I couple of posts ago I showed you how poor shortwave radio propagation was as a storm passed over the UK – so I thought I’d show you ‘what good looks like’!
Here is what an FM hamfax image looks like, transmitted from my home in south-east England tonight, and received across at the University of Twente, Enschede, Netherlands:
For comparison I also transmitted this image using AM hamfax too:
The difference in picture quality between the Amplitude-Modulated image (lower) and Frequency-Modulated image (upper) is stark even when the shortwave band conditions are great, as they are tonight.
The AM image shows distinct banding where my signal faded occasionally as it was reflected by the upper atmosphere. Compare that to FM where you notice a small increase in noise corresponding with the deepest amount of signal fading.
Given that the AM fax receiving software draws fainter signal as ‘blacker’, the slightest variance in signal strength is noted in the resulting image. Since the information contained in an FM signal is nothing to do with signal strength, only the most severe changes caused any disturbance.