There is growing interest in the UK space sector for communications, imaging and earth observation. Key to this is sending and receiving electromagnetic waves. To enable higher communication rates and get greater accuracy in imaging often higher frequencies are used. This project will develop new structures using microfabrication techniques to develop novel antennas and polarizers for satellites and the earth segment over frequencies from 28 GHz up to 1 THz. This frequency range overlaps and extends the currently used frequencies. ANISAT will address these five technical challenges: 1) Designing anisotropic metamaterials; 2) Exploiting these properties to design novel antennas, polarizers and RF devices; 3) Developing novel methods of measuring these properties; 4) Microfabricating heterogeneous anisotropic structures; 5) Combining these elements into a series of demonstrators. The above five points are addressed in more detail below: i) When an electromagnetic wave moves through a material it is slowed down by the dielectric properties. If an artificial dielectric can be composed of small (compared to a wavelength) rectangular or elliptical inclusions, then this composite material will behave differently when the incident electromagnetic wave has different polarizations. This can be exploited to create circularly polarized antennas where the electric field traces a circle in time. This is an advantageous property for space communications. ii) Currently, dielectric measurements only consider the dielectric properties for one polarization and effectively assume the materials are isotropic. ANISAT will develop a novel measurement system using resonant metasurfaces that can measure the properties along all three axes. This will open a new degree of freedom for antenna and radiofrequency engineers. iii) These anisotropic artificial dielectrics will be used to design novel circularly polarized antennas. It is currently challenging to feed antennas to create circular polarization at frequencies above 50 GHz due to the small scale of the feed structure. High gain multi beam cavity antennas and polarizers will be designed at a range of frequencies up to 1 THz. iv) Initial anisotropic artificial dielectrics will be fabricated using 3D-printing. This provides a simple and readily exploitable fabrication process. However, the upper frequency range is limited to approximately 40 GHz by the size of the small-scale air/metal inclusions inside the composite. Above this frequency the inclusions approach the scale of a wavelength and they become resonant. To extend the frequency range, novel microfabrication processes in clean rooms will be developed and exploited. These include fully metallised SU8 photoresist polymers and/or silicon layers with a high dimensional accuracy of the scale of a few microns. v) The learning process will be multidisciplinary and iterative as each stage innovates further advances. The close geographical proximity of the two universities will be highly beneficial in this regard. The plan is to create laboratory demonstrators that can be showcased to industry. These provisionally include: a novel dielectric measurement system; a high gain circularly polarized antenna at Ka band (26 - 40 GHz); a circularly polarized Fabry-Perot antennas at frequencies up to 110 GHz; and linear to circular polarizers and beam splitters from 220 - 300 GHz and at a central frequency of 640 GHz.

As communications move towards higher frequencies for higher data rates, concrete structures and buildings will significantly reduce the electromagnetic signal strength compared to windows. The overarching vision of TRANSMETA is to create transparent intelligent reflecting metasurfaces that could be placed on the windows of buildings or vehicles, and which would intelligently reflect the incoming electromagnetic wave from a base station directly to the user (either inside or outside) to improve signal reception quality. Metasurfaces can also filter certain frequencies, change the polarisation, or reduce the reflections from radar. The challenges to achieving this are: 1. For transparent conductors there is a trade-off between optical transparency and electrical conductivity in terms of layer thickness and frequency response which needs to be quantified. There are also practical challenges in how to connect these materials electrically and physically to the conventional opaque electronics. TRANSMETA will address this by investigating two approaches for the conductors: i) metallic meshes on the sub-micron scale where the lines are too small for the human eye to see; ii), if the results are not as required, a complementary technique using indium tin oxide will also be investigated. To test their performance, transparent antennas and static metasurfaces, such as frequency selective surfaces, will be fabricated and measured. 2. Novel metasurfaces must be designed based on the material properties. TRANSMETA will address this by carrying out extensive studies using commercial electromagnetic software with input from the earlier measurements. The effect of the ground plane at the rear of the metasurface will be investigated and we will aim to maximise the optical transparency. As an alternative to the reflecting metasurfaces, transmitting surfaces will also be designed where no rear ground plane is required. 3. The practical challenges of fabricating these metasurfaces must be investigated. TRANSMETA will initially make static (non-intelligent) metasurfaces which can reflect the signal between two fixed positions, tested by blocking the direct signal in the anechoic chambers at Loughborough University. This will be applicable if there were known communication dead zones in buildings which will become increasingly common as we move towards higher frequencies. Of course, optical transparency is not always essential for these novel metasurfaces, but it increases the scope of applications. 4. To make the metasurface intelligent, reconfigurability must be integrated into the system. TRANSMETA will address this with two techniques: i) vanadium dioxide where the properties change from being an insulator to a conductor when a direct current is applied, ii) PIN diodes. There are challenges in integrating these techniques into the system while also maximising the transparency. The direct current bias lines can be made transparent, but their optimum position and orientation are critical to the overall performance. 5. A further challenge in achieving the intelligence is being able to sense where the transmitter and user are located in order to reflect the signal in the correct direction. TRANSMETA will develop a sensing system that uses the pilot signals from the base station and user and then applies signal processing to retrieve the directions. A field-programmable gate array (FPGA) will control the metasurface behaviour accordingly. Finally, all these elements will be integrated together to create metasurface demonstrators which will be tested in real-world environments with support from our 12 industrial Project Partners. The impact of successfully completing this project will be improved capability for beyond-5G communication systems. Utilising transparent conductors will enable these intelligent metasurfaces to be employed in vehicles and building windows.