Digital Amateur Radio

Note: This page is a bit dated and doesn’t reflect some newer modes like FT8, but what’s here should generally be correct.

Introduction

People can use Amateur Radio not just for voice communication, but also for digital communication. This happens all over the bands, from HF through UHF. On VHF and UHF, digital might be D-Star or Packet Radio, which are not discussed here. This page mainly discusses the keyboard-to-keyboard HF modes.

These modes are used for keyboard-to-keyboard communication. That is, you are having a QSO (conversation) with another amateur using keyboards, similar to the concept of an instant message conversation.

What You Need

To operate with HF digital, most people will use a PC with some sort of audio interface to their radio. Cables can be made to hook up your sound card to the radio. Many people instead go for a SignaLink USB or similar interface, which is about $100 but doesn’t interfere with the regular use of your sound card, and probably provides a higher-quality interface as well.

For software: You will use a sound card interface software for this. I use fldigi because it is cross-platform (Linux, FreeBSD, Mac, and Windows), high quality, and free. Other options such as MixW and MultiPSK exist as well. The examples in this page refer to fldigi, because it’s what I know, but the concepts should translate well to others.

Basics of Operating

There are many different digital modes. PSK-31 is probably the most common. You can often find PSK-31 at these frequencies:

• 3.580 USB
• 7.035 USB
• 7.050 USB
• 10.142 USB
• 14.070 USB
• 18.100 USB
• 21.070 USB
• 21.080 USB
• 24.920 USB
• 28.120 USB

Other popular modes include RTTY and MFSK-16. Even PSK can be used in faster modes; for instance, PSK-63 runs at 63 baud.

For now, we’ll assume PSK-31. Please note: the most common PSK-31 mode is often called BPSK31 in software (QPSK is usually not what you want).

The Waterfall

Normally at the bottom of the display in your interface software, you’ll see a waterfall.

An SSB voice signal uses up to 3000Hz of bandwidth. When you tune your rig to, say, 14300KHz, your voice transmission actually uses, roughly, frequencies between 14300 and 14303kHz.

A PSK-31 signal is much narrower – instead of a bandwidth of 3000Hz, it only uses 31.25Hz. So, when you tune your rig to a given frequency in USB mode, you can receive dozens of PSK-31 signals at once. If you listen to the audio, they will sound like they are simply different pitches; lower frequency will be lower-pitched, higher frequency will be higher pitched. This is similar to the way that you can distinctly hear both a soprano and a baritone singing at the same time.

So, since tuning your rig doesn’t select one individual PSK-31 signal, but rather a 3kHz-wide window of many, many PSK-31 signals, you need a way to select the PSK-31 signal you want in software. This is where the waterfall comes in. The fldigi waterfall represents, by default, the 3kHz bandwidth that your rig will theoretically receive. (In practice, filters and the like reduce that width somewhat, down to perhaps the middle 2kHz.)

A few times a second, a new row of pixels is added at the top of the waterfal. The pixel at any given location represents the strength of a received signal at that frequency – brighter for brighter signals. So what you see is a recent history of transmissions across the 3kHz bandwidth. Ongoing transmissions will appear as vertical bars, and you can click on one to tune to it and decode it. In this example, an MFSK-16 transmission is being decoded; the red bars represent the boundary of the signal after the operator clicked on it.

Different modes look different on your waterfall. Here are some links to pages with screenshots of the appearnces of different modes:

• Sights and Sounds of Digital Modes

• Radio sounds - waterfalls pictures

• Digital modes used in ham radio

One other feature of fldigi and many other programs is what fldigi calls the “PSK browser”. It will simultaneously decode all PSK streams on your waterfall, and can optionally highlight ones that are calling CQ so you can easily find a station to talk to.

I’m not going to go into more detail about operating software here because there are plenty of good references available via Google or the online help for your program.

