1. A radio-frequency combiner-splitter, comprising: (a) a microwave cavity with an internal volume generally enclosed by a conductive skin; (b) a plurality of radio-frequency access ports placed at a set of locations and penetrating said conductive skin; and (c) a corresponding plurality of antennas each associated with individuals of said plurality of radio-frequency access ports and providing for near-field free-space intercommunication of radio signals within said internal volume amongst said radio-frequency access ports; wherein, individual members of the plurality of radio-frequency access ports are associated in groups, and any particular radio-frequency access port of the plurality of radio-frequency access ports presents a spatial signature to any grouping of the plurality of radio-frequency access ports that does not include the particular radio-frequency access port, and said spatial signatures occurring as a result of the particular way the plurality of radio-frequency access ports have been placed.