I’ve had a couple of emails about the FAX transmission test I performed a couple of days ago, where I was highlighting the challenging band conditions while a low pressure weather system passed over the UK, suspending summer for a few days.
I’m glad to say that we are back to (near) summer conditions again, and the messages wanted to learn more about what I was doing.
The FAX software I use is called HamFax, available for package installation on Linux and downloadable for Windows. More on this great little piece of software available from http://hamfax.sourceforge.net
FAX is usually a fairly resilient image transmission medium as long as it is set correctly. For amateur radio purposes, here are the settings:
…although note you can use ‘color’ [sic] or ‘mono’; colour taking twice exactly 3 times as long to transmit as monochrome, since each line is transmitted three times – each for red, green and blue components.
The other setting you must use is either ‘Adjust IOC’ or ‘Scale to IOC’ in the Image menu once you have loaded your image. IOC means ‘Index of Cooperation’, an agreed line width format that both sender and receiver of the FAX must use so as to not cause aspect ratio problems with the image. If the IOC value is not the same at both ends then, for example. any circles in the image end up being be stretched into ovals.
There are two agreed IOC values – 288 and 576. Amateur radio faxes tend to be sent with an IOC of 288, and some weather faxes found elsewhere on shortwave can have the IOC 576 setting. ‘Adjust IOC’ or ‘Scale to IOC’ requests a value – use 288 – and then either scales your loaded image up or down to match. ‘Adjust IOC’ stretches the image width without adjusting the height, and ‘Scale to IOC’ adjusts both, keeping the original image’s aspect ratio.
When you transmit a FAX with all these settings, you will use 800Hz of bandwidth centred at 1900Hz. Use USB (upper sideband) or the receiver will get an inverted shaded FAX.
There’s one very interesting area to look at, and that’s whether to use Frequency Modulated (FM) or Amplitude Modulated (AM) FAXes. Let me show you the difference with some FAX images received from audio that has been affected by band conditions. It’s pretty clear that FM gets through! The downside? Not much really – both use 800 Hz (AM could be half that but it transmits two sidebands).
AM fails more because shortwave radio paths tend to cause phase distortions and signal fading. FM is more resilient to these effects, which makes sense when you think about it> Imagine that for each picture element being scanned and transmitted:
- AM transmits ‘black’ as no signal and ‘white’ as maximum signal. So on fading paths where the signal strength ebbs and flows or where the phase gets distorted then the picture elements are going to variously darken and brighten regardless of the image being transmitted.
- FM transmits ‘black’ on a low frequency and ‘white’ on a high frequency – both with maximum signal. As you can see from the images compared below, it is much less susceptible to fading and phase distortion.
(same scale –
see FM for scale)
So why use AM FAX at all? Apart from the fun of experimenting with various modes, AM can used where a transmitter has limited capability for transmitting for a long time. FM requires transmission at maximum power for the full duration of the FAX send – up to 15 minutes (after which a callsign must be transmitted).
Older ham radio equipment may find 15 minutes on full power a challenge and overheat. My own very modern transceiver pulls over 30 amps of 12v DC power to transmit at 100 watts of radio input power to the antenna. I use a large caravan-style leisure battery to take the load and offer a smooth DC current. AM FAX transmissions use far less power because they only need full signal to transmit the ‘whitest’ pixel elements. An AM picture fax is more likely to take up half the average transmitter power of FM.
For more on FAX please have read of this great Wikipedia entry: https://en.wikipedia.org/wiki/Radiofax