The radio frequency spectrum is organized in bands (ranges of frequencies), with each band dedicated to one or more radio services (purposes and licensing systems). For example, U.S. broadcast radio is in two bands: the AM broadcast band is from 535 kHz to 1.705 MHz and the FM broadcast band is from 88 MHz to 108 MHz. (Here's a nice chart.) In many radio services, each frequency is assigned to a specific station in a specific area. For example, KOA in Denver is the only station allowed to transmit on 850 kHz with significant power at night in the continental US, and in the daytime in most of the western US.

The amateur radio service (ham radio) doesn't assign frequencies to specific stations. Amateur bands are open to anyone with an appropriate license, and it's up to amateur operators to avoid operating on a frequency that's already in use. This is normally fairly straightforward: listen first, then ask if anyone's using the frequency, then you can call CQ (ask people to call you). High frequency radio waves have a limited range though, and also a short-range "skip zone" where they can't be heard. So sometimes two people are calling CQ on the same frequency, but can't hear each other. I occasionally run into this situation with single sideband: one station in Florida and one station in Georgia might both be seeking contacts. If their timing is such that I can make out which is which by the sound of their voice, I can sometimes work both stations and tell them that another station is on the same frequency.

I've been practicing Morse code lately, and while some operators have a distinct "fist" (keying rhythm), often the only way to tell the dits and dahs of two transmissions apart is by the signal strength, if that. Today in the weekly K1USN SST slow speed contest I was listening to several rounds until I worked out the operator's callsign before calling them. An exchange is information given by the two parties in a contact. If K1USN is calling CQ and W1AW contacts them, the full sequence would be something like

CQ SST DE K1USN
W1AW
W1AW GA WATSON MA
TU WATSON HIRAM CT
TU HIRAM ES 73

with DE short for from, ES for and, GA for good afternoon, TU for thank you, MA/CT are state abbreviations, and 73 stands for kind regards, end of conversation. After a few passes, I'd written down W6RIF, called him, and got his exchange as WARREN IL (Warren in Illinois). I said TU WARREN TREVOR CO and moved on. I typed up my log file at home and ran it through a script I wrote to double check callsigns and states against the FCC database. I was surprised to discover that W6RIF is named Reed and lives in Virginia; neither the names nor the states sound similar in Morse code. I was pretty sure I'd copied the callsign correctly, and I relied on my phone to pick up the name. I searched QRZ for several variants with wildcards in various places, none of which turned up a more promising operator. I tried searching QRZ for just warren but in a hobby dominated by old white guys, there are a few thousand. I recalled finding a text file of SST operators and their exchanges, only one of whom is Warren from Illinois: KC9IL. I could confuse IF for IL (L and F both have three dits and a dah, with the dah one position different), but it's implausible that I misheard KC9 as W6R; none of those letters sound like the other. I had the insight to check the Reverse Beacon Network where people run software to automatically spot (announce that they heard) stations calling CQ in CW (Morse code) or digital modes. I looked up both callsigns, and saw they were both calling CQ on the same frequency in the same time range. It's possible that both of them responded to me at the same time, but I only picked up Warren's exchange. Maybe I ended up in both of their logs. I'm surprised they didn't notice each other on the same frequency: Virginia Beach and Chicago are far apart to be well out of the skip zone on 20 meters, but close enough to have a clear signal.

High frequency and medium frequency amateur radio is a curious hobby. In an era where you can place a phone call or send a short message to almost anyone on the planet for cheap, hams have to concentrate to pull out callsigns, names, and other details in the spaces between simultaneous transmissions, over atmospheric noise and static from thunderstorms, and signals fading in and out. Before I got my General class license, I was curious why someone would do this. The answer: it's fun in part because it's hard to communicate. It's a bit like a game of chance, strongly influenced by skill and appropriate use of technology.