And that six minutes total could be in 20 - second bursts. North America 2-way in this region is normally spaced in 20KHz steps. Many systems in Latin America are offset from this 20KHz step arrangement by customized amounts, e.g. 40.345MHz. Pagers in the U.S. with their mixed voice and digital tone signalling, run high power and high activity. These pagers constitute important first-level propagation indicators.
Some frequencies have several dozen different transmitters spread from Maine to California.
In both cases, there are a few additional channels above and below these limits. Frequency loading of any channel varies, as does transmitter power. The best channels will vary depending on the listeners location and on the type of opening. Even if you do verify the call letters, a current, complete listing of such transmitters is lacking. Instead you can derive propagation information from the simple presence or absence of activity.