The ITU standards were upgraded to grant the entire “K” prefix to the US, in addition to the existing “W” and “N” prefixes. (Remember that Germany had the “KAA” to “KCZ” prefixes issued previously). The Navy was reserved the “N” prefix, while starting in 1928 the “W” and “K” prefixes were authorized for civilian services, such as amateur radio. As new amateur licenses were issued, and old ones were renewed, the “W” prefix was simply added to the existing call sign. For example, the call sign of 6UO, (or the unofficial nu6UO), became W6UO. The “K” prefix at that time was reserved for US possessions, such as Alaska, Hawaii, and other islands. (Note that “A” block letters were unassigned until 1947, when the US received the “AA” through “AL” prefix blocks). The US amateur radio call sign had finally taken its modern shape we all know today.
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