We have looked at the free MultiPSK software before on this website when we showed how to use it to decode amateur SSTV pictures but with so many modes available on the program it can be used for so much more.
Having not looked at weather fax for a while and a little bored with RTTY and SSTV (if such a thing is possible) we thought it might be time to have a go at getting some nice HF fax pictures. No special interface is needed to decode fax provided you can wire the radio into the line in or mic socket on your computer although some sort of basic isolating transformer could stop stray noise from the PC interfering with your shortwave receiver.
Finding HF Fax Transmissions
Unlike decoding amateur radio digital signals which have to be hunted down throughout the various frequency allocations across the HF spectrum Fax signals have set frequencies and times. This makes it much easier and you’d be very unlucky not to find something worth listening to when you take into account the large amount of transmissions available.
For maximum coverage these signals are also backed up be a huge amount of power making the level of equipment to receive them effectively low and inexpensive provided your radio can receive sideband radio transmissions.
Starting down on 4.610 MHz we original found a signal that was just worth decoding but that annoying HF signal fading was making a mess of the fax picture. Hunting for something a little more stable we moved up to 8.040 MHz where the signal from Nortwood in the UK was coming in strong with almost zero fade.
For more info on where to listen to fax transmissions HF Underground have a wealth of information at their website.
How to use MulitPSK for Fax
Start up MultiPSK and after closing the configuration window select the Fax mode in the panel in the top left of the screen.
This will bring up a display window where your decoded signals are going to be displayed and the classic waterfall feature above to help with fine tuning the signal. Fax is very slow at giving a complete picture but this can be used to our advantage by being able to slowly tune the shortwave radio to give best results.
With every radio being slightly different and the offset needed to get a good signal when using a sideband mode you’ll have to move either side of the stated fax station frequency until the contrast between black/white on the fax picture is at its best.
Volume also plays a big part in the stability of the finished fax picture and even though its tempting at first to turn the audio from the shortwave radio up as high as possible this usually only results in distortion creeping into the decoding process. Start with the audio low then work your way up to what you can see is the optimum sound volume for your setup.
MultiPSK is an interesting free program that can take a little time to learn but with the amount of data modes it can be used for, it is in our opinion time well spent.