# Weather Fax At the end you have weather charts for ships as picture files on your computer. You also have the text warnings that coast stations send on 518 kHz. You need a shortwave receiver with upper sideband, an antenna and one free program. Spurious Emissions · Rev A · checked 2026-10-10 · https://spurious-emissions.net/guides/weather-by-radio/ Some links are affiliate links: if you buy through them, I may earn a commission at no extra cost to you. As an Amazon Associate I earn from qualifying purchases. ## 1. What this guide builds A receiver, an antenna and a computer that turn two marine broadcasts into files. The first broadcast is radiofax, also called weather fax: a weather chart sent as a sliding tone. The second is NAVTEX: short text warnings from coast stations, all on one frequency. | Signal | What you get | Frequency | Antenna | Radio | Decoder | Computer | |---|---|---|---|---|---|---| | Radiofax | Weather charts as pictures | Shortwave, 2 to 23 MHz. The list for the US is in section 7 | Wire or loop | Any receiver with upper sideband | fldigi, mode WEFAX-IOC576 | Any PC or Mac with a sound input | | NAVTEX | Text warnings and forecasts | 518 kHz | The same | The same, if it tunes to 518 kHz | fldigi, mode NAVTEX | The same | In this guide, "USB" always means the computer socket. I write "upper sideband" in full for the receiver mode. Your radio and fldigi label that mode "USB". ``` wire or loop -- coax -- receiver (upper sideband) -- sound -- computer -- fldigi dial = assigned frequency | minus 1.9 kHz for a chart +-- charts: PNG files dial = 517.000 kHz for NAVTEX +-- warnings: text ``` My own station does this with other parts. The receiver is an RX-888 Mk II with the program ka9q-radio. The decoders are small programs I wrote myself, and they are not public. Section 11 shows what you can do on that hardware with public programs. ## 2. What you need A part marked ◆ is what I use. A blank box is for your pencil. Prices are US dollars unless the cell says otherwise, as listed by the named seller on 2026-10-07. They change. ### The lowest-cost path Most licensed operators own every part of this path. It costs nothing new. | ☐ | Part | Job | Connector | Note | |---|---|---|---|---| | ☐ | A shortwave transceiver or receiver with upper sideband | Tunes one broadcast at a time | Your antenna socket | It must show the dial frequency to 0.1 kHz | | ☐ | A wire antenna for shortwave, or the antenna you transmit with | Receives the signal | As fitted | Outdoors is better than indoors | | ☐ | A sound path from the receiver to the computer | Carries the tone to fldigi | A USB cable if the radio has a built-in sound device. If not, a 3.5 mm cable to the computer's line input | Section 5 | | ☐ | A computer with Windows, macOS or Linux | Runs fldigi | | Any PC of the last 10 years. I did not measure the load | | ☐ | [fldigi](http://www.w1hkj.com) | Decodes charts and NAVTEX | | Free, license GPL-3.0. Version 4.2.13 was the newest on 2026-10-10 | What this path gives up: it hears one frequency at a time. You choose the frequency for the time of day, by hand. My station listens on four chart frequencies and on 518 kHz all day. ### What I use | ☐ | Part | Job | Connector | Price | Note | |---|---|---|---|---|---| | ☐ | ◆ [MLA-30+ active loop](https://www.amazon.com/dp/B095K89WND?tag=spuriousemiss-20) with its power injector | Shortwave and 518 kHz | SMA | See listing, Amazon | Mine is still indoors, the worst place for it | | ☐ | ◆ [Turn Island Systems F3060](https://turnislandsystems.com/product/f3060-filter-shelf-30-60-mhz-lpf/) low-pass filter | Removes signals above 30 MHz | SMA | $80, Turn Island Systems | Keep its shelf filter bypassed for 518 kHz | | ☐ | ◆ [TAPR RF Limiter](https://tapr.org/product/rf-limiter/) | Protects the receiver input | SMA | $20, TAPR | Needed if you also transmit | | ☐ | ◆ [RX-888 Mk II](https://opensourcesdrlab.com/products/rx888-mkii-16bit-sdr-receiver-radio-ltc2208-adc-upgrade-rx888?DIST=QU5PHQ%3D%3D) | Receives all of shortwave at once | SMA in, USB 3.0 out | $152.79, OpenSourceSDRLab, 2026-10-11 | Needs a PC and ka9q-radio | | ☐ | ◆ [ka9q-radio](https://github.com/ka9q/ka9q-radio) | Cuts one channel for each broadcast | | Free | The build steps are in [Build the station](/guides/build-the-station/), section 11 | What this path adds: every frequency at once, all day, with no hand on the dial. What it costs: a second guide to build it, and decoders that run without a screen (section 11). A narrow SDR is a third path. I did not test one for this job, so this guide gives no parts list for it. Section 5 says what such a path needs. ## 3. Before you start **Skills.** You tune a receiver in upper sideband. You install a program. You choose a sound device in a settings window. No command line is necessary for the lowest-cost path. **Time.