  In this paper, we propose a closely spaced multi-antenna system with
\textit{passive} self-interference cancellation (\textit{p}-SIC) of $\approx
90$ dB between the transmitter and receiver antenna for full-duplex
application. The \textit{p}-SIC is achieved by field confinement near
individual antennae using shorted metallic vias and the application of U-shaped
perturbation in the ground plane. The \textit{p}-SIC technique is initially
implemented in a 1-Tx and 1-Rx antenna system and explained using transmission
line-based theory. Further, it is extended to 1-Tx and 2-Rx configurations.
Here the proposed full-duplex antenna system is designed at $5.9$ GHz
($5.855-5.925$ GHz, IEEE 802.11p / WAVE technology) intelligent transportation
system (ITS) application band using a microstrip patch configuration. The
individual antenna exhibits an impedance bandwidth of $93$ MHz ($5.850-5.944$
GHz), $5.63$ dBi gain at $5.9$ GHz operating frequency and X-pol level less
than $20$ dB in the broad side direction. The proposed FD configuration
exhibits $|S_{ij}|$ of less than $-50$ dB over the complete operating band and
$\approx -90$ dB is achieved at the operating frequency between the Tx and Rx.
Similarly, $|S_{ij}|$ of less the $-30$ dB is achieved between 2-Rx antennas
for a three-element FD configuration. The design procedure of the proposed FD
configuration is explained and verified using fabrication and measurement. An
experimental demonstration of the self-interference channel and its suppression
using the proposed \textit{p}-SIC technique is also provided. Further, to study
the diversity performance of the proposed multi-antenna configuration, the MIMO
performance metrics such as \textit{ECC} and \textit{CCL} are evaluated using
simulation and measurement.
