The simple mechanical design of the proposed OMT makes it particularly appealing for additive manufacturing techniques, as demonstrated with a variant of the design having folded single-mode waveguides, which preserves the RF properties of the original design. orthomode transducer; passive waveguide component; dual-polarized phased array antenna; broadband communication satellite +
Footnote †: journal: Sensors  
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Berkeley, CA 94720, USA  
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not suited for closely spaced arrays required for future broadband communication satellite systems [12], as the side-coupling port results in a footprint twice that of the waveguide's broad wall along the coupling direction. Existing OMT solutions with their footprint fitting within the common waveguide cross-section are typically based on septum designs. These components were first introduced and are still mostly used in waveguide antenna designs to provide dual-circular polarizations [13; 14; 15; 16]. The septum is sized to produce a 90\({}^{\circ}\) phase difference between the two orthogonal linearly polarized modes of the common waveguide, resulting in the two single-mode ports producing right-hand and left-hand circular polarization signals. The OMTs described in [17; 18] make use instead of a 180\({}^{\circ}\) septum design that provides slanted dual-linear modes in the common waveguide with reference to the single-mode waveguide orientation (i.e., \(\pm 45^{\circ}\) with reference to the fundamental modes of the common waveguide). The OMT design described in [19] advantageously combines a 90\({}^{\circ}\) septum and a 90\({}^{\circ}\) iris polarizer to provide flexibility in the orientation of the linearly polarized modes in the common waveguide. This solution also reports a broader frequency bandwidth operation when compared to [17]. However, the designs based on septums are generally quite long, with an in-line dimension that is typically 3 to 4 times the waveguide's broad wall dimension. A promising alternative design with a reduced footprint would comprise a folded standard turnstile junction as proposed in [20]. A turnstile waveguide OMT would still lead to a lengthy component because of the combination network required. Two-probe waveguide OMT designs with transversal coupling ports, which may be seen as simplified turnstile junctions, have been reported in [21; 22]. Combined with the folding approach described in [20], it could produce the most compact waveguide OMT designs. However, the solutions proposed in [21; 22] rely on small features, such as small cylindrical or cross-shaped pins, requiring very high precision milling.