** About one hour to the first chart, if a broadcast is on the air. A chart takes about 7 to 10 minutes. NAVTEX may need a wait until night. **Accounts.** None. **Values to choose first.** | Placeholder | Meaning | Example form | |---|---|---| | `` | The radiofax station nearest to your ocean | A call sign from section 7 | | `` | One assigned frequency of that station, in kHz | From section 7 | | `` | `` minus 1.9 kHz | Section 7 | | `` | Your grid locator, 4 characters are enough | fldigi uses it to name NAVTEX stations | **The law.** This guide is for reception only. Never transmit on these frequencies. I am not a lawyer, and this is what I read for the United States. The charts are works of the US National Weather Service, so they are in the public domain. NAVTEX is a safety broadcast for ships. US law (47 U.S.C. section 605) limits what a person may pass on from a radio message that is not meant for the public. I show that a warning arrived and what kind it was. I do not publish its text, because a warning can name a vessel or a person. ## 4. The antenna and the radio-frequency chain Both broadcasts are strong signals made for small ship antennas. A simple antenna is enough. Its position matters more than its type. 1. Use the shortwave antenna that you have. A dipole, an end-fed wire or a vertical all work for reception. 2. If you have no antenna, put a wire of 10 m or more outdoors, as high as you safely can. 3. Keep the antenna away from the electronics in the house. Switching power supplies and network equipment make most of the noise. 4. Connect the antenna to the receiver with 50 ohm coax. > **Caution.** An outdoor antenna cable can carry a surge into your equipment. Ground the cable where it enters the building, and bond that ground to the building's own grounding electrode system. In the US the rule is National Electrical Code Article 810. Use a licensed electrician if you are not sure. ### If you use the active loop Build the chain in this order, antenna first. ``` loop -- RG-6 -- power injector -- F3060 (30 MHz low-pass in, shelf filter bypassed) -- RF limiter -- RX-888 Mk II, HF input ``` - Put the injector first on the loop cable. Every part after it then sees no DC. - Keep the F3060 shelf filter bypassed. It cuts 28 dB below 1 MHz, and NAVTEX is at 518 kHz. - For the loop, height matters little. Distance from the electronics in the house matters a lot. ### 518 kHz is not shortwave NAVTEX is just below the AM broadcast band. Check three things before you expect a signal. 1. Check that your receiver tunes to 517 kHz in upper sideband. Many general-coverage transceivers do. Some are less sensitive there. 2. Check that no high-pass filter or broadcast-band filter is in the antenna line. 3. Expect a weak signal from a short antenna. A long wire or an active loop helps at this frequency. ## 5. The sound path into the computer fldigi decodes sound. It needs the receiver's audio on a sound input of the computer. ### A radio with a built-in sound device 1. Connect the radio to the computer with a USB cable. 2. Install the radio maker's driver, if the maker's manual asks for one. 3. Result: the computer shows a new sound input with the radio's name. ### A radio with only a headphone or line output 1. Connect the radio's line output or headphone socket to the computer's line input with a 3.5 mm cable. 2. Use the line input, not the microphone input, if the computer has both. 3. Set the radio's volume low. Raise it in section 6 until fldigi shows a signal with no overload. ### An SDR An SDR has no sound output of its own. Two more programs do that job. - An SDR program that tunes the SDR and demodulates upper sideband. - A virtual audio cable: a program that makes a sound output of one program appear as a sound input of another. Set the SDR program to upper sideband with a filter of 3 kHz or wider. Send its sound to the virtual audio cable. Select the same cable as the input in fldigi. I did not run this path, and I name no programs for it (section 16). ### Receiver settings for a chart Set these on the receiver before you start fldigi. | Setting | Value | Why | |---|---|---| | Mode | Upper sideband | The schedule says so | | Filter | 2.4 kHz or wider, 3 kHz if you have it | The tone moves from 1500 Hz (black) to 2300 Hz (white) | | Noise blanker and noise reduction | Off | They can damage the tone | | Automatic gain control | Slow, or as you like it | I did not compare the settings | | Transmit | Disabled, or the microphone unplugged | Reception only | ## 6. Install and set up fldigi fldigi is a free program for many amateur digital modes. Two of its modes do this job: WEFAX-IOC576 for charts and NAVTEX for the text warnings. 