Simplex
Simplex is direct, radio-to-radio communication on a single frequency, with no repeater in between. Each station transmits and receives on the same frequency, taking turns. It is the simplest form of operating, the way every HF contact works, and on VHF and UHF it is what you fall back on when a repeater is out of reach, out of service, or simply unnecessary.
Why use simplex
A repeater is a shared resource, and a long conversation ties it up for everyone in range. If two stations can hear each other directly, custom and courtesy say to move to simplex. Simplex also needs no infrastructure, which is why it matters in emergencies, at public events, on hikes and in convoys. And it's a test of your station: if you can work a friend across town simplex, your antenna is doing its job.
Simplex frequencies
On FM, simplex operation happens on agreed channels rather than anywhere in the band, so that you don't accidentally transmit on a repeater's input. In the US:
| Band | National calling frequency | Other common simplex channels |
|---|---|---|
| 2 m | 146.520 MHz | 146.550, 146.580, 147.420, 147.540 |
| 70 cm | 446.000 MHz | 446.050, 446.100, 445.950 |
| 6 m | 52.525 MHz | 52.540 |
| 1.25 m | None in the ARRL band plan | 223.400–223.520 MHz (FM simplex segment) |
The calling frequency is for making contact; once two stations find each other they should move to another simplex channel and leave it clear. Stations often monitor 146.520 while traveling, and it's the frequency to try when you arrive somewhere new.
On HF there is no such distinction. SSB, CW and digital contacts are all simplex by nature.
Range
Simplex range on VHF and UHF depends on antenna height and terrain more than on power. Two handhelds with stock antennas might manage a mile or two in a city and five or more across open ground; the same handhelds on two hilltops can talk fifty miles. A mobile radio with a roof-mounted antenna reliably works other mobiles 10 to 20 miles away.
Duplex and half-duplex
Repeater operation, where you transmit on one frequency and listen on another, is called duplex or, strictly, half-duplex, since you still can't do both at once. Satellite operating is full duplex: you listen to the downlink while you transmit on the uplink, which lets you hear your own signal coming back from space.