The oldest digital mode
What is CW?
Another name for a Morse code transmission.
Morse is a digital mode — a carrier switched on and off in a coded pattern. It predates everything else here by a century, occupies about 150 Hz, and remains one of the most effective ways to get a message through a poor path with simple equipment.
No US amateur licence has required Morse proficiency since 2007. It is worth learning anyway, for the same reason it survives: it works when nothing else does.
Weak-signal digital: FT8
What is FT8?
A digital mode capable of low signal-to-noise operation.
FT8 sends a fixed, tiny message — call signs, grid square, signal report — in a precisely timed 15-second slot, occupying about 50 Hz. Its decoder works well below the noise floor, roughly 20 dB under what a human ear can pull out of static.
The consequence is that contacts happen on paths that carry nothing else. The trade is that the message is fixed-format: you exchange call signs and a report, and that is the entire conversation.
Keyboard-to-keyboard: PSK
What does the abbreviation “PSK” mean?
Phase Shift Keying.
The carrier’s phase is shifted to encode data, rather than its amplitude or frequency. PSK31 — the popular variant — occupies about 31 Hz, roughly the bandwidth of a typed conversation at human speed, and it is a real keyboard-to-keyboard chat mode rather than a fixed exchange.
Packet radio and APRS
Packet radio breaks data into addressed packets. The pool asks what is included in a packet transmission and answers “all these choices are correct” — a check sum, the sending station’s call sign, and an address. That combination is what makes packet a network rather than a link: packets can be routed and errors detected.
APRS — the Automatic Packet Reporting System — is packet put to a specific use.
What kind of data can be transmitted by APRS?
All these choices are correct — GPS position data, text messages, and weather data.
Which of the following is an application of APRS?
Providing real-time tactical digital communications in conjunction with a map showing the locations of stations.
That is exactly what it does in public-service work: every station’s position on a shared map, updating live, with short messages alongside. For anyone who has flown a drone under Part 107, the mental model is familiar — situational awareness as a shared map rather than a radio net.
Digital voice: DMR
Which of the following describes DMR?
A technique for time-multiplexing two digital voice signals on a single 12.5 kHz repeater channel.
The key word is time-multiplexing. DMR splits a channel into two alternating time slots, so one repeater channel carries two independent conversations. That is why DMR repeaters seem to do more with the same spectrum, and why talkgroups and time slots both appear in DMR programming.
Mesh networking
Which of the following best describes an amateur radio mesh network?
An amateur-radio data network using commercial Wi-Fi equipment with modified firmware.
Consumer Wi-Fi hardware operates in bands that overlap amateur allocations, and with amateur firmware it can run at amateur power levels and with directional antennas, under an amateur licence. The result is a self-organising IP network — video, file transfer, chat — over paths no consumer Wi-Fi would manage.
Television
What is meant by the term “NTSC?” An analog fast-scan color TV signal
The legacy US broadcast television standard, about 6 MHz wide, still used by amateurs on the higher bands where there is room for it.
ARQ: getting it right
What is the role of ARQ in a transmission system?
An error correction method in which the receiving station detects errors and sends a request for retransmission
Automatic Repeat reQuest. Rather than adding enough redundancy to correct every possible error in advance, ARQ detects that something is wrong and asks again.
It is the right choice when the link is two-way and the data must be exact — a file, a form, an emergency message. It is the wrong choice when there is no return path or when latency matters, which is why broadcast-style modes use forward error correction instead.
Winlink, the amateur email system, is built on ARQ protocols for exactly this reason: an email that arrives with a corrupted address is worse than no email.
The mode-to-purpose map
| Need | Mode |
|---|---|
| A contact on a nearly dead path | FT8 |
| A typed conversation | PSK31 |
| Position reporting and tactical mapping | APRS |
| Exact text or files, error-free | Winlink over ARQ |
| Two voice conversations on one channel | DMR |
| Broadband data over a link | mesh |
Check yourself
- Which mode would you reach for to make a contact on a path where you can hear nothing at all?
- What does DMR time-multiplexing buy?
- Why does Winlink use ARQ rather than forward error correction?
Answers
- FT8 — capable of low signal-to-noise operation, roughly 20 dB below what the ear can decode.
- Two independent voice conversations on one 12.5 kHz repeater channel, via alternating time slots.
- The message must be exact, and there is a return path to ask again with. ARQ retransmits on detected error rather than padding every message with correction overhead.