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Abst ractIn communication satellite transponders for mobile users, very powerful transmitters with multicarrier (linear) operation capability and high RF to DC efficiency are demanded. The travelling wave tube amplifier (TWTA) is the best candidate for high efficiency, but the linearity has to be improved by using a predistortion linearizer. The diode-bridge type linearizer followed by a limiter is the optimum approach in terms of linearization effectiveness close to saturation, stable operation in satellite environment, simple construction and high reliability. Designing and using such a linearizer with a high efficiency TWT especially developed by AEG, the achieved linearization is close to the calculated for the "ideal" case. Just the limiter by itself improves the multicarrier behaviour of the TWTA close to saturation. Considering all the experience obtained by the work to be described, a proposal for a multicarrier transmitter is made finally which copes with most of the demands of present and future mobile satellite communication systems. IntroductionIn present and future satellite systems with mobile earth stations (Mobile Satellite Systems = MSS) single channel per carrier (SCPC) transmission is the predominant mode of operation. This means that the satellite repeater equipment has to cope with multicarrier RF signals. As a consequence the transmit amplifier capabilities cannot be fully exploited because the nonlinear behaviour of the amplifier requires operation at a certain backoff below the saturation point. This in turn leads to a relatively low RF output power and a poor DC to RF efficiency. So, due to the mode of operation the primary power bus system and the thermal design of the satellite becomes complex with considerable drawbacks in terms of mass, volume and fuel. On the basis of the above mentioned background new ideas have come up for the design of modern MSS repeaters and especially for the transmit amplifiers. It turns out that most linear solid state amplifiers which had been used so far are less favourable than travelling wave tube amplifiers (TWTAs) provided that the tubes are equipped with self-radiating multistage collectors and with linearizing networks. In order to verify this approach ESA placed a contract with ANT Nachrichtentechnik the goal being to study the linearity and the efficiency and to design and build a space applicable linearized L-Band TWTA. In this paper the main results of this contract are reported. RequirementsThe basis for this work were requirements oriented on Inmarsat 1I demands: Centre f requency 1537,5 MHz Note 1: Frequency band -20 MHz EOL operation with RF output power 188 Watt (note 1) 125 equal level and Carrier/Intermod. ratio (C/1) 15 dB min (note 1) spaced carriers 15,5 dB min. (note 2) Note 2: Phase shift 40 deg. max. (note 3) Test value for 8 Overall gain 30 dB min. carriers Total DC to RF efficiency 30% min Note 3: Operational temperature range Single carrier and -TWT 0 to 70 deg. C 0 to Sat. input -linearizer, driver amp. and EPC 0 to 45 deg. C powe...

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