Just passed my Tech exam and starting to listen on HF. I keep hearing operators use three-letter codes like QTH, QRL, and QSY. What are the most essential Q-codes I should learn to understand conversations? I know they're shorthand from CW days, but which ones are still commonly used in SSB and FM? Also, when someone says QRL? on a frequency, what's the proper response if I'm already using it?

Start with the big five: QTH (location), QRL (frequency busy?), QSY (change frequency), QRZ (who's calling?), and QRT (stop transmitting). For QRL? - if you're using the frequency, just reply "QRL" or simply "C" for "correct, it's busy." These will cover 90% of what you'll hear on the bands!

Don't forget QRN (static/atmospheric noise) and QRM (man-made interference) - you'll hear those constantly during band conditions discussions. Q codes make it easier to send common messages, like how people recognize ETA as shorthand for "estimated time of arrival."

Amateur radio operators use a subset of the full international Q-code extensively, and in many countries learning this code is necessary to obtain a ham radio license. I'd suggest making flashcards with the common ones - it becomes second nature pretty quickly once you start using them on the air.