Weather Facsimile—commonly known as WEFAX, HF-FAX, or Radiofax—is easily one of the most rewarding niches in the shortwave hobby. There is a certain “magic” in tuning your receiver to a scheduled frequency and watching a high-resolution synoptic chart or satellite image slowly materialize on your screen, line by line, over several minutes.
While it feels like a relic of a pre-internet era, Radiofax remains a vital, free resource for mariners and a fascinating challenge for radio enthusiasts. This guide covers everything you need to get started in 2026, from understanding the signal to troubleshooting your first decode.
Weather Facsimile (WEFAX), also known as HF FAX or Radiofax, remains a vital, free resource for mariners and radio enthusiasts. It provides high-resolution meteorological charts, including surface analyses, sea state charts, and satellite imagery, transmitted via High Frequency (HF) radio.
What is a weather fax signal?
A WEFAX transmission is an analog slow-scan facsimile stream sent over HF. Typical HF radiofax emissions use upper-sideband operation and an audio subcarrier where image brightness is encoded as audio frequency (frequency-shifted tones). The commonly cited tonal mapping is roughly black ≈ 1500 Hz and white ≈ 2300 Hz, with intermediate gray levels between those tones. Transmissions begin with a start tone and a sequence of phasing (synchronizing) lines; those are used by decoders to align lines on the receiving side and measure line timing. Bandwidth is on the order of a few kilohertz and most HF facsimile is sent as USB with the audio fed to the decoder. These signal characteristics are described in multiple facsimile references and decoder manuals.
Practical takeaway: tune the receiver so the fax audio is centered in your soundcard / SDR audio passband, set the decoder to USB mode, and expect ~3 kHz audio bandwidth for best fidelity.
The Mechanics of HF-FAX
HF-FAX works on a simple yet effective analog principle: the image is transmitted line by line. To decode it properly, the receiving station must synchronize with the transmitter’s “drum speed” (typically 120 lines per minute or RPM) and the Index of Cooperation (IOC 576). The signal is Frequency Modulated (FM), where different audio tones represent specific shades. By convention, a 1500 Hz tone represents black, while a 2300 Hz tone represents white. The frequencies in between create the gray scales necessary for satellite-derived imagery.
Because these are transmitted over High Frequency (HF) using Single Side Band (SSB), you must tune your receiver to the Upper Side Band (USB). A critical operational detail is the “tuning offset”: you should typically tune your receiver 1.9 kHz lower than the listed carrier frequency in the official schedules to center the FM audio sub-carrier within your filter’s passband.
WEFAX is essentially an FM signal sent over SSB, you must tune 1.9 kHz lower (e.g., 9108.1 kHz) in USB mode to center the 1500–2300 Hz tones correctly in the audio passband.
Receiving chain Weather Fax — from antenna to image
At its simplest the chain is: antenna → HF receiver or SDR front end → audio (line-out / virtual audio) → decoder software → saved image (optionally geo-referenced in navigation apps).
Hardware choices:
- Legacy receivers or ham transceivers with good SSB/USB stability work well; feed the audio out (line or headphone) into your PC soundcard (or into an SDR receiver if you use a panadapter setup).
- SDR receivers simplify tuning and visual waterfall/FFT inspection. Popular modern HF SDRs used by hobbyists include the Airspy HF+ family and SDRplay RSP series; lower-cost RTL-SDR dongles paired with an HF upconverter (e.g., NooElec “Ham It Up”) are a budget path. Which to pick depends on dynamic range and local RF noise: if you live near strong signals or need deep sensitivity, choose a higher-performance HF SDR (Airspy/SDRplay) over a cheap RTL dongle.
Software connection:
- If using an SDR, route audio via the SDR application’s virtual audio output or via virtual audio cable to the decoder.
- If using a conventional receiver, use a good soundcard with line-in (or a USB audio interface) and appropriate isolation (audio transformer or small isolation cable) to prevent ground loops.
A good reference on upconverters/SDR options and practical tradeoffs is the RTL-SDR / Airspy community content and product pages.
For the modern hobbyist, an SDR (Software Defined Radio) like the RTL-SDR Blog V4 or an Airspy HF+ Discovery is the gold standard for reception. These devices offer excellent sensitivity and visual waterfalls that help you identify the characteristic “ticking” sound of a FAX signal. If you prefer traditional hardware, desktop receivers like the Icom IC-7300 or portable units such as the Tecsun PL-880 work perfectly, provided they have stable SSB capability.
