Weather Fax on HF: How to Receive Weather Charts With Your Radio

Long before websites and smartphone apps could show the pressure map of the Atlantic, ships at sea received weather charts through their radios. The technique is still alive. Several national weather services continue to broadcast charts as a radio signal on the shortwave bands, and with an ordinary HF receiver, a computer and free decoding software, you can watch a chart appear line by line on your screen. This is weather fax, also called radiofax or WEFAX, and it is one of the most satisfying listening projects for a radio amateur or a shortwave enthusiast. It needs no transmitter and no licence to receive, it works on any HF receiver that has single sideband and it teaches you a lot about weather and about how a signal is turned into a picture. This guide explains how it works, how to tune it, what software to use and how to read what you receive.

How Radiofax Works

A fax machine turns a picture into a series of lines and each line into a sequence of tones. Radiofax uses the same idea over a radio link. The chart is scanned line by line, and the brightness of each point on the line is converted into an audio frequency: black is represented by 1500 Hz and white by 2300 Hz, with grey shades in between, around a centre of 1900 Hz. The audio tone is sent as a frequency modulated subcarrier, and the result is transmitted in single sideband, so the receiver sees a signal that moves slightly in frequency as the picture changes. The Wikipedia article on radiofax gives the history and the modulation, and mentions the services that still send charts on shortwave.

The sequence at the start of a transmission includes a start tone, a phasing signal and the image itself, and ends with a stop tone. The start tone tells the receiver which scanning parameters the chart uses, and the phasing lines allow the decoder to align the start of each line with the left edge of the picture. A well-built program detects these automatically, but it is useful to understand what they do when you must set them by hand.

Key Parameters at a Glance

The index of cooperation is a measure of the resolution. An IOC of 576 gives a picture about 1800 pixels wide, since the width equals the IOC multiplied by pi, and this is the usual choice for the weather charts of the main services. A value of 288 gives half of that and was used for coarser pictures and for satellite relays.

What You Need to Receive It

The equipment list is short. You need an HF receiver or transceiver that can receive upper sideband, an antenna that can hear the signal, a connection between the audio output of the radio and the sound card of a computer and a decoding program. A modern SDR receiver can do all of this inside a single computer, with virtual audio cables between the receiver software and the decoder. If you already use your radio for digital modes, your interface for these modes is the one that you need. Our article on the digital interface with a sound card explains how to connect it and how to prevent ground loops from putting hum into the picture.

The antenna does not have to be special. A long wire or a dipole is enough for strong stations, and the quality of the picture depends mainly on the strength of the signal and on the noise on the band. Weather fax stations send on several frequencies in the low and middle HF bands, because the best frequency changes with the time of day and the season. The usual approach is to use a lower frequency for night and a higher one for day, and to try the frequencies in the published schedule of the station. Look up the current schedule of the issuing service, because the lists of stations and frequencies change over the years; the broadcast times and frequencies are published by the national weather services themselves, for instance the German weather service DWD, and they should be taken from the source and not from an old list.

Tuning It Correctly

The most common mistake is to tune exactly to the published frequency. Weather fax stations publish the centre frequency of the signal, often called the carrier frequency, and the transmitter sends it in single sideband with the suppressed carrier 1.9 kHz lower. If you set the receiver to upper sideband and tune it to the published frequency itself, the centre of the signal appears at 3800 Hz in the audio instead of 1900 Hz, and the decoder cannot find the tones. So the rule is to set the receiver to USB and to put the dial 1.9 kHz below the published frequency. For a station listed at 8,000.0 kHz, tune to 7,998.1 kHz. The result is that the black and white tones appear at 1500 and 2300 Hz in the audio. If your radio shows the frequency in a different way, you can check the tuning by looking at the waterfall of your decoder: the signal should be a narrow band between 1500 and 2300 Hz. The receiver bandwidth should be about 2.4 to 3 kHz. A narrower filter cuts the sideband tones and reduces the picture contrast, while a wider filter lets in noise. The relation between sidebands and frequencies is explained in our article on USB and LSB, and for this mode the choice is always the upper sideband, whatever the band.

Choosing the Software

A number of programs decode weather fax, and many of them are available at no cost. The program fldigi, which many digital mode operators already use, includes WEFAX modes with the usual speeds, and our guide to setting up fldigi shows how to connect the sound card. Other programs, such as dedicated radiofax decoders for Windows and Linux, offer extra features such as automatic slant correction, filtering and the storage of charts. For a first try, any of them will do. The important functions are the automatic start and stop on the tones, the correction of the slant and the choice of LPM and IOC.

