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This work presents two high-gain, multi-director Yagi-Uda antennas for use within the 24.5 GHz ISM band, realized through a multilayer, purely additive inkjet printing fabrication process on a flexible substrate. Multilayer material deposition is used to realize these 3D antenna structures, including a fully-printed 120 µm thick dielectric substrate for microstrip to slotline feeding conversion. The antennas are fabricated, measured, and compared to simulated results showing good agreement and highlighting the reliable predictability of the printing process. An end-fire realized gain of 8 dBi is achieved within the 24.5 GHz ISM band, presenting the highest gain inkjetprinted antenna at this end of the mm-wave regime. The results of this work further demonstrate the feasibility of utilizing inkjet printing for low-cost, vertically-integrated antenna structures for on-chip and on-package integration throughout the emerging field of high-frequency wireless electronics.

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| Inkjet printing on flexible paper and additive manufacturing technologies (AMT) are introduced for the sustainable ultra-low-cost fabrication of flexible radio frequency (RF)/microwave electronics and sensors. This paper covers examples of state-of-the-art integrated wireless sensor modules on paper or flexible polymers and shows numerous inkjetprinted passives, sensors, origami, and microfluidics topologies. It also demonstrates additively manufactured antennas that could potentially set the foundation for the truly convergent wireless sensor ad-hoc networks of the future with enhanced cognitive intelligence and ''zero-power'' operability through ambient energy harvesting and wireless power transfer. The paper also discusses the major challenges for the realization of inkjet-printed/3-D printed high-complexity flexible modules as well as future directions in the area of environmentally-friendly ''Green'') RF electronics and ''Smart-House'' conformal sensors.

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