Shortwave reception fading in and out? - UK Vintage Radio Repair and Restoration Discussion Forum

Voxophone · 20th Mar 2017, 2:02 pm

On all my sets with shortwave bands, I've noticed that the signal seems to fade in and out slightly during listening. Reception is strong and clear, but seems to drift off station at regular intervals every few seconds, causing the sound to become muffled or distorted.

Since this happens on all my sets and on all shortwave stations at both ends of the band, I'm guessing it's a problem with the reception in my area or the type of aerial I'm using.

Is there anything I can do to improve the reception to keep it steady? I've thought about building a tuned loop aerial for SW. Would this be likely to help?

I'd appreciate any suggestions.

Cheers

Liam

michamoo · 20th Mar 2017, 2:05 pm

It is just the nature of Short Wave listening. Maybe you are more used to listening to FM / DAB stations

Nuvistor · 20th Mar 2017, 2:14 pm

A short article which may help you understand what's occurring.
http://www.hamradioschool.com/t3a08-skip-signal-fade/

Frank

Sideband · 20th Mar 2017, 2:54 pm

It happens on MW as well particularly at the high frequency end. It's the nature of the transmission. We all accepted it back in 'the day' especially when listening to Radio Luxembourg on 208 metres. Much worse at night.

Voxophone · 20th Mar 2017, 3:06 pm

Thanks all. That's very useful.

I would have thought though that the varying signal strength could be evened out by something like an AGC circuit? Then again, I've noticed that some sets with shortwave omit AGC on the SW band only. Is there a reason AGC can't be used to minimise the varaition?

Nuvistor · 20th Mar 2017, 3:29 pm

AGC can only help within its range, if the varies outside that level it won't help.
The problem it cannot cope with is selective fading, I.e. Fading on the frequencies within the channel, that produces distortion.
Selective fading can be reduced by the receiver being able to receive one sideband.
Frank

russell_w_b · 20th Mar 2017, 3:44 pm

AGC in the listeners' receiver is the prerequisite upon which AMC (Amplitude-Modulated Companding) worked on the BBC HF bands. 6db compression is applied at the transmitter so that the peak of modulation never exceeds that of unmodulated carrier power.

This gratuitous reliance on receiver AGC was developed by the BBC in the 1980s and deployed on their HF transmitters (RIP, nearly...) from about 1992 to save money, as the radiated power never exceeded that of the unmodulated carrier. An increase in efficiency by typically 8% was noted.

I can't speak for the modulation methods other HF broadcasters, but the AMC system is well suited to PWM transmitters like the Marconi B6126 and I would expect the more modern DRM-capable transmitters to use something similar as a matter of course

20th Mar 2017, 6:27 pm

Selective fading, (a bit like a slow whah whah pedal on a guitar) part of the fun of shortwave. A decent SDR (Software Defined Receiver) will remove most of it. Although the use of shortwave is less than it was there are some quite interesting stations from the east (taken with a pinch of salt to counteract the propaganda) and lots of religious ones from America. Also given the right conditions the African 'local' tropical band http://www.pateplumaradio.com/genbroad/tropical.htm ones can be a breath of fresh air.

20th Mar 2017, 2:02 pm	#1
Voxophone Hexode Join Date: Oct 2015 Location: Sunderland, Tyne and Wear, UK. Posts: 340	Shortwave reception fading in and out? On all my sets with shortwave bands, I've noticed that the signal seems to fade in and out slightly during listening. Reception is strong and clear, but seems to drift off station at regular intervals every few seconds, causing the sound to become muffled or distorted. Since this happens on all my sets and on all shortwave stations at both ends of the band, I'm guessing it's a problem with the reception in my area or the type of aerial I'm using. Is there anything I can do to improve the reception to keep it steady? I've thought about building a tuned loop aerial for SW. Would this be likely to help? I'd appreciate any suggestions. Cheers Liam

20th Mar 2017, 2:05 pm	#2
michamoo Hexode Join Date: May 2006 Location: Cambridge, Cambs. UK. Posts: 469	Re: Shortwave reception fading in and out? It is just the nature of Short Wave listening. Maybe you are more used to listening to FM / DAB stations __________________ If smoking is so bad for you, how come it cures kippers?

20th Mar 2017, 2:14 pm	#3
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: Shortwave reception fading in and out? A short article which may help you understand what's occurring. http://www.hamradioschool.com/t3a08-skip-signal-fade/ Frank

20th Mar 2017, 2:54 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,566	Re: Shortwave reception fading in and out? It happens on MW as well particularly at the high frequency end. It's the nature of the transmission. We all accepted it back in 'the day' especially when listening to Radio Luxembourg on 208 metres. Much worse at night. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

20th Mar 2017, 3:06 pm	#5
Voxophone Hexode Join Date: Oct 2015 Location: Sunderland, Tyne and Wear, UK. Posts: 340	Re: Shortwave reception fading in and out? Thanks all. That's very useful. I would have thought though that the varying signal strength could be evened out by something like an AGC circuit? Then again, I've noticed that some sets with shortwave omit AGC on the SW band only. Is there a reason AGC can't be used to minimise the varaition?

20th Mar 2017, 3:29 pm	#6
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: Shortwave reception fading in and out? AGC can only help within its range, if the varies outside that level it won't help. The problem it cannot cope with is selective fading, I.e. Fading on the frequencies within the channel, that produces distortion. Selective fading can be reduced by the receiver being able to receive one sideband. Frank

20th Mar 2017, 3:44 pm	#7
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Shortwave reception fading in and out? AGC in the listeners' receiver is the prerequisite upon which AMC (Amplitude-Modulated Companding) worked on the BBC HF bands. 6db compression is applied at the transmitter so that the peak of modulation never exceeds that of unmodulated carrier power. This gratuitous reliance on receiver AGC was developed by the BBC in the 1980s and deployed on their HF transmitters (RIP, nearly...) from about 1992 to save money, as the radiated power never exceeded that of the unmodulated carrier. An increase in efficiency by typically 8% was noted. I can't speak for the modulation methods other HF broadcasters, but the AMC system is well suited to PWM transmitters like the Marconi B6126 and I would expect the more modern DRM-capable transmitters to use something similar as a matter of course __________________ Regds, Russell W. B. G4YLI.

20th Mar 2017, 6:27 pm	#8
Guest Guest Posts: n/a	Re: Shortwave reception fading in and out? Selective fading, (a bit like a slow whah whah pedal on a guitar) part of the fun of shortwave. A decent SDR (Software Defined Receiver) will remove most of it. Although the use of shortwave is less than it was there are some quite interesting stations from the east (taken with a pinch of salt to counteract the propaganda) and lots of religious ones from America. Also given the right conditions the African 'local' tropical band http://www.pateplumaradio.com/genbroad/tropical.htm ones can be a breath of fresh air.