A note on Amateur Call Signs          Richard Rogers, VK7RO

               Following World War I, when experimental radio 
       licenses were issued in Australia and other countries the 
       call signs allocated were a single numeral followed by two 
       letters, for example 7AA.   This soon was found inadequate, 
       as the introduction of valve transmitters and receivers 
       soon led to communication between countries and consequent 
       confusion.   Calls such as "CQ Aussie de 3AA NZ" had to be 
       used.
               An idea introduced from USA was that of using 
       "international intermediates". The idea was that other 
       letters should replace the letters "de" used between call 
       signs, for example "3JU aa 7AA" when two Australian 
       stations were in contact or "3AA 3AA 3AA az 3AA 3AA 3AA" 
       when a New Zealander called an Australian.
       (see Wireless Weekly 28/8/25).

       The first actual example of this in the logbook of Trevor 
       Watkins, 7AA, was on 19th January 1924:
               "1150pm  4aa nz de 2zg
                1155pm  2zg de 4aa
                        3bd az 4aa
                        4aa de 3bd
                        4aa nz a 3bd
                        3bd az 4aa"
       Note that the New Zealand station, 4AA, is using the 
       intermediates but that the Australian, 3BD, has used "a" as 
       a prefix to his call.  The idea of using prefix letters to 
       identify different countries appears to have come from UK. 
       (see The World at Their Fingertips p100)

       Some other examples from the log book are:
            2nd March 1924
                    "1aa za 3bd"
            3rd August 1924
                    "cq a 3ot"
       This usage for international communication continues until 
       the book ends in July 1924.

       The intermediates were not a popular idea as the usage "Z3AA 
       de A3AA" became common, presumably because the repeated 
       prefix letter was less easily obliterated by static.
               Single letter prefixes soon were found inadequate 
       as inter-continental communication increased and additional 
       letters were proposed (see Listener In 29/1/27) .   These 
       were used in the form of a prefix "OA7CH" for a Tasmanian 
       or "NU6AKW" for a North American station in the United 
       States.   The O for Oceania and N for North America remind 
       me of the current packet radio addresses.
               The prefix VK for Australia was introduced on 31st 
       December, 1928, when new regulations were introduced which 
       also banned the playing of gramophone records during the 
       hours when normal broadcasting stations were operating.