Simplex means both stations transmit and receive on the same frequency, taking turns, with no repeater in between. It is the simplest way to use a radio: no infrastructure, no tones, no offsets. It is also how radios keep working when a repeater, or the power to it, fails.
Same frequency both ways = simplex. 146.520 MHz is the 2 m national FM simplex calling frequency.
How far will it reach?
VHF and UHF travel in straight lines, so range is set mainly by how high each antenna is. Each antenna can "see" about 1.41 × √height (in feet) miles to its radio horizon, and the two add. A handheld at head height sees about 3 miles; an antenna 50 feet up sees about 10.
Each antenna sees about 1.41 × √height (feet) miles. Add the two. Hills, buildings and trees only shorten it. Masts not to scale.
A sees3.5 mi
B sees10.0 mi
That is a best case: hills, buildings and dense trees shorten it, and a good antenna on a high point often beats a high-power radio at street level. Moving a few feet, or standing at a window, can make the difference.
Using it
Calling frequencies are for calling. Make contact on 146.520 MHz, then move to a clear simplex channel (the Band plans lists them) so the calling frequency stays free.
Listen first, then ask if the frequency is in use. Nobody owns a frequency.
To call: "CQ, this is W1ABC, listening on 146.52." Use phonetics for your call sign if conditions are poor.
Identify at least every 10 minutes and at the end of a contact.
Use a vertical antenna, matching the near-universal vertical polarization on FM.
Why bother?
Linked: one repeater hears you, all of them transmit. Simplex: within range, leave the repeater alone.
It keeps repeaters free. If you can talk direct, don't tie up a machine that others in a wider area need.
It still works when infrastructure doesn't. Emergency groups rely on simplex for that reason, and often assign simplex channels in advance.
It teaches you your radio's real range, which is worth knowing before you need it.