Intra-vehicle wireless communication applications, e.g. the wireless sensor network, are in need of antennas with uniformly radiated power for signal transmitting and receiving with their randomly distributed neighboring targets. This demand can be met with power quasi-isotropic antennas with low angular gain variation (GV). In this paper, two types of practical implementation for conventional U-shaped lambda/4 resonators, which are supposed to have theoretically ideal (approaching zero) GV but have intrinsically low radiation resistance, have been proposed. One is a dual-layer U-shaped radiator. The extra added layer introduces a resonance that can be utilized for a good impedance matching with a commonly used 50 lambda feeding line. A two-element array model and the two-port network are used to show the working mechanism and design considerations. The other is a U-shaped RFID tag antenna, which can be considered as the miniaturized form of a U-shaped lambda/4 resonator. With a structure evolution starting from a simple triple element model, the tag can have a good conjugate impedance matching with the terminated chip. Compared to their respective counterparts, both proposed antennas have the lowest measured GV. Meanwhile, their attracting features such as extremely low profile, small lateral size, light weight, simple configuration and fabrication process, and the possibility of a conformal implementation make them excellent candidates for future intra-vehicle wireless communications.