1. Download fldigi from or from . | System | File on 2026-10-10 | |---|---| | Windows | `fldigi-4.2.13_setup.exe` | | macOS | `fldigi-4.2.13.dmg` | | Linux | Your distribution's package, or build `fldigi-4.2.13.tar.gz` | 2. Install it and start it. 3. Result: on the first start, fldigi may show a setup wizard. It makes a working folder: `fldigi.files` in your user folder on Windows, `.fldigi` in your home folder on macOS and Linux. 4. Open the menu **Configure**, then **Config Dialog**. 5. Open the page for the operator. Enter `` in the locator field. A callsign is not necessary for reception. 6. Open the sound card page. Select **PortAudio**. On Linux with PulseAudio, the fldigi manual recommends PulseAudio. 7. Select your receiver's sound input as the capture device. 8. Save the configuration and close the window. 9. Tune the receiver to any busy frequency. 10. Result: the waterfall at the bottom of the fldigi window shows moving signals. The waterfall is a picture of the sound: frequency from left to right, time from top to bottom. 11. Adjust the sound level. The waterfall background must be dark, and strong signals must be bright. A waterfall that is bright everywhere means too much level. If the waterfall stays black, the sound does not arrive. Go to section 14. ### The two settings pages for this guide Both pages are in the Config Dialog, under **Modem**. The defaults are correct for US stations. Read them once. | Page | Setting | Default | Meaning | |---|---|---|---| | Modem/Wefax | Center freq | 1900 Hz | The middle of the fax tone in the sound | | Modem/Wefax | Frequency shift | 800 Hz | Black is 400 Hz below the middle, white is 400 Hz above. The fldigi manual names one German station that uses 850 Hz | | Modem/Wefax | Max Image Rows | See the page | fldigi saves the picture and starts a new one at this number of lines | | Modem/Wefax | Filter | Start with medium (fldigi manual) | Narrow, medium or wide | | Modem/Wefax | Fax images destination directory | The `images` folder in the working folder | Where the charts go | | Modem/Navtex | Log Navtex messages to Adif file | Off | Keeps each message in the fldigi log file | ## 7. Find your station and its schedule The US National Weather Service makes the charts. Four US Coast Guard stations and one Defense Department transmitter site in Honolulu send them. One document lists every radiofax station in the world: "Worldwide Marine Radiofacsimile Broadcast Schedules", . The copy I read is dated March 7, 2025. ### The US stations Assigned frequencies in kHz, and the hours (UTC) for a frequency that is not on all day. | Station | Call sign | Ocean | Assigned frequencies | |---|---|---|---| | Point Reyes, California ◆ | NMC | North-east and tropical Pacific | 4346 (0140 to 1608), 8682, 12786, 17151.2, 22527 (1840 to 2356) | | Kodiak, Alaska | NOJ | North Pacific, Alaska waters | 2054, 4298, 8459, 12412.5 | | Honolulu, Hawaii | KVM70 | Central Pacific | 9982.5 (0519 to 1556), 11090, 16135 (1719 to 0356) | | Boston, Massachusetts | NMF | North Atlantic | 4235 (0230 to 1039), 6340.5, 9110, 12750 (1400 to 2239) | | New Orleans, Louisiana | NMG | Gulf, Caribbean, tropical Atlantic | 4317.9, 8503.9, 12789.9, 17146.4 (1200 to 2045) | I receive Point Reyes. In about a week, three of its frequencies gave me charts: 4346, 8682 and 12786 kHz. 17151.2 kHz gave none. I do not listen on 22527 kHz. ### How to find your own nearest station 1. Open . 2. Read the table of contents. The stations are in groups by region of the world, then by country. 3. Go to the stations on your ocean. Each entry has the call sign, the frequencies, the hours and the list of charts. 4. Choose two or three stations within a few thousand kilometers. Shortwave does not always favor the nearest one. 5. Write down each `` frequency and the hours when it is on. Some entries in that document are old. The document says so itself: its schedules came "from official and unofficial sources". For a US station, the station's own schedule file is the better source. The one for Point Reyes is . The chart pages of all five US stations are at . ### The tuning offset: 1.9 kHz The schedule gives the **assigned frequency**. That is the middle of the fax signal. A single-sideband receiver shows the **carrier frequency** on its dial. Every US schedule has the same note: the carrier frequency is 1.9 kHz below the assigned frequency. ``` = - 1.9 kHz, upper sideband ``` With this dial setting, the middle of the fax tone is at 1900 Hz in the sound. That is the default of fldigi. | Station | Dial frequencies in kHz, upper sideband | |---|---| | NMC ◆ | 4344.1, 8680.1, 12784.1, 17149.3, 22525.1 | | NOJ | 2052.1, 4296.1, 8457.1, 12410.6 | | KVM70 | 9980.6, 11088.1, 16133.1 | | NMF | 4233.1, 6338.6, 9108.1, 12748.1 | | NMG | 4316.0, 