Antennas are the most significant variable in image quality. A simple long-wire antenna or a dipole cut for the specific band (usually 8 MHz or 12 MHz) is effective. However, for those in noisy urban environments, a magnetic loop antenna or an active loop (like the MLA-30+) can significantly reduce local electromagnetic interference, leading to cleaner, “snow-free” images.
Software and Decoding Tools
Several actively used and maintained decoders exist. The ones you’re most likely to choose depend on platform and desired features (automatic scheduling, image saving, geo-overlay, SDR integration):
- FLdigi— free, cross-platform (Windows/Linux/macOS via compatibility layers), widely used by amateurs for WEFAX decoding. FLdigi decodes facsimile, handles phasing/start/stop, and offers auto-alignment and image saving options. It’s a common first choice for HF WEFAX because it integrates directly with many SDRs and soundcards.
- Black Cat Systems HF Weather Fax — a polished commercial app for Windows/macOS focused specifically on HF weather fax, with features tuned to marginal reception (advanced filtering and auto-tune). Black Cat’s HF Weather Fax releases have continued to be updated in recent years.
- JVComm32 — long-standing Windows program that decodes HF-FAX and many other maritime modes (NAVTEX, RTTY, SSTV). It’s popular among coastal listeners.
- SeaTTY— useful for NAVTEX and HF-FAX, commonly used in maritime contexts.
- MultiPSK — Windows multimode tool which supports many legacy and current modes including FAX; recommended for experimenters who decode many digital modes.
Which to pick: for a single, free solution try FLdigi; for the best out-of-the-box experience with marginal signals consider Black Cat HF Weather Fax; if you need many maritime/utility decoders on one pane, JVComm32 or MultiPSK are solid.
A virtual audio cable acts as a digital bridge, routing the audio output from one application, such as SDRuno, directly into the input of a decoding software like fldigi or HF Weather Fax. This ensures the signal remains entirely in the digital domain, preventing the quality loss and background noise typically associated with physical cables or speakers.
Setting up the decoder — practical knobs and settings
When you have audio feeding into the decoder, these are the common settings to check:
- Mode and sideband — set the radio/SDR to USB. The decoder expects the fax audio to appear in the audio band.
- Center the fax audio in the audio passband — use your SDR waterfall to place the phasing/start tone in the decoder’s center frequency if the program uses a center-frequency control. Some decoders have “auto center” or let you feed the audio and click the waterfall peak.
- Audio sampling and bandwidth — ensure the audio sample rate and soundcard are standard (48 kHz or 44.1 kHz) and that your SDR/audio application isn’t applying aggressive filters. A 3 kHz audio pass is good.
- Auto-alignment / phasing — enable the decoder’s automatic alignment; this compensates for small timing differences between transmitter and receiver. FLdigi and Black Cat have reliable auto-align routines.
If the decoded image shows slant or tilt, small corrections to the line-length (lines per minute) or sample rate can be used; many programs offer “skew” or “line length” adjustments to correct consistent slanting.
To decode WEFAX correctly using standard software, you must tune your SSB receiver 1.9 kHz lower than the “published” frequency. For example, to receive Boston on 9110 kHz, tune your dial to 9108.1 kHz USB.
Antenna, RF and noise tips
WEFAX signals can be transmitted from multiple stations worldwide on schedules; they are strongest offshore and at times near sunrise/sunset when HF propagation favors the path. For best reception:
- Use a longwire, elevated inverted-V or a tuned dipole for the bands you target; a dedicated receiving loop or active loop can help in urban noise environments. Offshore or coastal listeners benefit from unobstructed low-loss feedlines.
- SDR users: add a good HF preselector or choose an SDR with front-end filtering (e.g., Airspy HF+ has good preselection). If using an RTL dongle, consider an HF upconverter plus a proper antenna, and be mindful of strong local AM broadcast band signals which can overload cheap SDRs.
- Use moderate RF gain; excessive AGC/gain invites intermodulation and images. If you see spurs in the waterfall, reduce gain or enable notch filters.
- Keep cables short, use ferrite chokes on USB and audio lines, and isolate audio paths to avoid hum/ground loops.
Common artifacts, why they occur, and how to fix them
- Skew/Slant: transmitter and decoder timing mismatch or doppler-like instability; fix with line-length or sampling adjustments and ensure accurate soundcard clocking.
- Missing lines / dropouts: RF fade, interference or selective fading of the modulation tones; a better antenna, filtering, or receiving at another time often helps.
- Poor contrast / washed-out tones: audio filtering clipping or AGC problems; reduce compression and widen the audio passband.
- Phasing problems: incorrect start/stop detection or noisy phasing pulses; manual re-align or increase squelch/threshold in the decoder to ignore noise spikes.
