Antenna isolation is a measure of how much signal leaks from one antenna element to another, quoted in dB. Higher isolation means less leakage. Antenna isolation matters wherever several antennas share a housing or sit close together, as they do in MIMO antennas and combination antennas.

Antenna isolation keeps the elements of a MIMO antenna working as separate paths. MIMO carries more data by sending different streams over different paths, and elements that couple strongly pick up near-identical signals, which reduces the benefit. Designers raise isolation by spacing elements further apart and by giving each a different polarisation, which is why a cross-polarised antenna can fit two well-isolated elements in one compact housing.

Antenna isolation also protects one service from another in a combination antenna, a single housing with separate elements for cellular, Wi-Fi and GNSS. A cellular transmitter is far stronger than the satellite signals a GNSS element must receive, so poor isolation lets cellular transmissions interfere with GNSS reception, and with Wi-Fi.

Antenna datasheets quote isolation between each pair of ports, usually for each band. A related figure, the envelope correlation coefficient (ECC), measures how similar the signals on two MIMO ports are, from 0 (completely different) to 1 (identical), and lower is better. When installing separate antennas, spacing them apart helps isolation.