How to Receive Marine Radiofax Charts
The National Oceanic and Atmospheric Administration (NOAA) runs the National Weather Service (NWS) which is the government agency responsible for monitoring and reporting weather conditions. Many of your local weather forecasts are made based on this information. The NWS has several methods of distributing weather info including the Internet, National Weather Radio (NWR) and via fax over radio. This is called Radiofax, Weatherfax or Wefax for short.
Radiofax works like the fax machine in your office but instead of going over phone lines it goes over radio waves in the HF part of the electro-magnetic spectrum. The technology is about a century old but is still used by ships at sea for weather updates. Radiofaxes can be received inland as well, making them an alternate source of information when other lines of communication are not available. For detailed information, visit these links:
Equipment specialized for this function can be purchased, or you can do it with fairly common equipment you may already have.
- A Linux, Mac or Windows computer with a sound card.
- The fldigi computer program for receiving digital modes, available for all platforms. It’s Open Source.
- A shortwave radio capable of receiving in USB mode (Upper Side Band) between 2MHz and 23.4MHz.
- A patch cable to get audio from your radio to your computer’s sound input.
- An antenna good enough to pull in at least a moderately strong signal at your location.
This guide is based on using fldigi in Linux Mint and a Sangean ATS-909 radio. Some details might change using other equipment but the concepts are the same.
Setup your radio and antenna. We need a relatively strong signal in order to receive a clear fax. If you’re close to one of the Radiofax transmitter sites, the telescoping antenna on the radio might be more than enough. You might even need to turn down the RF gain (if so equipped) or shorten the antenna if the signal is overloading the radio. If you’re hundreds or thousands of miles away, you’ll probably need more antenna. First, a little radio theory: Long distance HF radio transmissions are dependent on radio propagation. In a nutshell, radio waves reflect/refract through the ionosphere similar to how light bends through water. It’s how we can hear a station from far over the horizon. It’s a complex subject, just know that characteristics change based on time of day, season, weather, frequency and sun activity. That’s why there’s so many frequencies to choose from because what works great at noon probably won’t at midnight. If you’d like to dive into the details, check this article:
I’m in the Phoenix Metro area ~1000 miles from the nearest transmitters at Pt. Reyes or New Orleans. A simple coax-fed dipole antenna on the roof works well here. You might get a good signal with a few yards of wire thumb-tacked to the wall or you might need to go into the attic or outdoors with a longer run of wire. Generally, the longer and the higher the better. The exact length isn’t critical and you can make your own using spare wire and RG6 cable TV coax for feed-line. Watch out for power lines! If you can clearly receive at least one NIST time service transmission from the WWV on 2.5, 5, 10, 15 or 20 MHz then you should be good with the radio and antenna part.
Connect the line-out of your radio to the line-in of your computers sound card using a standard 1/8″ mini to 1/8″ mini cable. If your radio doesn’t have a line-out, use the headphone jack and set the radio’s volume low.
Tune in one of the Radiofax frequencies with a strong signal. Make sure your radio is set to Upper Side Band (USB) and tune 1.9kHz lower than the advertised frequency. Tuning 2kHz down is close enough. For example, the NWS station at Pt. Reyes is listed at 12786 kHz so tuning to 12784 kHz puts you right on target. At this point you should be hearing a fax signal, which can be best described as a repetitive digital pulsing sound that resembles a metronome.
Adjust your levels using your sound card’s control panel, activate the line-level input and set the levels the same way you would if you wanted to make a recording. If your control panel has meters, make adjustments so the sound level peaks at ~0db. Too high (in the red) creates distortion, too low will create noise in the image.
Install fldigi. The procedure is easy and intuitive. Most Linux users can find this within the Synaptic Package Manager while Mac and Windows users can find it at:
Start fldigi. From the top menu choose Op Mode > WEFAX > WEFAX-IOC576. In the button bar just above the waterfall display confirm that LPM is set to 120, Slant to .018 and tick the box next to the Align slider to Enable automatic image centering. If all is well, you should see a waterfall display and a chart slowly scanning in above it. Note that it can take up to ten minutes or more to receive some charts.