FLdigi’s WEFAX manual explain many of these adjustments and show how auto-alignment mitigates skew and timing drift.
Weather Fax – Operating Schedules and Frequencies
Knowing when and where to listen is half the battle. Major stations like NMC (Point Reyes, California), NMG (New Orleans), and DWD (Hamburg, Germany) broadcast 24/7.
Always check the latest NOAA Worldwide Marine Radiofacsimile Broadcast Schedule (available as a PDF on weather.gov) for updated times, as schedules change seasonally and include specific windows for surface analysis, sea state, and satellite imagery.
The detail list of frequencies and station schedule can be find in publication Marine Worldwide Radiofacsimile Broadcast Schedules, which is published by the National Oceanic and Atmospheric Administration’s (NOAA) and it’s freely available to download.
Radio facsimile Worldwide Marine Broadcast Schedules: https://www.weather.gov/media/marine/rfax.pdf
check the Grey Line. Transmissions from the DWD (Germany) or Kyodo News (Japan) might come in crystal clear in India during dawn or dusk. Suggest using a lower frequency (3-5 MHz) at night and a higher one (12-16 MHz) during the day.
Weather Fax –Updated Schedules & Links
Schedules change periodically. It is recommended to download the latest PDF guides to know exactly when specific charts (like “Surface Analysis” or “Wind/Wave”) are being transmitted.
- NOAA Worldwide Guide (The “Bible” of WEFAX): Worldwide Marine Radio facsimile Broadcast Schedules (PDF)
- National Weather Service (NWS) Radiofax Home: NWS Marine Radiofax Charts
- German Weather Service (DWD): DWD HF-Fax Schedules,broadcast facsimile
- JMA (Japan): Tokyo (JMH) Schedule
WEFAX –Frequently Asked Questions (FAQ)
Q1. Do I need a ham radio licence to receive weather fax transmissions?
No licence is required. WEFAX/Radiofax stations are broadcast services — anyone with a shortwave receiver or SDR can receive and decode them freely. No transmission is involved on your end.
Q2. I have an RTL-SDR dongle already. Is that good enough to receive weather fax, or do I need a better SDR?
An RTL-SDR can work, but it has limitations on HF. You will need an upconverter (such as the NooElec Ham It Up) to bring HF signals into its receive range. If you live near strong AM broadcast transmitters, the cheap RTL-SDR front end can overload and produce spurious signals. For a noticeably cleaner experience with fewer headaches, an Airspy HF+ Discovery or an SDRplay RSP is a better investment for HF work — but the RTL-SDR with an upconverter is a perfectly valid starting point on a tight budget.
Q3. Why do I need to tune 1.9 kHz lower than the published frequency? Can I not just dial in the listed frequency directly?
WEFAX is an FM signal riding on an SSB carrier. When you tune to the published carrier frequency in USB mode, the 1500–2300 Hz audio tones that carry the image end up shifted outside the ideal audio passband of your decoder. Tuning 1.9 kHz lower places those tones correctly in the centre of your audio window. For example, if the schedule lists 9110 kHz, tune your dial to 9108.1 kHz USB. Skipping this step is the single most common reason beginners get garbled or blank output.
Q4. Which decoder software is the best starting point for a beginner?
FLdigi is the most practical first choice — it is free, runs on Windows, Linux, and Mac, and handles all the WEFAX-specific functions like auto-alignment, phasing detection, and automatic image saving. If you find FLdigi’s interface confusing or your signals are marginal and noisy, Black Cat Systems HF Weather Fax is a polished paid alternative with better filtering and a simpler workflow. JVComm32 and MultiPSK are worth exploring later if you want to decode other maritime digital modes alongside WEFAX.
Q5. How do I get the audio from my SDR software into the decoder without using a physical cable or playing it through speakers?
Use a virtual audio cable application. This creates a software loopback — the SDR application sends its audio output to a virtual device, and the decoder reads from the same virtual device as its input. On Windows, VB-Audio Virtual Cable is a free and widely used option. Once set up, both the SDR app and the decoder see it as a normal audio device; no physical cable or microphone is needed and there is no background noise introduced.
Q6. My decoded image comes out slanted — everything is tilted at an angle. What is causing this and how do I fix it?
Slant or tilt means the decoder’s line timing is slightly out of sync with the transmitter. Most decoders have a “skew” or “line length” adjustment — nudge it in small steps until the image comes out straight. In FLdigi, the slant correction is found in the WEFAX mode settings. The root cause is usually a small mismatch between your soundcard’s actual sample rate and what the decoder expects, or Doppler-like instability on a weak signal path. Once you find the right correction value for a particular station, it tends to stay stable.
