The Quantar hybrid build, NAC and talkgroup, and a flat no on roaming in conventional use. Working notes, v1.1, 10 September 2026.
P25 is the public safety mode, and amateur P25 is built almost entirely out of surplus public safety hardware. It is the only one of the six where the usual amateur build is a hybrid: commercial RF, open source network. Its answer on roaming is a flat no, and P25's own technology interest group says so in print.
The same four questions are asked of every mode, in the same order, so the seven parts stay comparable:
MMDVMHost plus a board, gateway is P25Gateway from G4KLX's P25Clients, with a
parrot alongside. The reflector end is P25Reflector. Linking is by talkgroup
number, and every reflector you want to reach needs a line in P25Hosts.txt,
which is why a large P25 board is expensive in registrations rather than in code.
Overwhelmingly the Motorola Quantar, surplus commercial gear that turns up cheap. KPARC runs Quantars on 2m, 33cm and 70cm. Kenwood and Tait also appear.
A pattern different from the other five. P25 amateurs commonly run a hybrid, commercial hardware for the RF and open source software for the network. W9YB is a clean example: a Motorola Quantar UHF-R2 at 45 W into a Wacom duplexer, connected out through the MMDVM P25 reflector network, defaulting to TG 6395 with NAC $396. The Quantar never knows the internet exists.
P25 Phase 2 is trunked only, and MMDVM_HS firmware supports Phase 1 only, so amateur P25 could not be Phase 2 even if someone wanted it.
| Mode | Channel bandwidth | Modulation | Bit rate |
|---|---|---|---|
| P25 Phase 1 | 12.5 kHz | C4FM, a form of 4 level FM. CQPSK also permitted | 9600 bps, 4800 symbols per second |
| P25 Phase 2 | 12.5 kHz | H-CPM inbound, H-DQPSK outbound, two slot TDMA | 12 kbit/s, 6000 symbols per second, two voice paths |
Two things in that table that get printed wrong. P25 Phase 1's modulation is named C4FM, the same name Yaesu uses for its own mode, which catches out anyone who assumes the two labels cannot refer to the same thing. And Phase 2 is not simply "TDMA": it is H-CPM on the inbound and H-DQPSK on the outbound, two different modulations in the two directions.
Phase 2 divides the time rather than narrowing the RF. Wikipedia is blunt about it: voice "transmits with the full 12.5 kHz per frequency allocation, however it does so at a faster data rate of 12 kbit/s allowing two simultaneous voice transmissions." So Phase 2 sits in the same camp as DMR, and Phase 1, which amateurs actually run, sits in neither: full 12.5 kHz, one conversation.
Every mode has two different fields and they get confused constantly. An access field, which works like CTCSS and decides whether the repeater listens to you at all, and a routing field, which decides where your audio goes. P25 splits them the same way DMR does.
| Mode | Access field | Routing field | Range |
|---|---|---|---|
| P25 | NAC | Talkgroup | NAC 12 bit, TG 16 bit |
NAC is access and is the CTCSS analogue. Talkgroup is routing. The NAC never reaches the network at all, so the talkgroup is the only selector there is, and every room needs its own number in the host file. That is the practical difference from DMR, where the network sorts talkgroups out for you.
The talkgroup field is 16 bit, the same width as NXDN's destination ID and narrower than DMR's 24 bit talkgroup. The comparison table in part 7 puts all six side by side.
Roaming needs two halves: a network half, several sites linked so that being on any of them puts you in the same conversation, and a radio half, something the radio can measure plus logic to move by itself. P25 has neither. No beacon, no radio side voting.
This is not inference. PTIG's own white paper states it:
P25 conventional only supports the manual roaming method.
Automatic roaming in P25 needs a trunked system with a control channel to listen to and a radio that registers and affiliates with a site. A conventional repeater has none of that: no control channel, no registration, nothing for the radio to affiliate with. Since amateur P25 is conventional Phase 1 by necessity, the automatic path is closed at both ends.
Note how different this is from D-STAR's no. D-STAR at least offers a GPS-ranked list with the operator choosing. P25 conventional offers changing channel by hand, the same as any analogue repeater. It sits with M17 at the bottom of the roaming table.
P25 Phase 2 is trunked only. Trunking needs a pool of frequency pairs with one given over to a control channel that transmits continuously, the trunking infrastructure is commercial only, and it exists to solve congestion that amateur repeaters do not have. For P25 it is doubly out of reach, since the MMDVM route cannot reach Phase 2 at all. The full argument is in part 7.
No amateur P25 trunked system was found anywhere. Conventional P25 on ham bands is common. Trunked P25 is not. The only amateur trunked systems found in the whole search are DMR, and they are on the DMR page.
P25 Phase 1 is single channel, so a P25-locked repeater carries one user. Its MMDVMHost TXHang default is 5 seconds, the same as NXDN. The mode hang timers are in part 7.
Note though that the usual amateur P25 build is a Quantar with its own controller rather than an MMDVM board, so the usual P25 build is not a multi-mode machine.