Operating Practices

There are a few things to keep in mind on digital:

• At any given moment, only one station can transmit. This is similar to voice on HF, and there are similar challenges: needing to make sure each station knows when the other is done transmitting, and ready to listen to your transmission.
• Some of the modes are fairly slow. PSK-31 is roughly 50 words per minute, which is slower than the typing speed of many touch typists.
• The need to identify yourself still exists
• PSK-31 is not an error-free mode. You can see corruption in the received text.

As such, there are some practices and terms that help people.

Digital Lingo

Much of the link is related to CW (Morse) conventions. There are plenty of lists, but I want to give you just a few that you’ll need to know for PSK-31 operation.

A sample QSO

Here’s a sample conversation. I’ll call the stations BB0BB and CC0CC just for example, and label the transmissions appropriately. First, BB0BB is calling CQ.

  BB0BB transmits:

  CQ CQ CQ de BB0BB BB0BB BB0BB
  CQ CQ CQ de BB0BB BB0BB BB0BB pse K

Like a SSB CQ, you want this to be long enough that people find it. Also, the callsign is important to get right, so it specifically is repeated multiple times in case it gets corrupted due to noise or QSB.

BB0BB gets no answer, so calls CQ again:

  BB0BB transmits:

  CQ CQ CQ de BB0BB BB0BB BB0BB
  CQ CQ CQ de BB0BB BB0BB BB0BB pse K

Now CC0CC heard this and answers:

  CC0CC transmits:

  BB0BB BB0BB de CC0CC CC0CC CC0CC KN

This means “BB0BB from CC0CC, go ahead BB0BB”

Again, the C station repeats the callsign multiple times in case it gets corrupted due to noise.

At this point, the QSO begins. It might go like this:

  BB0BB transmits:

  CC0CC de BB0BB

  Good evening to you.  Your RST is 599 599.  My name is Simon Simon.
  My QTH is Wyoming, grid FJ87 FJ87.

  How copy?

  BTU CC0CC de BB0BB KN

Each QSO begins with “othercall de mycall”. Then the conversation. An RST, name, and QTH are given. BB0BB repeats the name and grid twice since they’re short and could get corrupted by noise.

  CC0CC transmits:

  BB0BB de CC0CC

  Hi Simon.  U R also 599 599.

  Name here:   Russell Russell
  QTH:         Washington, DC  Washington, DC
  Rig:         Kenwood Something-5351
  Antenna:     Vertically-mounted dipole at 50'

  I copy you 100% error free.  How are things in Wyoming?

  BTU BB0BB de CC0CC KN

Some software have macros, so you can hit a certain key to transmit often-needed information, such as the BTU line, the information about names, rigs, etc. Both people may have used macros in this QSO. So let’s say that the QSO has gone on for awhile and they’re ready to say goodbye. Here’s how that may happen:

  BB0BB transmits:

  CC0CC de BB0BB

  Well OM, time for me to go.  73 73 to you and have a good weekend.

  CC0CC de BB0BB SK SK SK

  CC0CC transmits:

  BB0BB de CC0CC

  OK, thanks for the QSO.

  BB0BB de CC0CC SK

Sometimes if conditions are noisy, important signals to the other operator like KN or SK are repeated to help avoid confusion about when one station is done transmitting. You saw that here.

See Also

• PSK-31 guide, a good resource for getting started

• Fldigi manual

• PSK31 article on Wikipedia

Links to this note

• Signalink USB

Ths SignaLink USB is a small radio interface for use with Digital Amateur Radio. It appears to your computer as a USB sound card. It has a completely isolated radio interface and is sold in various configurations with prebuilt cables for many popular rigs. It’s less than $100, which means it’s very popular.

• Packet Radio

Packet radio, also known as AX.25, is a specific type of Digital Amateur Radio. Packet radio works somewhat like the Internet in that it splits communications into discrete packets, performs error checking on these packets, automatically requests retransmision of packets that arrived with errors, and thus provides a reliable and error-free communication channel.

• Amateur Radio

Amateur radio is a radio service in which people are allowed and encouraged to build their own radios, antennas, and so forth. It can be used to communicate all around the globe without any intervening infrastructure such as satellites or cables.