Q7. The image starts decoding but then loses sync partway through, producing garbled lines. What should I check?
This is almost always caused by RF fading or interference mid-transmission. HF propagation on a fixed path can drop suddenly, especially on shorter paths during the day. First, confirm you have the phasing/auto-alignment feature turned on in your decoder — this helps it recover quickly from brief dropouts. Second, try a different frequency from the same station’s schedule (most stations broadcast on two or three frequencies simultaneously) and pick the one with the strongest and most stable signal on your waterfall. Third, check that your SDR gain is not set too high — excessive gain causes intermodulation, which looks like dropouts.
Q8. What is the best antenna for WEFAX if I live in a noisy urban area?
In a high-noise urban environment, a receiving magnetic loop or an active loop antenna (like the MLA-30+) is significantly better than a simple long wire. These antennas are directional and reject electrical noise that comes from directions other than the target station. A simple long wire or inverted-V dipole is excellent in a quiet rural or suburban setting but tends to pick up all the local electrical interference along with the signal in a city. Even indoors, a small active loop near a window can outperform a large outdoor wire in a noisy location.
Q9. Which frequency band should I use — 4–8 MHz or 12–16 MHz — and how do I decide?
Use lower frequencies (4–8 MHz) at night and higher frequencies (12–16 MHz) during the day. The ionosphere behaves differently at different times — at night, lower bands travel further and higher bands often go quiet; during daylight hours, higher bands open up and support long-distance paths. For listeners in India, dawn and dusk are particularly productive windows to try European stations like DWD Germany or Asian stations like JMA Japan, because the grey line (the twilight zone between day and night) greatly enhances HF propagation at those moments.
Q10. Where do I find the current transmission schedules and frequencies for WEFAX stations?
The definitive reference is the NOAA Worldwide Marine Radiofacsimile Broadcast Schedules PDF, freely available from weather.gov. This document lists every active station, their frequencies, transmission times, and what type of chart is sent in each slot (surface analysis, sea state, satellite imagery, etc.). Always download the latest edition because stations occasionally shift frequencies or adjust their schedules seasonally. Links to the NOAA PDF and schedules for DWD Germany and JMA Japan are provided in the article above.
Q11. I am getting a good audio signal but the image contrast looks very washed out — everything appears grey with no clear blacks or whites. What is wrong?
This is usually caused by audio compression or AGC (automatic gain control) in your receiver or SDR software squashing the dynamic range of the tones. The decoder needs to see a clear difference between the 1500 Hz (black) and 2300 Hz (white) tones — if AGC is smoothing those levels together, the output looks flat and grey. Turn off any audio compression, reduce AGC aggressiveness, and widen your audio passband slightly so it cleanly passes the full 1500–2300 Hz tone range without clipping either end.
Q12. Can I use a regular ham transceiver like the Icom IC-7300 instead of an SDR for WEFAX reception?
Absolutely — a stable SSB-capable HF transceiver works very well for WEFAX. The IC-7300 in particular is an excellent choice because its built-in panadapter and waterfall make it easy to visually confirm you are tuned correctly. Connect the audio output (line out or headphone jack) to your PC soundcard or a USB audio interface, set it to USB mode, tune 1.9 kHz below the published frequency, and feed the audio into your decoder exactly as you would with an SDR. Portable receivers like the Tecsun PL-880 also work provided they have stable SSB reception.
Q13. Does a no-licence shortwave listener (SWL) have any practical use for WEFAX charts, or is this mainly for sailors?
Plenty of value for a land-based listener too. Weather fax charts include synoptic surface analysis maps, upper air charts, sea state forecasts, and satellite imagery — all the same data that professional meteorologists and sailors rely on. For someone in a coastal region, an island, or a remote area with unreliable internet, receiving these charts directly from an HF broadcast provides real weather intelligence that is completely independent of cellular or internet infrastructure. Beyond the practical side, many hobbyists simply enjoy the satisfaction of pulling a crisp meteorological chart out of thin air using only a receiver and a piece of wire.
Q14. Do I need an internet connection to receive WEFAX?
No. WEFAX signals are transmitted directly over radio, so you can receive weather charts even without internet access. This is one reason it is still used on ships at sea.
Q15. Why do WEFAX images sometimes appear slanted or distorted?
Slanted images usually happen due to: Incorrect sample rate, Wrong lines-per-minute (LPM) setting, Slight tuning errors. Most decoding software allows adjustments to fix the alignment.
Q16. Why must the receiver be tuned slightly below the published frequency?
Weather fax transmissions are sent using SSB audio tones, so the receiver should usually be tuned about 1.9 kHz below the listed frequency in USB mode for proper decoding
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