[SOC] MCW - modulated continuous wave , A2 now A2A All you ever wanted to know and more.

[g4jht]

All you ever wanted to know and more about MCW - modulated continuous 
wave or A2, now A2A.

MCM stands for modulated continuous wave. was A2 now A2A.
A2 was required to be used on Marine MF telegraph transmitters and 
lifeboat sets.

How is it produced:
                    The  RF  carrier is modulated with an audio tone in 
the range 500 to 800 cycles at a depth of 80 to 100% and then keyed. 
Usually using a Class C PA choke modulated with self oscillating modulator.
Advantage of MCW is you can hear it on an ordinary (diode) envelope 
detector such as a crystal set without a BFO. The ITU state such a 
signal keyed at 35 WPM will not have a band width wider than 2.1 Kc/s 
(kHz). This was a development of ICW.

There is another type of MCW used for MF D/F beacons. Here the carrier 
runs continuously and only the audio tone is keyed with the beacon 
callsign. Unfortunately this also covered by the ITU definition for A2A, 
but fairly obviously when you tune one in.

ICW pre-dates MCW and was produced by interrupting the HT supply to the 
PA (again probably Class C) by means of a "tone wheel". The tone wheel 
had insulated segments and a brush, which chopped the supply to  
producing a square wave at an audio rate depending on the number of 
segments and the speed of the disk in revs per second. The disadvantages 
of this system were it required maintenance,  of the brush and segment 
hight like a commutator. the note produced was rather rough so difficult 
to listen to and the bandwidth would be wider than MCW, which replaced it.

Hope that helps 73,
                    Dave  G4JHT/EI0DB/VP8ART  2600 nautical miles East 
of the Brooklyn Bridge.

Friends all over the globe by skywave W/T; CW the one true art; in an 
over digiefied world.