RF systems based on modular architectures have the potential of better re-use of technology, decreasing development time, and decreasing life cycle cost. Moreover, modular architectures provide scalability, allowing low cost upgrades and adaptability to different platforms. To achieve maximum benefit from modularity and scalability, the challenge is to define the front-end concept in such a way that it supports multiple functions and is applicable in multiple environments and platforms. This paper describes concepts for such modular systems that may be used for RF functions in maritime, airborne and land based systems. The first part discusses general scalability properties of various RF functions, the second part discusses example system concepts in a 'joint' scenario (land/sea/air). The focus of this paper is on X-Ku band systems: considering essential RF functions like radar and communication, and the types of platforms in maritime, land-based and airborne operations, this frequency range provides the highest potential for application in all these domains and platforms. Conceptual designs for weapon location radar, SAR/GMTI, surveillance radar and tactical data link are presented.