Your own MMDVM hotspot: hardware, simplex vs duplex, and why we don't run Pi-Star

A hotspot is built from two parts: a Raspberry Pi mini-computer and an MMDVM radio modem. The market is full of dozens of options with similar names, and it's easy to buy the wrong thing. Let's go over what you actually need for DMR, how the modems differ, and why at DMRhub we gave up on manually configuring Pi-Star in favour of an image that configures itself.

Which Raspberry Pi to get

For GPIO modems (which mount directly onto the pins) the Zero form factor is more convenient. On the Pi 3/Zero the image disables Bluetooth automatically to free up the hardware UART /dev/ttyAMA0 for the modem.

The MMDVM modem: the key decision

The heart of the modem is the ADF7021 radio chip. The number of these chips is exactly what determines whether you have simplex or duplex.

MMDVM_HS_Hat — one ADF7021 (simplex)

The most common and cheapest option. Receive and transmit happen on a single frequency, taking turns — just like an ordinary radio. For a home hotspot this is enough: it works on timeslot TS2. This category also includes the Nano hotSpot USB "dongles" and most cheap boards.

MMDVM_HS_Dual_Hat — two ADF7021 (duplex)

Two independent radio sections: receive and transmit on different frequencies simultaneously, with both timeslots TS1+TS2 available. You need this if you want a "home repeater" with a frequency split. It's more expensive, runs hotter, and is usually overkill at home.

GPIO or USB?

• GPIO HAT (onto the pins, UART) — more compact and tidier, but it needs a free hardware UART (the image handles this itself).
• USB modem — easier to plug into any Pi/mini-PC, and the port is detected automatically.

Antenna, band and power

• Band. The modem and antenna must match on band (most often 70 cm / UHF). A few boards are dual-band.
• An antenna is mandatory. Even the tiny "rubber duck" that comes in the box is better than transmitting without an antenna (you can burn out the output stage). Want more range? Build a 1/4λ whip.
• Power. A good-quality 5 V / 2.5–3 A PSU and a proper data cable. Voltage sags = the undervoltage "lightning bolt" and unpredictable failures.
• OLED display (0.96″, optional) — shows status, who it's hearing, and the IP. Handy, but not required.

Pi-Star — and why we took a different path

The classic hotspot software is Pi-Star (and forks like WPSD). It's powerful, but it requires manual fuss: flash the image, open the web configurator, choose the modem type, enter frequencies, the Color Code, network passwords, your DMR ID, and connect to the right network. For a beginner, that's dozens of fields where mistakes are easy to make.

At DMRhub we reduced all of that to zero. On first boot, the RadioStar image detects the modem type itself, writes MMDVM.ini, obtains a DMR ID and links to the portal. From then on, frequencies, talkgroups and updates all come from your dashboard, over the air. No SSH, no hand-editing config files.