Connecting a balanced antenna like a ZS6BKW to a coax feedline can cause serious problems be means of a high noise levell, for the cabinet of the transceiver is connected to the antenna.
This means that all nearby QRM is transported to the trx by the coax schield.
A galvanic isolator helps reducing this noise. Also unbalanced vertical antennas can profit from this device.
11 turns of side by side 1mm transformer wire on a FT140-61 or 4C65 toroid form the coupler.