F-81 / F81 BARREL CONNECTORS / FITTINGS. F-81 RG6 Couple Adapter Connectors for SD HD 4K 8K. (15) Coax Blue Centered 3GHz Female Cable Barrels. 15 NEW 3Ghz Blue Interior Coax Connectors aka Barrels. The F-81 type connector is available in three styles to accommodate outdoor splicing, wall plates and demarc applications.