# Tunable RF Filters Based on Liquid Crystal for Space Applications
## 1 Introduction  
The benefit of tunable filters in communication systems also applies to satellite communications, and especially also to the payload on the satellite itself. Typically, such a payload contains numerous RF filters operating mostly in frequency ranges from the L-band (1-2 GHz) to the Ka-band (26-40 GHz) [1] and dealing with signal strengths from low-RF power levels (in the order of mW) up to power levels as high as 100 W (as needed to transfer the signal back from the satellite to an earth-based receiver). Depending on those exact needs, various filter technologies have developed over the last decades and are routinely manufactured and employed in satellite missions [2]. It is noted that although concepts for tunable filters in various technologies have been developed and presented, untunable filters are still state-of-the-art for satellite communication. Obstacles for the introduction of tunable filters are:  
1. The lifetime of a geostationary communications satellite is typically fifteen years, and a new technology will only be considered if sufficient confidence has been gained that it will remain functional over this period of time. 2. The application on a satellite is in a harsh environment with respect to, for example, mechanical robustness, thermal environment, radiation, or electromagnetic compatibility, which further strengthens the need for thorough design and testing phases beforehand. 3. In order to operate a tunable filter, some kind of control electronics is needed. This control electronics draws power from the satellite power supply and translates received telecommands into signals that set the tunable filter to the desired state. Although such kind of electronics isroutinely used for various applications on the satellite and its design therefore does not present fundamental hurdles, it considerably adds complexity to the switchable filter system in terms of reliability, power consumption, thermal design, and telemetry/telecommand transfer to and from an earth-based operator  
Filters based on liquid crystal technology can be operated by static electrical fields, and hence, they consume little power, which is advantageous for satellite applications. Another remarkable aspect is that RF losses are relatively low, especially for higher frequencies. Further, liquid crystal devices do not rely on the mechanical movement of parts, which is a natural advantage in satellite application. The application of liquid crystal technology to filters is based on pioneering work on high birefringence liquid crystals [3] and its use in mmW phase shifters [4; 5]. This article describes work that was done on a tunable filter operating at 20 GHz (Ka-band downlink), which is a frequency range that is widely used in satellite communications. The filter has a fixed bandwidth and can be tuned in center frequency. The application of a liquid crystal device on a satellite presents another challenge, which is the liquid crystal itself. Being a liquid, it must be absolutely assured that it always remains in its assigned container. If this container showed leakage and liquid crystal could penetrate to the outside, it would not only degrade the function of the tunable filter itself (which would still be a failure confined to a specific part of the communication systems), but it could even harm other units of the satellite itself, such as the solar panels. Therefore, much emphasis is devoted to the container. Work on the filter has been ongoing for many years, and design aspects have been published before [6]. Some relevant details are replicated here, in order to set a base for the description of new work in Section 2. The filter is based on the TM010 mode, which allows the favorable introduction of the liquid crystal container and of the biasing electrodes. Conventional tuning screws can be inserted as needed to overcome manufacturing tolerances and deficiencies in simulation. Further, this geometry allows the coupling between adjacent resonators as needed for a multi-pole filter.