  Simultaneous transmit and receive (STAR) allows full-duplex operation of a
radio, which leads to doubled capacity for a given bandwidth. A circulator with
high-isolation between transmit and receive ports, and low-loss from the
antenna to receive port is typically required for achieving STAR. Conventional
circulators do not offer wideband performance. Although wideband circulators
have been proposed using parametric, switched delay-line/capacitor, and N-path
filter techniques using custom integrated circuits, these magnet-free devices
have non-linearity, noise, aliasing, and switching noise injection issues. In
this paper, a STAR front-end based on passive linear microwave circuit is
proposed. Here, a dummy antenna located inside a miniature RF-silent absorption
chamber allows circulator-free STAR using simple COTS components. The proposed
approach is highly-linear, free from noise, does not require switching or
parametric modulation circuits, and has virtually unlimited bandwidth only set
by the performance of COTS passive microwave components. The trade-off is
relatively large size of the miniature RF-shielded chamber, making this
suitable for base-station side applications. Preliminary results show the
measured performance of Tx/Rx isolation between 25-60 dB in the 1.0-3.0 GHz
range, and 50-60 dB for the 2.4-2.7 GHz range.
