CQ is ham-speak for “calling any station”. For all modes— voice, Morse, visual, or digital, it signifies a radio amateur looking to make a contact.
The basic, general CQ means looking for any station to respond.
It can be refined (a directed CQ) to be more specific, often a location. Common examples are out-of-country long distance (CQ DX), a particular prefix, country or state (CQ VT), a contest or event (CQ SOTA), or perhaps a CW specialty such as using a straight key (CQ SKCC).
When you hear a station calling CQ, it’s your chance to work them. Respond by sending their call sign and then your own, much like on a VHF/UHF repeater.
If a ham doesn’t hear any activity or anyone calling CQ it could mean the band is dead… or it could be a good opportunity to go fishing for a contact by calling CQ. New hams should get some experience with every new mode before calling CQ themselves.
When calling CQ , make sure that you are permitted to operate on the frequency you have chosen (and keep away from band edges). Also make sure that the frequency is not in use. Do not assume that if you hear quiet at a valid frequency that it is free; it could be that another ham is listening to a station you can’t hear.
Always check if a frequency is in use before calling CQ. The proper way to do this is to simply ask by voice with your ID (phone), or send the Morse prosign QRL? with your call (CW mode).
If someone else is working that frequency, they will let you know. Move to another frequency (QSY) should you receive a response to QRL.
If nobody replies to your QRL, go ahead with your CQ. There is no official CQ protocol but there are many suggestions out there. Some short, some long, and some in between. Also consider differences between modes. Without stirring up a passionate discussion, do what seems best for you after reading up on these CQ guidelines (some have helpful comments):
Always give your call sign phonetically at least once and it may help to give your general location. Also allow some time between CQs for a response. One general guideline (perhaps a starting point) is a 3x3x3 format where you call CQ three times, your call sign three times, repeated three times.
One last thought before some trivia— it is generally not appropriate to call CQ on a FM repeater. To initiate a random repeater contact, simply transmit your call sign or say “[call sign] monitoring”. If someone is listening and willing to chat, they will respond. If no response, you can speak your call again more specifically by asking for a QSO or demo or radio check. If nobody replies to that, there may not be anyone on that repeater frequency.
The root of the term CQ is often misreported, or has been Anglicized. When pronounced in French the letters CQ sound like the word sécurité (Secu = CQ) , which can mean “safety” or “pay attention”. Many early radio terms derive from French and they remain with us. In English it helps to think that it means “seek you”, but that is not the actual origin.