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Simplex means communicating directly from one radio to another on the same frequency, without using a repeater. All HF communication is simplex (or split-frequency simplex), and on VHF/UHF it is the simplest and most fundamental form of communication — no infrastructure required, just two radios on the same frequency.
In simplex operation, both stations transmit and receive on the same frequency. When you key up, you are talking directly to the other station with no relay in between. This means both stations need to be within range of each other, which on VHF/UHF typically depends on antenna height, power, and terrain.
This stands in contrast to repeater (or duplex) operation, where you transmit on one frequency and the repeater retransmits on another.
Most countries designate specific calling frequencies on VHF and UHF where operators can make initial contact before moving to another frequency to continue their conversation. These are not frequencies to have long QSOs on — they are meeting points.
| Band | Frequency | Region / Notes |
|---|---|---|
| 2 m FM | 146.520 MHz | North America national simplex calling |
| 2 m FM | 145.500 MHz | Europe, UK, Australia, and many other countries |
| 2 m SSB | 144.200 MHz | SSB calling frequency (North America) |
| 2 m SSB | 144.300 MHz | SSB calling frequency (Europe, ITU Region 1) |
| 70 cm FM | 446.000 MHz | North America national simplex calling |
| 70 cm FM | 433.500 MHz | Europe, UK, Australia |
| 6 m FM | 52.525 MHz | North America national simplex calling |
| 6 m SSB | 50.125 MHz | North America SSB calling |
Always check the band plan and calling frequency conventions for your specific country, as these vary.
The proper procedure is:
This keeps the calling frequency clear for others to use. Lengthy conversations on the calling frequency are poor practice, as they block other operators from making initial contact.
On VHF and UHF, simplex range is primarily determined by:
Antenna height — This is the single biggest factor. Radio signals at VHF/UHF travel essentially in straight lines, so the higher your antenna, the farther you can communicate. An antenna on a hilltop can reach far more stations than one at ground level in a valley.
Power — More power extends range, but with diminishing returns. Going from 5 watts to 50 watts makes a noticeable difference; going from 50 to 500 watts makes much less difference compared to improving your antenna position.
Terrain — Hills, mountains, and buildings block VHF/UHF signals. Line of sight (or close to it) is generally required. Flat terrain or communication over water provides the best simplex range.
Antenna type — A good external antenna (a J-pole, a ground plane, or a directional antenna) significantly outperforms the rubber duck antenna that comes with a handheld radio.
As a rough guide, two stations using handheld radios at street level might only reach 1–3 km (0.5–2 miles) in an urban area. Two stations with mobile radios and good antennas on flat terrain might manage 30–50 km (20–30 miles). From elevated positions with external antennas, 80–160 km (50–100 miles) or more is achievable on 2 metres.
All standard HF operation is effectively simplex — both stations transmit and receive on the same frequency (or very close to it). The concept of simplex calling frequencies applies more to VHF/UHF, since on HF you simply find a clear frequency and call CQ, or tune across the band listening for stations to work.
The exception is split operation on HF, where a station (often a rare DX station) transmits on one frequency but listens on a different one. This is not true duplex in the repeater sense, but it does involve two frequencies. See DX Operating for more on split operating.
Some operators set up a simplex repeater (sometimes called a parrot or store-and-forward device). This device records a transmission and then replays it on the same frequency. It is not a true repeater (it does not operate simultaneously on two frequencies) but can extend range in some situations. Some commercial radios include a built-in simplex repeater function.