Duplex hotspot: who actually needs one

Stores carry two kinds of MMDVM boards: simplex ones (HS_Hat, ZUMspot) and duplex ones (Dual_Hat, MMDVM_HS Duplex). A duplex board costs more, runs hotter, and forces you to think about frequency offsets and antennas. Is it worth paying extra? The answer depends on what you want the hotspot for.

Inside: one chip versus two

The key hardware on both boards is the ADF7021 radio chip made by Analog Devices. It's an 80–650 MHz transceiver with FSK/GFSK modulation; it's what handles both receiving and transmitting the DMR signal.

• Simplex board — a single ADF7021 chip. Reception and transmission alternate on one and the same frequency. While the hotspot is transmitting, it cannot receive a new signal.
• Duplex board — two independent ADF7021 chips, one for RX and one for TX. Reception and transmission happen simultaneously on different frequencies — exactly like a real repeater.

Timeslots: the fundamental difference

A DMR channel is split into two timeslots (TS1 and TS2) — two time windows within a single radio channel that are completely independent at the protocol level. This is the ETSI DMR Tier II standard.

On a simplex hotspot, the convention is to work only with TS2. The reason is simple: while the board is "talking" on the air (TX), it is blind to incoming packets. Trying to hold two conversations at once — one on TS1, the other on TS2 — would immediately cause collisions. The software (Pi-Star/WPSD/RadioStar) resolves this conflict by passing only a single slot.

On a duplex board, RX and TX are separated in hardware. Both timeslots work simultaneously and independently: TS1 can carry a conversation with one group while TS2 carries another, and neither interferes with the other.

Frequency offset and antenna isolation

The main challenge of duplex operation is that the TX must not "drown out" its own RX receiver, which is working during those very same milliseconds.

On real repeaters this is solved with duplexer filters — a pair of cavity filters costing from several thousand rubles upward. On a low-power hotspot (transmit power is on the order of 20 mW) the requirements are lower, but you still need a sufficient offset:

• 70 cm band (430–440 MHz): recommended TX/RX offset — 5 MHz or more. For example, RX 433.500 MHz / TX 438.500 MHz.
• 2 m band (144–146 MHz): recommended offset — 600 kHz or more.
• An alternative is two separate antennas placed as far apart from each other as possible.

Configuration in Pi-Star / WPSD

After selecting the duplex board in the "Modem Type" menu, you need to change the controller mode. A typical path in Pi-Star:

Configuration → General Configuration
  Controller Software: MMDVMHost
  Controller Mode:     Duplex Repeater
  (or Half-Duplex on some firmware versions)

Frequencies are set separately:

DMR Configuration
  RX Frequency: 433.500  (the radio transmits here)
  TX Frequency: 438.500  (the radio listens here)

A diagnostic sign of a wrong offset or swapped frequencies is the TS Failed error in the MMDVMHost logs.

Heat and power draw

Two active ADF7021 radio chips instead of one draw noticeably more power. In practice:

• Simplex hotspot: the board runs warm, passive cooling is usually enough.
• Duplex hotspot: under continuous load (two active slots) the board gets noticeably hotter; a small heatsink and/or a ventilated case is recommended.
• Power: it's better to use a 5 V / 2.5 A power supply (not 1 A) — especially if a display is connected to the Raspberry Pi.

When duplex is justified, and when it isn't

A summary to help you decide:

• Personal DMRhub hotspot, one radio, one operator — simplex is entirely sufficient. TS2, one frequency, minimum hassle.
• Hotspot for several operators in a room/office — duplex gives two independent conversations at once.
• Mini-repeater for a cottage or garage — duplex, if you need a standard repeater-style frequency offset and the ability to use either of two radios without a "busy" condition.
• Mobile/field hotspot on a battery — simplex: less power draw, less heat.
• No callsign / license — neither one should be used on the air.