Simplex and Duplex Radio Explained

Two words appear in every radio quotation and are rarely explained to the person signing it. The difference between them determines what the system costs, what it covers, and whether it can grow.

In simplex, every radio transmits and receives on the same frequency. Press the button, everyone in range hears you directly. There is no infrastructure between you: the coverage of the system is exactly the coverage between two handsets.

That is a real limit and a real virtue. The limit is range — handset to handset, a few hundred metres inside a building, a few kilometres across open ground, and less if either party is in a bad spot. The virtue is that there is nothing to fail. No repeater, no antenna on a mast, no power supply on a roof, no site rent, and only one frequency to license.

Simplex suits operations that fit inside that range and are not planning to leave it: a restaurant, a small hotel, a single-floor workshop, a car park, a security team on one site, a construction project in its first weeks.

Duplex adds a repeater. The repeater listens on one frequency and simultaneously retransmits everything it hears on a second frequency. Handsets transmit on the first and listen on the second.

The gain is not really about the repeater's power. It is about the repeater's position. A handset transmitting from ground level between two buildings has a poor path to another handset in the same situation. Both, however, may have a clear path to an antenna on a roof or a mast. The repeater converts two bad paths into two good ones, and coverage grows by far more than an increase in handset power ever could.

This is the single most misunderstood point in radio purchasing. Buyers ask for more powerful handsets when what they need is one antenna placed higher.

The repeater works in both directions at once. The handsets do not. A user still holds the button to speak and releases it to listen, and only one person transmits at a time.

That is not a shortcoming carried over from old equipment. Team coordination works because everyone hears the same thing at the same time and only one voice occupies the channel. A telephone conversation is private and simultaneous; a radio conversation is shared and sequential. They are different tools, and the sequencing is why radio scales to a team while a phone call does not.

A repeater receiving and transmitting at once, on one antenna, has an obvious problem: its own transmitter is many orders of magnitude stronger than the signal it is trying to hear. The duplexer is the filter assembly that solves this, passing the transmit frequency to the antenna while keeping it out of the receiver.

Duplexers are physical, tuned, and unforgiving. A duplexer tuned for one frequency pair and then used with another will desensitise the receiver — the repeater will transmit perfectly and hear almost nothing. The symptom on site is distinctive and misleading: coverage that seems fine when standing near the site and collapses at distance, because only strong signals get through.

Anyone commissioning a repeater should ask for the duplexer tuning report alongside the transmit power measurement.

In DMR, two-slot TDMA puts two independent conversations through one repeater on one 12.5 kHz pair. Two teams can hold separate conversations at the same time through the same hardware and the same licence.

DMR also offers a direct mode that uses both timeslots between handsets without a repeater, and a single-frequency repeater mode that avoids needing a pair at all — with trade-offs covered in single versus dual frequency repeaters.

The practical consequence: a small operation that would once have needed two repeaters and two frequency pairs may now need one of each.

Simplex needs one frequency. Duplex needs a pair with a defined separation, and the pair has to be coordinated so it does not clash with anyone else's assignment in the same area.

In Türkiye these assignments are made by the BTK, per organisation and per site, and the coordination takes time. It is normal for the licence process to be the long pole in a repeater project while the hardware sits in a box. Buyers who treat licensing as paperwork to be done after installation discover this in the worst way. It belongs in the project plan from the first week — see radio licensing in Türkiye.

A duplex system is not a repeater; it is a site. The parts that decide whether it works:

• Antenna position and height — the reason the system exists.
• Feeder cable type and length — a long run of thin coax can consume most of what the repeater produces. Cable loss is specified per 100 metres at the operating frequency, and it is worth reading.
• Duplexer tuning to the assigned pair, documented.
• Lightning and earthing protection — a rooftop antenna is a lightning target, and a repeater is an expensive fuse.
• Power, with a defined behaviour on mains failure. A repeater that dies with the building takes the emergency communication with it.
• A talkaround channel programmed into every handset, so a repeater failure degrades the system instead of ending it.

Simplex if the operation fits inside handset range, has no growth plan, and can live with everyone hearing everything.

Duplex if the site is larger than handset range, spans floors or terrain, needs separate talkgroups, or is expected to grow. The threshold in practice is lower than people assume: a two-building site or a four-storey hotel is usually already past it.

And a point that costs projects money: retrofitting a repeater later is not simply adding a box. It changes the licence, the channel plan and every handset's programming. Deciding correctly at the start is cheaper than deciding twice.