PSK31 and Weak Signal HF Digital Modes
PSK31 (Phase Shift Keying, 31 baud) was the first widely adopted HF digital mode to use a computer sound card as the modem, and it opened the door to the explosion of digital modes that followed. PSK31 allows real-time keyboard-to-keyboard conversation on HF using only 31 Hz of bandwidth — narrower than a CW signal — making it extraordinarily efficient and capable of pulling readable signals out of noisy band conditions. Alongside PSK31, this guide covers WSPR (beacon propagation monitoring), WSPR-X, and other weak signal modes that complement on-air operating.
Phase shift keying
PSK31 encodes text using phase shifts in a continuous carrier signal. A phase shift represents a "1" and no phase shift represents a "0." At 31.25 baud, the signal is slow enough that it fits in just 31 Hz of bandwidth — you can fit a hundred PSK31 signals in the space occupied by a single SSB phone signal. The narrow bandwidth means that atmospheric noise, which is distributed across the spectrum, contributes very little power to the received signal, giving PSK31 excellent performance in noisy HF conditions.
PSK31 variants
BPSK31 (Binary PSK) is the most common variant and the standard for casual contacts. QPSK31 (Quadrature PSK) encodes two bits per symbol using four phases, doubling the data rate but requiring better signal quality. PSK63 and PSK125 are faster variants useful when band conditions are good. Most PSK31 software supports all variants. For day-to-day contacts, BPSK31 is the right choice — it has the best sensitivity and widest software support.
Get PSK31 software
Several excellent free programs support PSK31. Fldigi (fast light digital modem application) is the most feature-rich and supports PSK31 along with dozens of other modes including RTTY, SSTV, Olivia, and more. Download it free at w1hkj.com. WSJT-X does not support PSK31 — use Fldigi for PSK31 operation. JS8Call, Ham Radio Deluxe's Digital Master 780, and DigiPan also support PSK31.
Connect your radio
PSK31 uses the same sound card interface connection as other HF digital modes — either a dedicated interface like a SignaLink USB or your radio's built-in USB audio. Set your radio to USB mode on 14.070 MHz for 20m PSK31. Set audio levels so the ALC barely deflects when transmitting — PSK31 is sensitive to overdrive and overdriven PSK31 causes ugly splatter across the passband.
Tune in signals on the waterfall
The Fldigi waterfall display will fill with the vertical striped traces characteristic of PSK31 signals. Each pair of closely-spaced vertical lines is a PSK31 signal. Click on a signal to tune to it and Fldigi will start decoding the text. On a busy band like 20m around 14.070–14.073 MHz, you will see dozens of signals simultaneously.
Make a contact
To call CQ on PSK31, find a clear spot on the waterfall above 14.070 MHz, type your CQ message in the transmit window ("CQ CQ CQ de [callsign] [callsign] K"), and transmit. Keep the CQ brief and listen for a response. When someone responds, their text will appear in the receive window and you can type your reply. PSK31 QSOs have a conversational feel similar to typing messages in real time.
What WSPR does
WSPR (Weak Signal Propagation Reporter, pronounced "whisper") is an automated beacon mode for monitoring propagation. A WSPR station periodically transmits a short beacon containing your callsign, grid square, and transmit power. Other stations running WSPR software automatically receive and decode your beacon, then upload the spot to WSPRnet.org. The result is a real-time global map showing exactly which stations are hearing your signal and at what strength — invaluable for understanding your antenna's actual performance and current band conditions.
WSPR in practice
WSPR is almost entirely automated — you set it up, leave it running, and periodically check WSPRnet.org to see where your signal is being heard. Many operators run WSPR on a separate radio or a low-power setup 24/7 as a continuous propagation monitor. Even 200mW of WSPR can be heard around the world on 20m during a good opening. WSPR is supported by WSJT-X — select the WSPR mode from the mode menu. Primary WSPR frequencies are 14.0956 MHz (20m), 7.0386 MHz (40m), and 3.5926 MHz (80m).
| Band | PSK31 Centre | Range | Activity Level |
|---|---|---|---|
| 80m | 3.580 MHz | 3.580–3.590 MHz | Moderate — regional |
| 40m | 7.070 MHz | 7.070–7.075 MHz | Active — day and night |
| 20m | 14.070 MHz | 14.070–14.075 MHz | Most active PSK31 band |
| 15m | 21.070 MHz | 21.070–21.075 MHz | Moderate when open |
| 10m | 28.120 MHz | 28.120–28.125 MHz | Active at solar max |
Is PSK31 still active now that FT8 dominates?
PSK31 activity has declined significantly since FT8's introduction in 2017, but it is not dead. Operators who enjoy real-time keyboard conversations prefer PSK31 over FT8's fixed 15-second automated exchange. On 20m you can still find active PSK31 signals around 14.070–14.073 MHz, particularly on weekends. PSK31 contests also generate periodic activity surges.
What is Olivia and how does it compare to PSK31?
Olivia is a multi-tone FSK mode developed for keyboard conversations under very difficult conditions — it outperforms PSK31 in poor signal-to-noise ratios at the cost of wider bandwidth. Olivia 8/500 (8 tones, 500 Hz bandwidth) is a popular choice for difficult paths. Fldigi supports Olivia alongside PSK31. For good band conditions, PSK31 is faster and narrower. For marginal conditions where PSK31 struggles, Olivia maintains readable copy.
What is the difference between BPSK and QPSK?
BPSK31 (Binary PSK) uses two phases — 0 and 180 degrees — to encode one bit per symbol. QPSK31 (Quadrature PSK) uses four phases — 0, 90, 180, and 270 degrees — to encode two bits per symbol, giving it error correction capability and twice the data rate. QPSK31 is more robust against errors but requires better phase accuracy from both stations. For most contacts, BPSK31 is preferred because it works with a wider range of signal conditions and equipment quality.
Can I run PSK31 and FT8 simultaneously on the same computer?
Yes — you can run Fldigi (for PSK31) and WSJT-X (for FT8) simultaneously as long as your sound card interface supports multiple audio streams, or if you use separate audio devices for each. Alternatively, many operators run one mode at a time and switch between them. Virtual audio cable software (VB-Audio Cable, VAC) can split one physical audio interface into multiple virtual devices to support simultaneous software instances.