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For the airport in Mexico, see Querétaro International Airport.
The Q code is a standardized collection of three-letter message encodings, all starting with the letter "Q", initially developed for commercial radiotelegraph communication, and later adopted by other radio services, especially amateur radio. Although Q codes were created when radio used Morse code exclusively, they continued to be employed after the introduction of voice transmissions.
To avoid confusion, transmitter call signs have often been limited to restrict ones starting with "Q" or having an embedded three-letter Q sequence. The codes in the range QAA-QNZ are reserved for aeronautical use; QOA-QOZ for maritime use, and QRA-QUZ for all services.
Early developments
The original Q codes were created, circa 1909, by the British government as a "list of abbreviations...
prepared for the use of British ships and coast stations licensed by the Postmaster-General". The Q codes facilitated communication between maritime radio operators speaking different languages, so they were soon adopted internationally.
(This Convention, which met in London, was signed on July 5, 1912, and became effective July 1, 1913.)
The following table reviews a sample of the all-services Q codes adopted by the 1912 Convention:
First Twelve Q Codes Listed in the 1912 International Radiotelegraph Convention Regulations
Later usage
Over the years, modifications were made to the original Q codes to reflect changes in radio practice. In the original international list, QSW/QSX stood for "Shall I increase/decrease my spark frequency?", however, spark-gap transmitters were banned in the United States in the 1920s, rendering the original meaning of those Q codes obsolete.
Over a hundred Q codes were listed in the 'Post Office Handbook for Radio Operators' in the 1970s and cover subjects such as meteorology, radio direction finding, radio procedures, search & rescue and so on.
Some Q codes are also used in aviation, in particular QNH and QFE, referring to certain altimeter settings. These codes are used in radio conversations with air traffic control as unambiguous shorthand, where safety and efficiency are of vital importance.
A subset of Q codes is used by the Miami-Dade County, Florida local government for law enforcement and fire rescue communications, one of the few instances where Q codes are used in ground voice communication.
Many military and other organizations that use Morse code have adopted additional codes, including the Z code used by most European and NATO countries. The Z code adds commands and questions adapted for military radio transmissions.
For example, "ZBW 2" — change to backup frequency number 2 — or "ZNB abc" — my checksum is abc, what is yours?
Used in their formal "question/answer" sense, the meaning of a Q code varies depending on whether or not the individual Q code is sent as a question or an answer. For example, the message "QRP?" means "Shall I decrease transmitter power?", and a reply of "QRP" means "Yes, decrease your transmitter power".
This structured use of Q codes is fairly rare and now mainly limited to amateur radio and military morse code (CW) traffic networks.
Amateur radio
Selected Q codes were soon adopted by amateur radio operators. In December, 1915 the American Radio Relay League began publication of a magazine titled QST, named after the Q code for "General call to all stations".
In amateur radio, the Q codes were originally used in Morse code transmissions to shorten lengthy phrases and were followed by a Morse code question mark (..--..) if the phrase was a question.
Although considered bad operating practice, Q codes are commonly used in voice communications as shorthand nouns, verbs, and adjectives making up phrases. For example, an amateur radio operator will complain about QRM (man-made interference), or tell another operator that there is "QSB on the signal"; "to QSY" is to change your operating frequency.
Q Codes Commonly Used by Radio Amateurs
Some of the common usages vary somewhat from their formal, official sense.
QRL? is often sent to ask "Is this frequency in use (or busy)," though sending the American Morse letter "C" (didit dit) for "Clear?" is the traditional method used for doing this. In the question form, QNB?, is supposed to mean "How many buttons does your radio have?" A reply of the form QNB 45/15 means "45, and I know what 15 of them do." QRK is sometimes used to refer to the cost of something - "I would like an FT9000 but it is too much QRK".
QSK - "I can hear you during my transmission" - refers to a particular mode of Morse code operating in which the receiver is enabled during the spaces between the dots and dashes, which allows the receiving operator to interrupt transmissions.