Electromagnetically Consistent Monostatic ISAC Beamforming: Mutual Coupling, Active Mismatch and Tx-Rx Leakage in Compact Dipole Arrays
Beamforming for integrated sensing and communication (ISAC) is usually designed on ideal array manifolds with a norm constraint on the transmit signal. In a compact monostatic transceiver this hides three antenna effects: distorted embedded patterns, beam-dependent active mismatch at the power amplifiers, and transmit-to-receive leakage that raises the radar noise floor. We build a multiport scattering model of a shared-aperture dipole array directly from method-of-moments simulations and use it to formulate a Cramer-Rao bound minimizing ISAC beamformer with multiuser SINR constraints, per-port active reflection constraints and a leakage budget, solved as a semidefinite program with a one-dimensional budget search. Full-wave evaluation shows that coupling-unaware designs miss their SINR targets by up to 6.2 dB and drive amplifiers to an equivalent VSWR above 3.3 at small spacings. For spacings from 0.3 to 0.5 wavelengths, the proposed design meets all targets with an equivalent VSWR below 2 and lowers the root Cramer-Rao bound by 32 to 53 percent for 80 dB of self-interference cancellation and by a factor of 3.4 to 5.6 for 70 dB. An independent finite-difference time-domain model of the final layout in openEMS confirms that the leakage null and the ranking of the designs are preserved.