Rotary joints for SatCom

Modern mobile satellite communication systems provide bidirectional broadband network access for moving ground, maritime and airborne terminals. Applications enabled by mobile SatCom systems include broadband Internet access, video conferencing and data transmission. Currently such systems commonly called SatCom on-the move are being introduced on a large scale all over the world for commercial as well as for governmental (civilian and military) use. For mobile SatCom equipment the commercial Kuband is oftenchosen since this frequency range offers a trade-off between reduction of antenna size on the one hand, and reasonable resistance to adverse
weather conditions on the other.

A moving satellite terminal requires a highly sophisticated beam steering system in order to continuously track the satellite during operation. There are two basic technologies that can be employed for pointing an antenna beam: The electronically phased-array antenna and the mechanically steered pencil beam antenna. When the
latter system is used, the strong space restrictions imposed by a mobile SatCom terminal require the adoption of a small aperture antenna together with a low-profile pedestal. Likewise the rotary joints employed for signal transmission along the mechanical steering axes need to be small and often shaped specifically for the application.

Therefore, in order to provide equipment manufacturers with rotary joints tailored to the specific requirements of fixed and mobile SatCom systems, SPINNER has developed two new rotary joint product families single channel waveguide rotary joints for Ku-band, and dual channel coaxial rotary joints, which are suitable for Ku-band and intermediate frequency applications. Both product lines follow a common design philosophy that is characterized by well-thought-out modularity that allows a large variety of solutions with a minimum number of different piece parts and excellent electrical and mechanical performance, allied to a simple and highly reliable design.

Dual channel rotary joints

The newly introduced dual channel rotary joints belong to a universal product family that covers different preferential frequency ranges, different case styles and flanges. The predecessor to this family is the multi-purpose rotary joint (BN153118). Its main characteristics are summarized in the table on the left page. This dual band rotary joint and its derivatives are realized with galvanic contacts at the center and outer conductors. As a result of this design each channel en-ables combined RF and DC transmission. This DC transmission makes it possible to supply active components on the rotating part of a platform (e.g. an LNA).

Besides the I-shaped models (BN 153139), the new product family also includes rotary joints with L-shaped (BN 153146) and U-shaped housings that allow for a space-saving antenna pedestal design. These L-shaped and U-shaped variants significantly simplify the integration of the rotary joint. In order to cover a large variety of customer needs, the different basic case styles are available with standard and custom specific mounting flanges. The different housing variants described above are just one advantage. More-over, specialized versions are available that incorporate application specific tuning of the second channel in order to fulfill more stringent performance requirements within a dedicated frequency range. The L-shaped rotary joint is an example of a variant optimized for SatCom use.

As a result of this optimization channel 2 clearly outperforms channel 2 of the multi-purpose rotary joint with respect to all RF parameters. At the expense of a narrower usable frequency range, channel 2 of L-shaped rotary joint does now comply with a VSWR below 2.0 (compared to 3.5), a VSWR variation during rotation less than 0.5 (compared to 0.8), an insertion loss below 1.0 dB (compared to 1.5 dB) and a maximum insertion loss variation of 0.4 dB (compared to 0.8 dB).

All of the rotary joints are available in 3 different configurations I-style, L-Style and U-Style. The three shapes allow to build very low profile SatCom systems with high end performance.

For applications that require a contacting transmission of supply power (DC, AC) or low frequency signals (analog, data), the dual channel rotary joints are also available in combination with standard and customer specific slip ring assemblies.

Single channel rotary joints

These are available in U-shape, L-shape and I-shape, while the main characteristics of the Ku-band waveguide rotary joints are summarized in the following table. Although realized in a non-contacting design, the Ku-band family is characterized by wide frequency coverage (relative bandwidth of 30 percent and above). An advanced electromagnetic design employing two stepped impedance transformers makes it possible to cover the full satellite band portion of the Ku-band with a high return loss and low insertion loss.

These rotary joints are suitable for downlink frequencies (10.70 to 12.75 GHz) and uplink frequencies (13.75 to 14.50 GHz) as well as for the guard band in between. This last feature is being demanded by more and more customers. The graph shows that any parasitic resonances are placed clearly outside the specified transmission band.

Another significant feature of the single channel rotary joints is that they are able to transfer high average power levels safely. This feature is a result of the low insertion loss and the method of design, with large cross-section metallic parts that allow for efficient axial heat conductivity towards both ends of the joint. Besides good RF performance the Ku-band rotary joints are characterized by a dusttight design that is protected against jet water (protection level to IP65) by the inclusion of a dynamic seal. In order to accommodate this seal the torque is increased and the housing slightly lengthened, but both are kept to a minimum.

Conclusion

These two new SatCom rotary joints families that have been introduced by SPINNER define a reference standard in electrical and mechanical performance. The Ku-band single channel waveguide rotary joints stand out due to their extremely wide transmission band. The dual channel rotary joint family features a unique variety of shapes in conjunction with its excellent RF data.

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