An antenna installation comprises a plurality of antenna assemblies (10) mounted in respective enclosures (12). The enclosures are interconnected by flow connections (14) allowing passage of dry air supplied from a dry air panel (16). Having passed through all the enclosures, the dry air exhausts through a dry air return (18). The dew point of the air in the dry air return is detected by a dew point sensor (22). An air flow sensor (24) is disposed downstream of the dew point sensor and signals alarm if the air flow falls below a preset threshold.