Dave gave you a good explanation of the how's and why's of simplex/duplex, etc....and while he left out some details, I fear adding much more would just complicate things for you (and others here)....
But, as someone that built/owns/operates/maintains VHF repeaters myself, my 2m ham repeater is KA4WJA/R on 146.970/146.370mhz (and mine and Mario's 220 repeater is N4TSV/R on 224.1/222.5mhz and we maintains KA2MBE on 444.325/449.325mhz as well), I'd like to add that it isn't just the "complex equipment" that Dave mentioned, but a significant amount of actual engineering that goes into these systems...so much so that dozens and dozens books
have been written, and many classes
taught at universities, on just these subjects (duplex radio operations)....
Originally Posted by Auspicious
With a lot of complex equipment
(circulators, cavities, and filters) you can build a radio station that can simultaneously transmit on one frequency and receive on a second nearby frequency. You aren't going to see this gear
on a boat but it is fundamental to the operation of a repeater and to high-end shore stations.
Repeaters are automated coast stations that use circulators, cavities, and filters to allow them to continuously receive one frequency ('A') and transmit in near-real-time (there are some milliseconds of latency) everything heard on 'A' on a second frequency ('B').
No measureable latency at all in analog systems!!! (but those talking on modern digital cellular phones, understand the dreaded digital latency!!)
FYI, these are the exact same principles, procedures, engineering and equipment that go into ALL mobile phone
systems, ALL cellular/3G/4G/LTE/GSM, etc. since the 1940's....
Yes, that is correct, not a typo.....since the 1940's!!!
Although the exact frequencies, modes, etc. have changed....and recently we've done away with all consumer analog mobile phone
systems, there are still some commercial/business analog mobile radio/mobile phone systems still in operation, and many 1000's of ham radio "autopatch" interfaced vhf/uhf repeaters (including mine) operating 24/7/365, allowing users within range (typically 50 - 75 miles), to place non-business public phones calls (and in some systems, receive calls as well)....
Understand that ALL cellular/mobile phones, smart phones, etc. of ALL types, ALL brands, ALL systems, worldwide.... are all handheld duplex radios and work with a fixed base-station / repeater to interface with the Public Switched Telephone Network (PSTN), and/or connect to other cellular/mobile phones....
(yes, they are all handheld duplex radios...)
On HF, actually it's not more complex at all....it's just bigger and takes up acres of space!!
Originally Posted by Auspicious
On HF duplex is more complex on the shore side. Circulators, cavities, and filters aren't very practical due to size so remote
receive sites are used. AT&T High Seas radio had a huge infrastructure. To my knowledge Shipcom and other modern day successors use simplex.
Shipcom / WLO does use duplex frequencies and has "full-duplex" filtering and antenna
separation....(although not quite as sophisticated as the old. defunct AT&T systems, with dozens of antennas and multi-couplers at receive sites, located miles apart from their transmit sites...)
I cannot speak for Brunei Bay Radio, Monaco Radio, etc...but it's possible that Brunei uses "semi-duplex" (separate tx/rx freqs, but does not have the ability to receive at the same time they are transmitting)???
Bottom line here regarding maritime "duplex" channels, both HF and VHF, is that the rapid growth and expansion of "consumer electronics" (cellular phones, etc.), increase use of "text"/"data" communications
and decrease in Voice comms, decrease in costs/complexity of satellite
communications, etc. have all combined to render many/most of these "duplex" channels vacant / unused...
Yes, there are still some "public correspondence" Marine-VHF stations in Europe
, etc. and yes, there are the Sea-Tow's automated radio-check system, both on VHF......and yes on HF, there are still some users of these duplex channels (WLO, KLB, Monaco, Brunei, Aus/NZ, etc. as well as the USCG for their weather
broadcasts and working channels...)...
But, with dozens and dozens of unused HF duplex channels worldwide, (and many unused VHF duplex channels in most coastal locations), I see changes coming in the WARC....
I hope this helps a bit...
s/v Annie Laurie