. These are no longer widely used by the military and the surplus supply has mostly dried up. AB-621/G and AB-621A/G The big brother of the AB-577, this is a 6 " diameter, 50 foot mast... with significantly increased antenna load capacity. The mast is guyed in four directions. The more recent AB-621A/G is guyed in three directions and has an extension kit to 100...Antenna Masts Ontario Surplus, Inc. Home | FAQs | Contact | Site Map Antenna Masts AB-577/GRC A 5" diameter military mast originally used for point-to-point microwave applications...