The antenna "electrical lengths" are now electronically ( digitally ) adjusted, rather than a physical meshing/demeshing within the coupler unit.
The couplers were generally pressurised to prevent arcing and corrosion which often led to apparent failures in flight which were not apparent on the ground.
As a generalisation, the coupler was the most likely unit within an HF system to fail, moreso than the TX/RX or physical antenna.
You may benefit from reading Aircraft Radio Systems by James Powell ISBN 9780891003564
SF