A novel remote antenna unit, intended for application in the downlink of an analog radio frequency over fiber system, is proposed, whose radiated power originates entirely from the optical signal supplied by a single multimode fiber. By operating the photodetector of the optical receiver at zero bias voltage, while omitting typical active components, such as transimpedance amplifiers, a fully passive unit, requiring no external power supply, is obtained. Instead, a careful co-design is performed to maximize the power transfer from photodetector to antenna within the frequency range of interest using an impedance matching network. A wideband cavity-backed slot antenna is implemented in an air-filled substrate-integrated-waveguide technology. The antenna feed plane serves as an integration platform for the optical receiver. The resulting downlink remote antenna unit is compact, cost-effective, energy-efficient, and extremely reliable. Therefore, it is an ideal building block for novel, highly specialized, ultrahigh density wireless communication systems, which require massive amounts of remote antenna units, deployed in attocells as small as . A prototype operating in a wide frequency bandwidth ranging from 3.30 to 3.70 GHz is constructed and validated. In free-space conditions, a broadside gain of 0.2 dBi, a front-to-back ratio of 8.8 dB and linear polarization with a cross-polarization level of 28.8 dB are measured at 3.50 GHz. Furthermore, a 3 dB gain bandwidth of 500 MHz is observed. Finally, the prototype is deployed in a unidirectional data link, achieving a symbol rate of 80 MBd over a distance of 20 cm.