Inkjet printing, a solution-based patterning technology with high efficiency, shows enormous potential in the fabrication of antennas designed for flexible and wearable wireless communication systems. Owing to its high conductivity, low price, and compatibility with inkjet printing, copper particle-free printable ink is considered to be a promising candidate material for the conductive layer of the antennas. Currently, only a few developed copper particle-free inks are appropriate for inkjet printing technology, and straight-chain diamines are rarely considered in their synthesis process. Furthermore, little attention has been paid to the use of these inks in wireless electronic devices. Therefore, it will be interesting to carry out the research in this area. In this work, a copper ink with improved performance was formulated by utilizing 1,3-diaminopropane (DAP) as a complexing agent, octylamine (OA) as a morphology modifier, and copper formate (Cuf) as a copper precursor, which was stable, printable and capable of producing copper conductive films with dense microstructures and lower resistivity at 170 °C. The benefits of utilizing mixed amines in ink recipe were revealed, which not only improves the wetting ability of the solo Cuf–OA ink during the sintering process, but also contributes to the formation of a dense film structure composed of large particles filled with small particles. The copper ink was successfully used to fabricate a pre-designed flexible ultra-broadband (UWB) antenna via inkjet printing, which resulted in signal radiation in the targeted UWB frequency band, demonstrating the ink's feasibility for usage in wireless electronic devices.