Telecommunication satellites accommodate more and more Ka-band payloads for Multimedia systems, which require moderate power per feed access. In parallel, there is BSS, or reverse BSS, to respond to the HDTV systems (“Direct to Home”), which requires large frequency bands procured by the Ka-Band. The broadcast coverages are generally global (European, CONUS, Asia…) which imply a lower antenna gain for the users. Then, the EIRP has to be compensated by higher power at the antenna feed access to comply the power link budget. Therefore, the development of a “Dual polarised high power Ka-Band feed” is of great interest for these space applications.
Thanks to large platforms (> 12 kW), the satellites are able to drastically increase the number of transponders. Considering the required EIRP for Ka-Band BSS’s, some developments are ongoing in Europe. For instance a TWTA is under qualification (170W). Once the RF channels are combined in the output multiplexer (under development), it is transmitted to the feed antenna. Therefore, it is desirable to develop a feed chain capable of handling both dual-polarisation and high RF power. Assuming 36MHz channel bandwidth in contiguous configuration and 12 channels, the total power handling with two polarisations would be 4,0 kW.
The first major concern is the thermal power, close to 200 to 400 Watts, to be dissipated in this antenna feed. Moreover, this thermal power is localised in a very small volume inside the antenna feed. The main challenge is to extract this power by using a standalone radiator to maintain the feed temperature range to [-150; +150°C].
The associated challenge for the RF design is to minimise the feed losses as far as possible to limit the thermal power to be dissipated maintaining sufficient margin in terms of multipaction. Passive Intermodulation is a design and manufacturing driver.
The so-called Ka-band dedicated to fixed civilian communication via satellite ([27.5; 30 GHz] for uplink, [17.7; 20.2 GHz] for downlink) is well known for many years to be the suited frequency band for deploying wideband communications addressing consumer, professional or even institutional market, in complement to the terrestrial broadband offer. Ka-band is usually associated to multimedia satellite missions but its wide band can benefit also to broadcast ones by enlarging the frequency band required by HDTV and even more by 3-D broadcast. The ITU has standardized the Ka frequency band to differentiate the multimedia and broadcast satellite missions. The American regions mainly differ from the others by using "reverse BSS" frequency band (17.3-17.8 GHz).
This study will consider a feed operating at receive from 24.75 to 25.25 GHz and transmit from 17.3 to 17.8 GHz, with a minimum input power of 4kW.
Several antenna feed concepts has to be traded for providing a “dual polarised high power Ka-Band feed”.
a) Individual components assembly architecture
The feed concept with individual components is the first type of architecture developed by TAS-F. Each function has been optimised and manufactured individually before to be integrated. The transmit RF path presents high losses due to long waveguide paths between the different components, but the main advantage for this type of architecture is to present large thermal radiative surfaces and allows to implant more easily a new type of control like OSR or LHP if necessary.
b) Multilayer components assembly architecture
The multilayer architecture is the concept the most used for feed mechanical design, which has been patented by TAS-F. It presents the advantage to integrate several functions (diplexing OMT, magic-tees...) inside a same block like a BFN and reduces insertion losses and a good PIM level. Moreover, the cost and the delay for the manufacturing is competitive thanks to a classical countersinking. This concept has been used for Ku/Ku+ applications for an on going program operating in linear polarization and with a high power (4.2kW at transmit inputs).
c) Single bloc components assembly architecture
The last proposed solution with cooper electro-deposit offers several advantages with an integration of components more important than the multilayer concept. Its losses level is approximately equivalent to silver for multilayer. The electroforming is often used for ground station applications.
This study is divided in four phases:
1/ Antenna feed architecture consolidation and Trade-off,
This phase is devoted to the antenna feed architecture trade-off, focused on the high power handling in Ka-band. The objective is to define the best compromise between RF, mechanical and thermal aspects. The RF design will have to reduce the losses keeping, as far as possible, sufficient margins on the Multipaction levels. The mechanical design will have to optimise the RF with the thermal constraints (material, interfaces, field of view, radiadive surface…). The thermal design will have to control the antenna feed temperature between ±150°C. The thermal control shall be standalone from the platform. The dedicated radiator could be either mechanically coupled to the feed antenna or using heat pipes.
2/ Antenna feed RF, mechanical and thermal design,
This phase is devoted to the detailed design (RF, Mechanical and thermal) of the dual polarised high power Ka-Band feed based on the architecture retained from the trade-off.
3/ Antenna feed manufacturing,
This phase is devoted to the manufacturing of the Engineering Model.
4/ Antenna feed test and analysis.
This phase is devoted to the tests of the Engineering Model.
The activity has been successfully completed. The antenna feed architecture trade-off has been performed. The Multilayer components assembly has been selected for its compactness and its number of components, which has been minimised as far as possible. In addition, its versatility w.r.t thermal coupling or discoupling with the platform appears as a main advantage. The preferred thermal concept is the V-shape radiator one in association using thermal advanced technologies. Moreover, for this concept, the RF chain is very simple and offers [S] parameters compliant with the study requirements. During the detailed design, a new optimization of the concept has been performed to improve in particular the multipactor margins and axial-ratio. The Elementary Model composed of the Horn and the RF chain has been measured. ALL the [S] parameters and the radiating patterns are compliant with the requirements. The Multipaction analysis gives enough margins w.r.t the ECSS. The test has been performed on a representative part of the feed limited to the more sensible area and one polarisation. A power level close to 6 kW in pulse mode has been applied the breadboard without any Multipaction event which corresponds to 12kW effective on the full Ka-band feed. This level corresponds to 4.7dB margin w.r.t the operational input power.