The top pane is where received images are displayed. Grid lines should appear vertically and horizontally, adjust ‘Slant’ if needed. There should be a black bar running vertically along the left edge, adjust ‘Align’ if it’s not where it’s supposed to be.
The bottom is the waterfall display. Those two red bands you see inching down the screen visually represent the fax signal, the left one is for black and the right one for white with varying shades of grey in between. Typically they’ll appear within the center 1/3 of the 0-3000hz scale.
Fine tune the radio and note how it changes the waterfall display. You may need to fine-tune the radio to compensate for noise and interference. Note the red bar along the 0-3000hz scale: it should span the area between the red bands, indicating fldigi is properly tuned in for best image contrast. This will normally happen automatically when AFC (Automatic Frequency Control) is enabled. Sometimes AFC gets confused by weaker signals and you may need to disable it and manually tune by hovering over the area with the mouse and clicking when centered.
At this point, just let it do it’s thing. Images take at least several minutes to complete and Fldigi will automatically save images in .png format so you can review them later.
List of WEFAX frequencies and locations for North America. Listed and (tune to) frequency.
2054(2052) – Kodiak, Alaska [call sign NOJ]
3253(3251) – Iqaluit/Resolute Canada [call sign VFF/VFR]
4235(4233) – Boston, MA [call sign NMF]
4292(4290) – Inuvik, Canada [call sign VFA]
4298(4296) – Kodiak Alaska [call sign NOJ]
4317.9(4316) – New Orleans LA [call sign NMG]
4346(4344) – Pt. Reyes CA [call sign NMC]
4416(4414) – Sydney – Nova Scotia, Canada [call sign VCO]
6340.5(6339) – Boston MA [call sign NMF]
6915.1(6913) – Sydney – Nova Scotia, Canada [call sign VCO]
7710(7708) – Iqaluit/Resolute Canada [call sign VFF/VFR]
8456(8454) – Inuvik, Canada [call sign VFA]
8459(8457) – Kodiak, Alaska [call sign NOJ]
8503.9(8502) – New Orleans LA [call sign NMG]
8682(8680) – Pt. Reyes CA [call sign NMC]
9110(9108) – Boston MA [call sign NMF]
9982.5(9981) – Honolulu [call sign KVM70]
11090(11088) – Honolulu [call sign KVM70]
12412.5(12411) – Kodiak, Alaska [call sign NOJ]
12750(12748) – Boston MA [call sign NMF]
12786(12784) – Pt. Reyes CA [call sign NMC]
12789.9(12788) – New Orleans, LA [call sign NMG]
16135(16133) – Honolulu [call sign KVM70]
17146.4(17145) – New Orleans, LA [call sign NMG]
17151.2(17150) – Pt. Reyes CA [call sign NMC]
22527(22525) – Pt. Reyes CA [call sign NMC]
23331.5(23330) – Honolulu [call sign KVM70]
For detailed listings of worldwide marine radifax broadcast schedules, get it straight from the source at:
The Art of Tuning: See the two red bands scrolling down the waterfall display? The one on the left represents Black and ideally should be at 1500hz. The other one is for White and should be at 2300hz. (note the 800hz spread). As you can see from the screencaps, my tuning is a technically a bit off. In practice, it usually isn’t quite that precise of a matter. At times you may need to vary your tuning slightly both on the radio and in fldigi to get the cleanest possible image.
The best way to get proficient at this is to understand the theory and practice!
I’ve also found it handy to pre-program these frequencies into radio memory. During the daytime, start at the highest and work your way down the list until you find a strong station. At night time, search low to high. Remember that thing about propagation? In the daytime, higher frequencies travel further and at night the lower frequencies work better.
Experimenting with fldigi to receive wefax transmissions is a great way to get familiar with digital transmissions and how to decode them. NWS wefax transmissions are of high power, occur on a regular timetable and are consistent in frequency making them easy to find.
– Kenn Ranous