🔍 Key Concepts
• VHF marine channel plans: difference between U.S. and International (INT) channel usage
• Meaning of simplex vs duplex operation on marine VHF channels
• What the "alpha" channels (e.g., 07A, 19A, 21A) represent in U.S. coastal waters
💭 Think About
• Ask yourself: When your VHF is set to USA mode, how do the "alpha" channels behave compared to International mode—are you transmitting and receiving on the same frequency or different ones?
• Think about why the U.S. created USA vs INT channel sets: was it mainly to gain more duplex capability, or to use certain frequencies differently (e.g., for port ops, non-commercial comms)?
• On a typical small-boat VHF in U.S. coastal waters, when you select an "alpha" channel for routine traffic, do you usually use it as a ship-to-ship simplex channel or as a duplex (ship-to-shore) channel?
✅ Before You Answer
• Verify the definitions: simplex = same frequency for transmit/receive; duplex = different frequencies for transmit/receive
• Check how common "alpha" channels (like 07A, 19A, 21A) are actually used in U.S. practice—ship-to-ship or ship-to-shore?
• Confirm whether the main purpose of the USA-INT switch is to add more duplex channels, or to adapt international assignments so they function as simplex in U.S. coastal waters