In answer to your initial question, did I know about simplex and duplex channels, I would say, "yes" I did know about it for about 10 minutes then flushed it from my brain as useless knowledge. In Canada
is required to run a VHF radio. I just took the course about a year ago, so I grabbed my course book - Maritime Radio Course, Canadian Power & Sail Squadrons, 1st edition 2004 - and tell you what it says. The reason I flushed the info out of my brain was that my Standard Horizon Quest is simplex and the majority of VHF radios are simplex so I decided it was a waste of time to remember it: (I now quote the manual)
"2.7.2 Channel types and designations
A marine VHF radio channel may be a simplex channel or a duplex channel. A simplex channel is a channel on which a radio transmits and receives on the same frequency. If two stations attempt to transmit simultaneously, the frequency will be jammed for all users.
Simplex channels may be used for both ship-to-ship and ship-to-shore communications subject to the restrictions identified by the regulatory body governing channel usage in the area of operation.
Some channels are designated for duplex operation. A duplex channel is a channel on which a radio transmits on one frequency but receives on another frequency. For example Channel 24 (Canadian setting) is a duplex channel. When tuned to Channel 24, a vessel's marine VHF transmits on a frequency of 157.200 MHz, but it receives on a frequency of 16.800 MHz. It is convenient to think of the receive frquency as being offset or shifted 4.6 MHz above the transmit frequency on any duplx channel.
Duplex channels may only be used for ship-to-shore communications with authorized coast stations whose transmit and receive frequencies will be opposite to those of the vessel. On a duplex channel, an authorized coast station will recived on a vessel's transmit frequency, and transmit on the vessel's receive frequency. It should be readily apparent that duplex channels cannot be used for ship-to-ship communications.
Why duplex channels? The marine VHF radios typically used by recreational boaters are only capable of "semi-duplex" opration. They are able to transmit on one frequency and receive on another, but they are "incapable of doing both at the same time."
A channel containing the letter "A" as a suffix (sometimes called an Alpha channel) is the simplex counterpart of the duplex channel without the "A" suffix. For example, 22A is the simplex counterpart of duplex Channel 22. The duplex Channel 22 and simplex channel 22A have the same transmit frequency, but Channel 22 has a shifted receive frequency while 22A does not.
Simplex "A" channels are generally only used in Canada
and the United States, and their used is normally not recognized or permitted outside of North America.
Unique to Canada channel group are four channel numbers followed by the letter "B" on which marine VHF radios do not have a transmit capability. These channels are used by CCG for "continuous marine broadcasts."