Digital broadcasting can provide excellent quality on shortwave. But recently I have discovered mysterious Radio Kuwait fading of some sort.
Increasingly, shortwave broadcasters are switching to digital, or more specifically, DRM. Most days at lunchtime, I tune into Radio Kuwait on 13.650 MHz. With a signal-to-noise ratio of 15 dB or better, audio quality is perfect. With digital modulation, you either get a perfect sound or nothing. Here is an example.
Recently I started to notice a mysterious Radio Kuwait fading that was unlike “normal” slow or fast fading. As shown below, the signal fade was quite periodic, with a significant drop roughly every six minutes. The fade would last about 3 seconds, and then normal strength would resume.
Commensurate with the SNR drop below 15 dB, the digital audio would also disappear for a few seconds. (Decoding is done with an AAC codec using I/Q data from my Flex 6300.)
I recorded 100 minutes of Radio Kuwait data and plotted it in the SNR graph shown at the top of this page. You can see the periodic fades with six minute separation, quite different from the normal slower signal ups and downs.
It’s still a mystery to me, as I cannot explain this with any types of the normal fading we expect on shortwave.
Mysterious Radio Kuwait Fading – Perhaps Not
So, what is causing the mysterious Radio Kuwait fading and dropout every six minutes? My conclusion is that the cause is not ionospheric fading. It is likely a problem in encoding software or at the transmitter site.
At first, I thought it more likely that Radio Kuwait is reducing power for a few seconds, or perhaps switching to a different transmitter or antenna periodically. But watching spectrum display on receiver did not indicate any change in power level.
So, that leaves some sort of bug in the encoding or decoding process. Somehow the digital frames are losing their lock periodically, resulting in the “fade”. This could be happening at the Kuwait end, or in my Dream decoding software.
Perhaps one of you has noticed these artifacts and can provide an explanation.