Q signals used in amateur radio

Location. As a question, QTH followed by a question mark asks where you are; as a statement it introduces your own location. It is one of the handful of Q signals that is genuinely useful on voice as well as CW, because it is shorter than the alternatives and universally understood including by operators whose first language is not English.

QRM is interference from other transmissions, meaning other stations, splatter or electrical noise from equipment. QRN is natural atmospheric noise, principally static from distant lightning. They call for different responses: QRM is often solved by moving frequency, while QRN affects the whole band and calls for a different band or a noise blanker.

A few of them, sparingly. QTH, QSL, QSY, QRZ, QRM and QRN are all understood universally and are shorter than the plain language equivalents. Almost everything else is quicker to say in plain English on voice, and a transmission built mostly out of Q signals reads as an affectation rather than as efficiency.

Because 73 is a number from the old landline telegraph code meaning best regards, and it is already complete. Adding an s pluralises something that is not a countable noun. It is a small thing and experienced operators do notice it. The same applies to 88, which traditionally goes to a spouse rather than to everybody.

It asks whether the frequency is in use. Before calling CQ on a frequency that sounds empty, send QRL with a question mark and listen for a few seconds, because a station working somebody you cannot hear will answer. On voice the equivalent is asking whether the frequency is in use, which takes the same time and does the same job.