Ray, W2RJJ, presented the following on the air during the Millennium Net on Sept 15, 2000
HF WEFAX ON-AIR PRESENTATION - SIX-METER MILLENNIUM NET
Download the Wefax software discussed in this article
This presentation is made to introduce fellow radio enthusiasts to the technology for receiving and displaying radiofacimile transmissions, specifically those transmissions know as High Frequency Weather Facimile. This is more commonly called HF Wefax. We've all had the experience of wondering about those strange electronic sounds that seem to appear all over the short-wave spectrum. In the ham bands we recognize many of those sounds as Pactor, RTTY, and packet. But there are many more, unfamiliar sounds heard in between the ham bands. Many of those odd sounding signals are radiofacimile transmissions, and today most ham shacks and SWL listening stations can easily capture and decode their mysterious message.
Radiofacimile is a technology which permits sending maps, charts, and photos to a distant point using radio waves. The technology has been used by news organizations and the military going back to the first half of the 20th century. Today, its use is critical for the maritime community, where the safe passage of ships upon the world's oceans is dependent upon oceanographic and weather forcasts and warnings. The high seas is not an environment where internet access is practical. HF radiofacimile allows the transmission of graphical information in the form of detailed weather and sea surface maps, and retransmission of satellite photos. For the radio amateur and SWL alike, decoding and studying these transmissions can be a fascinating aspect of the radio hobby.
Weather charts originate with the National Weather Service which collects and analyzes weather and oceanographic data for dissemination to the maritime community. Current conditions and forcasts are assembled into a suite of maps, charts and satellite photos which are transmitted to ships at sea throughout the day and night. Transmitting facilities are provided by the United States Coast Guard.
The high frequency AM transmission contains a frequency modulated audio tone subcarrier. It is the subcarrier which contains the image data for displaying the weather chart. The subcarrier is frequency modulated between a 1500hz tone which represents black, and a 2300hz tone which represents white. Tones will be modulated between these two extremes when it is necessary to convey gray-scale graduations, such as for displaying satellite photos. Software running on your PC works with the sound card to convert the received tones into gray-scale values, which are then used to print the image on your monitor.
When radiofacimile is transmitted over HF, frequency modulation of the subcarrier is used in order to immunize the image data against the effects of signal fading which is common on HF. If amplitude modulation were used to convey data, false gray-scale values would result due to unpredictable conditions in the atmosphere which normally vary the amplitude of short wave signals. Image data at the receiving station would be too corrupt to be useful. FM modulation of the subcarrier avoids this problem by relying on tones, rather than amplitude, to convey image data.
In order to receive the FM audio subcarrier, your receiver is set to USB mode and is tuned 1.9khz below the published frequency. That is, you take the published frequency in kilohertz, and subtract 1.9. The difference is the frequency to which your VFO is tuned.
When you are listening for a radiofacimile transmission, you will hear the following transmit format:
1> 300hz start tone for 5 seconds - alerts receiving equipment that a frame is coming;
2> phasing interval for 5 seconds where the 1500hz black tone is interrupted by pulses of the 2300hz white tone- this allows the decoding system to synchronize to the upcoming frame;
3> frame - this is the image transmission which lasts from 10-16 minutes depending on the image. Weather charts are transmitted in less time than a satellite photo;
4> 450hz stop tone for 3 sec - this signals the end of the transmission.
The start tone, phasing interval, and stop tone described above allow automated, unattended reception and storage of weather charts when using software such as WXsat.
HF Wefax Receiving Station
Most amateur and SWL radio stations with a late model computer can become Wefax receiving stations.
The following are needed:
1> a general coverage shortwave receiver with an antenna suitable for shortwave reception;
2> a pentium class pc with a sound-blaster compatible sound card;
3> a shareware program called WXsat. WXsat is a pc/Windows-based program that listens to the pc sound card. It converts digital image data from the sound card to the image you see on the screen;
4> a cable that connects the receiver's audio-output to the sound card's microphone or line-level input. This most likely will be a two-wire cable with mini-plugs on either end- center-pin connects to center-pin; shield connects to shield. PSK31 operators already have this cable in place.
Those are the components for the HF Wefax receiving station.
A Word About WXsat
WXsat may be downloaded in the following manner. Locate the transcript for this presentation on the 6-Meter Millennium Net web site. The URL is WWW.QSL.NET/KB2ZPE. There you will find a link to the WXsat download site. It takes only several minutes to download and install WXsat. It comes with a comprehensive, easy-to-read, on-line user guide. This software will allow manual and unattended fax reception.
Here are the steps to listen:
1> Adjust the soundcard receive audio level using the testing tool built into WXsat. The audio level is adjusted by tweaking the windows mixer and rf gain (or vol.) control on the receiver. WXsat will tell you when you have set the correct audio level;
2> Put WXsat into the proper decode mode. There are 12 modes. The correct mode for HF WEfax is FM120;
3> Put WXsat into the recording mode;
4> Tune the VFO to the radiofacimile frequency and wait for a transmission. As you listen to the fax being received by your radio, you will see the weather chart appear on your monitor as each line of data is turned into an image by WXsat.
Where to Listen
I will list four (4) frequencies used by the USCG to transmit weather data to vessels navigating the North Atlantic Ocean. Transmissions on these frequencies are made from station NMF in Boston, and are easy to copy throughout the day. Tune your VFO to these frequencies:
6338.6 khz 9108.1 khz 4233.1 khz 12748.1 khz
The following frequencies are used by the Canadian Forces Metoc Centre and are transmitted from Halifax, Nova Scotia:
4269.1 khz 6494.5 khz 10534.1 khz 13508.1 khz
I suggest that you store these frequencies into memory channels on your receiver. This way you can quickly toggle between each frequency and determine which is active and which is delivering the strongest signal.
Once you are a committed HF radiofacimile listener, you may find additional frequencies used by stations from all over the world on the internet. I can also send you my frequency list via email.
This concludes the Wefax presentation. Thank you for listening. This is W2RJJ.
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