8502.0, 12788.0, 17144.5 | ### Which frequency at which hour Every frequency of a station carries the same chart at the same time. Choose the one that reaches you. - At night, try the low frequencies first (2 to 6 MHz). - By day, try the high frequencies first (12 to 22 MHz). - Near sunrise and sunset, try the middle (8 to 9 MHz). ### When a chart starts The Point Reyes schedule of 2026-10-10 has six blocks a day. Each block starts with a test pattern at these times (UTC): 0140, 0655, 1120, 1400, 1820 and 2320. After the test pattern, a new chart starts about every 10 minutes. The schedule file names each chart: satellite pictures, surface analyses, wind and wave forecasts, 500 millibar charts. At 1124 and 2324 the station sends its own schedule as a chart. ## 8. The first chart ### What you hear A chart has four parts. Learn the sound of each one. | Part | Sound | Length | Job | |---|---|---|---| | Start tone | A rasp: black and white 300 times a second | About 5 seconds | Tells the receiver that a chart starts | | Phasing lines | A steady tone with a short click twice a second | About 30 seconds | Shows where each line starts | | The chart | A scratchy tone that repeats twice a second | About 7 minutes for 800 lines | 120 lines a minute, half a second each | | Stop tone | A rasp at 450 times a second | About 5 seconds | Ends the chart | ### Receive a chart from its start 1. Find the next start time of your station in its schedule. Start 5 minutes early. 2. Set the receiver to upper sideband. 3. Tune the receiver to ``. ``` Example for NMC on 8682 kHz: dial 8680.1 kHz, upper sideband ``` 4. In fldigi, open the menu **Op Mode**, then **WEFAX**, then **WEFAX-IOC576**. 5. Result: the text area of fldigi changes to a blank picture with a row of controls. The waterfall cursor moves to 1900 Hz. 6. Check that the **LPM** control shows `120`. Every US chart is sent at 120 lines a minute with IOC 576. 7. Check that **Auto** is on. fldigi then centers the picture from the phasing lines. 8. Wait for the start tone. Do not press a button. 9. Result: the label of the picture shows the state of the reception. It changes when fldigi hears the start tone, then the phasing lines. 10. Look at the picture. Lines appear from the top, two each second. The **Row** number counts them. 11. Wait for the stop tone. 12. Result: fldigi saves the chart as a PNG file and clears the picture for the next chart. IOC is the index of cooperation: a number that fixes the shape of the picture. All charts in the US schedules use 576. ### Start in the middle of a chart You tuned in late, and the picture stays blank. fldigi still waits for a start tone. 1. Press **APT**. fldigi stops the wait for the start tone. 2. Press **Phase**. fldigi stops the wait for the phasing lines and draws the picture now. 3. Result: lines appear, but the chart is split. Its left edge is somewhere in the middle of the picture. 4. Turn **Auto** off. 5. Change **Align** until the edge of the chart is at the edge of the picture. The **Cont'** control sets continuous reception. In that mode fldigi does not wait for a start tone. It cuts the picture into pieces of equal length (the Max Image Rows setting). ### Phasing: why a chart comes out split The sender and your computer do not agree on where a line starts. The phasing lines fix that. Each one is black with one short white pulse at the line start. fldigi finds the pulse and puts it at the edge of the picture. If noise hides the phasing lines, the chart is whole but shifted: the right part is on the left. Use **Align** to move it. ### Slant correction: why a chart leans The sender's clock and your sound device's clock do not run at exactly the same rate. Each line then starts a little early or a little late. Over 800 lines the small error adds up, and every vertical edge leans to one side. 1. Receive a chart with a clear vertical edge. The border of a chart is a good one. 2. Look at the edge. If it leans, change **Tilt** by a small step. 3. Look at the next lines, or at the next chart. If the lean is larger, change **Tilt** in the other direction. 4. Repeat until the edge is vertical. 5. Result: fldigi keeps the Tilt value in its configuration. You do this one time for each sound device. fldigi can also measure the clock error of the sound device with its WWV mode. The fldigi manual has the steps. I did not run them. ### Save the chart fldigi saves a chart by itself when it detects the end. You can also press **Save** at any moment to save the picture as it is. | Item | Value | |---|---| | Folder on Linux | `$HOME/.fldigi/images` | | Folder on Windows | `fldigi.files\images` in your user folder | | Folder on macOS | The `images` folder in `.fldigi` in your home folder. I did not confirm this one | | File name | `wefax__