Tutorial Weather fax

Tutorial: Weather fax

What it is. Weather fax (WEFAX, HF radiofax) is a slow analogue picture service. National weather services send weather maps, forecast charts, wave and ice charts and sometimes satellite pictures on HF, mostly for ships. A picture of several hundred lines takes 10 to 20 minutes. OnAir turns the audio tones into an image as the lines arrive.

Fax, NAVTEX and DSC share one mode panel (group Maritime).

What you need

• Radio. An HF-capable SDR or a dongle with an upconverter. 2 Msps is the default; 250 ksps is the lowest working rate. See Supported-radios.
• Antenna. A long wire or an end-fed antenna, away from noise. Strong stations are heard with a modest wire.
• Frequencies from the repository's frequency database. The listed carriers are given with the USB dial 1.9 kHz lower. Examples: DWD Pinneberg 3855, 7880 and 13882.5 kHz (dial 3853.1, 7878.1 and 13880.6 kHz); NOAA Boston NMF 4235, 6340.5, 9110 and 12750 kHz; NOAA New Orleans NMG 4317.9, 8503.9, 12789.9 and 17146.4 kHz; NOAA Kodiak NOJ 2054 and 4298 kHz. The mode starts at 7.8781 MHz.

Before you start

• Set upper sideband (USB) tuning: the dial is 1.9 kHz below the assigned carrier. Clicking a station in Fax stations tunes the right dial frequency.
• Gain: moderate; HF noise fluctuates.
• Schedules change; times marked with a tilde in the panel are approximate.

Step by step

1. Mode list, group Maritime, entry Weather charts, HF.
2. Open the Fax stations view (a region list narrows the directory: all regions, Europe, North America, Asia, Oceania, South America, Africa) and click a station; the panel says what to tune, for example "tune the dial to ... kHz USB". Or type a frequency.
3. Service can stay on auto: other frequencies than NAVTEX and DSC ones are treated as fax.
4. Press Start and select the Fax view.
5. The status line reads "listening: a picture starts by itself on the start tone, the phasing, or a picture already under way". Wait for the start tone; the line then changes to "phasing", and the picture grows.

What you should see

The picture appears from the top line by line, black and white. The status line gives IOC, lines per minute (lpm), number of lines, width in pixels, slant in ppm, the frequency of black and the last tone.

Reading the screen

• lpm list: detect, 60, 90, 120 or 240 lines per minute.
• IOC list: start tone, IOC 576 or IOC 288 (index of cooperation, which sets the width).
• auto slant measures the sample clock error from the phasing lines and straightens the picture. The slant slider adds a manual correction on top (it applies to already-received lines too; double-click to type a value, 0 resets).
• fit and 1:1 zoom; save PNG saves the picture.
• Tuning shows black at 1500 Hz and white at 2300 Hz, start tones (576 and 288) and the stop tone.
• Listen plays the fax tones.

Options and settings

All listed above. Picture saving: use save PNG. To start a new picture, wait for the next start tone.

Troubleshooting

• Picture slanted. Turn on auto slant or set the slider.
• Mistuned (shown as "mistuned"): black not at 1500 Hz. The status shows how many Hz off. Retune the dial.
• Picture too wide or too narrow. Wrong IOC; choose IOC 576 or 288 by hand.
• Fading streaks. HF fading; wait for the repeat of the chart.
• Nothing happens. Wrong time or frequency; check the schedule and the sideband.
• Missing start tone. Choose lpm and IOC by hand to begin without it.

Tips

Try a station that sends all day (check its schedule). Use the closest strong station first.

Legal and privacy notes

Weather fax is a public broadcast for navigation. Receiving is legal in most places. See Legal.

Under the hood

Frequency modulation of a 1900 Hz sub-carrier: 1500 Hz black, 2300 Hz white; start tone 300 Hz (IOC 576) or 675 Hz (IOC 288) and stop tone 450 Hz; phasing lines; 120 lpm is the usual speed. Defined in ITU-R recommendation for facsimile (the WMO manual also describes it).

Related

Mode-Weather-fax, Tutorial-NAVTEX, Tutorial-DSC, Tutorial-Weather-satellite-images, Troubleshooting