This dissertation proposes a novel air-filled substrate-integrated-waveguide (AFSIW) antenna technology for highly-efficient ultra-wideband next-generation wireless systems. First, AFSIW technology is leveraged to obtain highly-efficient antennas for highly-accurate impulse-radio ultra-wideband (IR-UWB) localization systems. By means of an novel full-wave/circuit IR-UWB co-optimization framework, a cavity-backed slot antenna in AFSIW technology is developed to provide minimal pulse distortion in channel 5 and 7 of the IEEE 802.15.4a standard. Additionally, a coupled PIFA antenna is proposed as a sector antenna in IR-UWB localization systems, exhibiting minimal orientation-specific, antenna-induced ranging bias. Second, the potential of AFSIW technology in (beyond) 5G wireless systems is assessed by scaling it towards mmWave and THz frequencies. Here, a hybrid on-chip antenna in AFSIW technology is developed at 28 GHz and demonstrates a high efficiency over a very large impedance bandwidth in a compact footprint, making it especially suited for 5G antenna arrays. Next, a polymer-enabled AFSIW technology for highly-efficient and broadband on-chip THz antenna systems is proposed at 300 GHz, demonstrating great potential for beyond 5G multi-antenna systems.