A slanted picture is the most common problem. It means that the speed of the sound card clock differs slightly from the speed of the station, and each line starts a little later than the one before. A good program lets you adjust this by a few parts per million, or it detects it automatically from the phasing lines. If the picture is a ghosted, double image, the IOC or LPM is wrong. If it is stretched or compressed, the same settings are wrong. Check the start tone and select the matching IOC.

Matching the Schedule to Propagation

A weather service usually sends the same chart on several frequencies at once or in sequence, and the reason is propagation. A frequency of 4 MHz is useful for a ship a few hundred kilometres from the transmitter at night, while 12 MHz or more reaches ships on the other side of an ocean in daylight. As a listener you can use the same logic. Start with the lowest frequency of the station that is listed for your time of day, and if the signal is weak, try the next one up. Pay attention to the minutes before the start: a strong signal with the start tone is a good sign, and a signal that fades in and out in the middle of a chart will leave bands across the picture, so it is better to wait for the repeat of the chart on a better frequency than to keep a bad copy. Keep a short log of the frequency, the time and the quality of each reception, and in a few weeks you will know which channel suits your site best.

What the Charts Show

Most charts that are sent by radio are meteorological analyses and forecasts for the sea. The typical one is a surface analysis with lines of equal pressure, called isobars, high and low pressure centres marked H and L, and fronts drawn as lines with triangles or semicircles. Others show wind and wave forecasts, ice conditions, the temperature of the sea surface and upper air maps. The basic reading rules are simple. Isobars that are close together mean strong winds, and the wind blows nearly along the isobars with low pressure on the left in the northern hemisphere. A cold front marked with triangles moves in the direction that the triangles point, while a warm front is marked with semicircles. The Wikipedia article on weather maps explains the symbols and the notation. To make a useful forecast from the charts you should watch several in sequence, and compare the analysis with the later forecast charts.

The text broadcasts for sailors are a different service, such as NAVTEX on the medium frequencies, and they are described in the article on Navtex. Radiofax gives the picture, and the text services give the warnings and the written forecasts.

Getting Good Pictures

• Pick the right frequency for the time of day. The lower frequencies are usually better at night, and the higher ones in daylight, as the ionosphere changes.
• Reduce the noise. A noisy receiving site makes grain and streaks in the picture. A quiet antenna away from the house wiring improves it.
• Set the audio level carefully. A level that is too high clips the tones and ruins the grey scale, and a level that is too low increases the noise in the picture. Most decoders show an input meter.
• Use the automatic start where possible. The start tone and phasing make sure that the picture is aligned from the first line.
• Save the charts. A folder with the date and the name of the chart allows you to follow a front over several days.
• Compare with local observation. Check the pressure and the wind at your location against the chart, and you will learn how to read the symbols quickly.

Why It Is Worth the Trouble

Weather fax has value even in the age of the internet. It works wherever you can receive an HF signal, with no mobile phone coverage and no satellite subscription, which is the reason why sailors on long passages still use it. For the radio amateur it is a good way to test the receive chain of a station: if the charts come in clean, the antenna, the receiver and the interface work as they should. It also trains the ear and the eye to recognise the pattern of a signal, and it is a pleasant introduction to the world of utility stations, which our article on shortwave listening describes in more detail. Finally, the pictures are a good project for a club: a screen with a live chart at a public demonstration draws attention more than most of the other exhibits.

The Bottom Line

Weather fax, or radiofax, sends weather charts as audio tones in single sideband, with 1500 Hz for black and 2300 Hz for white, usually at 120 lines per minute and an IOC of 576. Set the receiver to USB, put the dial 1.9 kHz below the published frequency and feed the audio to a decoder such as fldigi. Use the current schedule of the weather service, pick a frequency that suits the time of day, keep the audio level moderate and correct the slant if the picture leans. The charts of pressure, fronts, wind and waves can then be read with the help of a basic guide to weather maps, and the whole project requires no transmitter.

Frequently Asked Questions

Do I need a licence to receive weather fax?

Listening and decoding do not require a transmitting licence in most countries, but check the rules in your own country. You only receive, and nothing is transmitted.

Which sideband should I use?

Upper sideband, with the receiver dial set 1.9 kHz below the published frequency, so that the subcarrier tones fall at 1500 to 2300 Hz in the audio.

What are LPM and IOC?

LPM is the number of lines per minute, normally 120. IOC is the index of cooperation, normally 576, which fixes the width of the picture.

Why is my picture slanted?

The clock of your sound card runs at a slightly different speed from the station. Use the slant correction of your decoder or adjust the sample rate by a few parts per million.

Can I receive weather fax with an SDR?

Yes. Any SDR that covers HF and has upper sideband can feed the audio to a decoder through a virtual audio cable.

Where do I find the current frequencies?

Check the broadcast schedule of the national weather service that issues the charts. Frequencies and times change, so always use the current list from the source.