Tuesday, November 17, 2020

Interesting signals on the 20 meter band: Probable Radio Habana Cuba transmitter malfunction - not jamming

 I happened to be looking at the various receivers at the Northern Utah WebSDR - as I'm wont to do (since I maintain them!) and noticed a few strange-looking signals that hadn't been there before:

Figure 1: 
Obvious QRM (interference) in the 20 meter amateur band.  The signal repeated every 66 kHz or so, allowing its source - below the 20 meter band - to be easily divined. 
Click on the image for a larger version.

The first thing that I did was to check other receivers - both on-site and across the U.S. - to make sure that this wasn't some sort of local problem (overload, image, nearby source) and found it elsewhere - but the selective fading visible in the waterfall display made me quite sure that this was ionospherically propagated and not local.  The errant signal was practically nonexistant in the Eastern U.S. - but with the known skip distance of 20 meters, that might have meant that those receivers were closer to the source, geographically.

When tuned in using AM, there was a very obvious audio tone (approximately 363 Hz) associated with the signal with a vestige of distorted speech underneath and the RF signal itself wasn't stable frequency-wise.  The tell-tale sign that this was more likely a spurious signal of some sort was the fact that this seemed to appear at intervals - roughly 65-70 kHz - so I decided to "follow the money", tuning lower in frequency and finding stronger and stronger instances.

Figure 2:
YouTube clip with audio from the errant spurious signal.  This clip - from one of the instance of spurious signal "nearby" the original - clearly contains Spanish-language audio - a clue as to a possible source!

Adjacent to the 20 meter amateur band is the 22 meter Shortwave Broadcast Band, and there I found the culprit:  A Radio Habana Cuba signal with the same sort of tone on it, symmetrically flanked by the same sidebands.  Using the TDOA feature of the KiwiSDR network clinched the diagnosis:  I tuned to one of the lower-frequency components of this signals, ran the analysis and came up with the results, below:

Figure 3:
  Several TDOA runs on the WebSDR network yielded the same results:  The errant signal appeared to be coming from western Cuba.  The main signal was not actually on 13563 kHz:  It was slightly higher up the band (probably 13700 kHz) - I just picked this particular spurious component because it was one of the strongest ones and "in the clear" - not atop another signal. 
Click on the image for a larger version.

Clearly, the program material matched the location!

While writing this, the spurious signal suddenly disappeared at around 1503 UTC:  Perhaps someone noticed the problem and switched the errant transmitter off (or fixed something) - or maybe whatever it was that had been failing finally gave up the ghost?



The same problem was noted again on 18 November (during the 1500 UTC hour) with spurious signals appearing on the 22 and 19 meter shortwave broadcast bands with interference again appearing in the 20 meter amateur band.  Again, the KiwiSDR TDOA network showed the likely source of the signal to be Cuba.

Either the folks at Radio Habana Cuba are unaware of the problem, or don't care enough to fix it/curtail transmissions to avoid causing issues across the HF spectrum!

The most likely source of the interference is the transmitter on 13700 kHz as it is symmetrically flanked with spurious signals above and below, spaced about 68 kHz (variable).  There is clearly something wrong with the 11760 kHz transmitter as well based on its long-term issues of very poor audio quality.

This page stolen from ka7oei.blogspot.com


1 comment:

  1. Ahhh...That moment when you realize that hobbyists in another country can now know intimate details about your technical competence and/or maintenance budget. How funny!



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