Hotspots
| Hotspot (digital voice) | |
|---|---|
| What it is | A very low-power transceiver that links a digital voice radio to internet networks |
| Modes | DMR, D-STAR, System Fusion and others, depending on the model |
| Typical hardware | An MMDVM modem board on a Raspberry Pi, or a ready-made commercial unit |
| Software | Pi-Star, WPSD and other MMDVM-based images |
| On the air | |
| Usual band | 70 cm |
| Typical power | A few milliwatts to a few tens of milliwatts |
| Connects through | Home Wi-Fi or Ethernet, or a phone's mobile data |
| Avoid | The 435–438 MHz satellite segment, repeater pairs and the national simplex frequency |
A hotspot is a small, very low-power radio that connects a digital voice transceiver to the internet-linked networks of DMR, D-STAR, System Fusion and similar modes. The ham's own radio talks to the hotspot across the room, and the hotspot passes the traffic to a network server over a home internet connection. A hotspot gives access to talkgroups and reflectors around the world without a digital repeater in range, and for many hams it is the main way they use digital voice. Most hotspots are inexpensive, about the size of a deck of cards, and run on a few watts from a USB power supply.
How a hotspot works
Inside most hotspots are two parts. A modem board contains a tiny UHF transceiver and a microcontroller that converts between radio signals and digital data. A small computer, very often a Raspberry Pi, runs the software that talks to the modem and to the network. The most common modem design is MMDVM, short for Multi-Mode Digital Voice Modem, an open-source project originally written for repeaters and adapted for hotspots. A single MMDVM hotspot can handle several modes, and many can also bridge between them, so that a Fusion radio can reach a DMR talkgroup.
A simplex hotspot uses a single frequency and handles one conversation at a time; on DMR it uses only one time slot. A duplex hotspot has separate transmit and receive frequencies, and on DMR it can carry both time slots, like a miniature repeater. Ready-made commercial hotspots, such as SharkRF's openSPOT series, and some recent radios that connect to networks directly, do the same job without the need to build and configure a Raspberry Pi.
Software
Pi-Star is a free software image for the Raspberry Pi that turns a computer and an MMDVM board into a working hotspot. Everything is set through a web page, called the dashboard, opened from a browser on the same network: the call sign, DMR ID, frequencies, modes and the networks to connect to. The dashboard also shows who is talking, the network connection and the received signal. WPSD is a newer derivative of Pi-Star with a redesigned dashboard and additional features. Both are built on the same MMDVM host software.
Setting up a hotspot usually involves:
- Writing the software image to a memory card and configuring Wi-Fi.
- Entering the call sign and, for DMR, the operator's DMR ID.
- Choosing the hotspot's frequency and programming the same frequency into the radio, with matching settings such as the DMR color code.
- Selecting the networks, such as Brandmeister for DMR, a D-STAR reflector or a YSF reflector for Fusion.
Inexpensive modem boards often need a small frequency offset correction before they work reliably. The dashboard reports the bit error rate (BER) of received signals; a high figure usually means the offset needs adjusting.
Choosing a frequency
A hotspot is a transmitter, and its signal, though weak, can be heard by nearby stations, so its frequency should be chosen with care. In the US there is no single national hotspot frequency. Good practice is to:
- Stay out of the 435–438 MHz satellite segment, where a hotspot can interfere with satellite uplinks and with nearby stations listening for weak satellite downlinks.
- Avoid local repeater inputs and outputs, the national FM simplex calling frequency of 446.000 MHz, and other well-known frequencies.
- Follow the local band plan, which may set aside frequencies for simplex digital use, and check whether the area's frequency coordinator recommends particular hotspot frequencies.
Under Part 97, a hotspot is part of its owner's amateur station. It needs a licensed control operator, and the station must identify as on any other mode. Power should be kept as low as practical; a few milliwatts is usually enough to cover a house.
Security
A hotspot is a small computer on a home network, and it deserves the same care as any other. Pi-Star ships with a well-known default password for its dashboard and remote login, and it should be changed as soon as the hotspot is running. The dashboard should never be exposed directly to the internet through router port forwarding, since anyone who reaches it could change the frequency, connect the hotspot to other networks or transmit under the owner's call sign. Networks such as Brandmeister let each user set a separate hotspot password in their account, which keeps others from connecting a device under that call sign. Keeping the software updated closes known problems.