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WEFAX Mode
Overview
Black and white weather facsimile charts referred to as ―WEFAX‖ by HF radio operators,
have been transmitted by shore stations for years and received by commercial ships,
yachts, and radio amateurs. By coincidence, the signal format used for WEFAX
broadcasts, 800 Hz shift key (FSK), is compatible with the modem housed within most
Kantronics TNCs (e.g., KPC-3, KPC-3 Plus, KAM Plus). In order to receive WEFAX
charts, which are typically hand drawn maps of atmospheric pressure, surface
temperature, or wind velocity/direction over a wide range of sea, you must use a special
program for your computer. Such programs enable your PC to receive 8 bit data from
the TNC and format it properly for display on your computer screen.
These maps, unlike satellite photos with a full gray scale range, are often produced by a
particular weather service at their own site. Two predominant stations are NAM, the
Norfolk US Navy Fleet Weather Service station, and CFH, the Canadian Forces Halifax
station.
Background: HF WEFAX Transmissions and Resolution
Full-size charts are eighteen (18) inches wide and vary in length from 10 to 18 inches.
In past years, many charts were generated on a rotating drum at 60 scan lines per
minute, but modern day charts are digitized (placed in computer storage) at the rate of
120 scan lines per minute on a flat-bed scanner. The scanner digitizes with a resolution
of 96 lines or pixels per inch. So, the maximum resolution of a hand-drawn synoptic
chart, when digitized, is nearly 1800 by 1800 pixels!
When the WEFAX maps and charts are transmitted on HF, some of the resolution is
lost. This occurs because the FSK modulation scheme used by all manufacturers of HF
WEFAX transmission equipment will not support the baud needed for full horizontal
resolution. The FSK format has been kept, however, to make today‘s equipment
compatible with that produced earlier, and received-map resolution is acceptable.
Finding WEFAX Broadcasts
There are approximately 50 commercial and governmental WEFAX transmitters located
in over 20 countries around the world. Most broadcast on HF (between 3 and 30 MHz).
Many broadcast continually while others are on a sporadic schedule.
These are some typical frequencies where WEFAX signals can be found, if ionospheric
conditions permit:
LOCATION FREQUENCIES (MHz)
Halifax, NS 4.275, 6.630, 9.890, 13.510
Norfolk, VA 8.080, 10.865, 16.410
San Diego, CA 8.646, 17.410
Mobile, AL 9.158
San Fran, CA 4.346, 8.682, 12.730, 17.151
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