Here's a short report I wrote recently on indoor noise in the FM broadcast band... but it also applies to other parts of the spectrum.
- - - - - - - - - Indoor Radio Noise in the FM Broadcast Band The impact of indoor noise sources for AM reception is well known, but less so for FM thanks to the FM receivers’ ability to hide the noise. But the noise masks weaker FM signals just the same – and probably digital HD as well - it just isn’t as apparent to the listeners why the signal is “weak”. And my strong impression is that local indoor noise sources have been the cause of many of our recent home-reception complaints. Last year I made an informal study of some of our listener and staff homes using a battery powered spectrum analyzer (a radio receiver with visual display of strength versus frequency) and a loop of stiff wire on a short length of coaxial cable as a pickup antenna. I wanted to get a better sense of the noise encountered in the 88-108 MHz range in a variety of indoor situations and see if it might explain the troubles these folks were having getting reliable radio reception. I found that the several urban apartments I checked were awash in noise, much higher than the background level in the parking lot outside. The strongest sources I noted were recently-manufactured “wall-wart” switch-mode power supplies used for charging b atteries in cellphones and digital cameras. Some made a broad “hash” while others produced a series of noise peaks on discrete frequencies through the band, probably related to the switching frequency. Some computer monitors, HDTV sets, and DVD players were also very noisy in the FM band, maybe from their power supplies as well but with their internal supplies it was impossible to be certain. Some personal computers and digital clocks and telephones were quite noisy in the FM band too. Suburban homes were quite noisy too – but with more square feet of room the noise sources were more “spread out”. Same line-up of noise sources, and similar increase in the overall noise from outside in the driveway to indoors. Urban offices on our University campus were checked too: they were much noisier in the FM band than outdoors on the adjacent sidewalk. I found it harder to find the specific causes of the noise in this environment. Some computers were noisy, as were quite a few of the telephones. Printers with their switching-mode power supplies were cranking out the noise as well. But the noise in the office areas was harder to pin-point – I’m thinking there are more sources in this environment, on various floors and rooms, with more reflection and multipath on the noise signals from the metallic structures, all of tens to “blur” the source. Putting numbers o n this noise is difficult, as moving the antenna around greatly varies the absolute strength of the noise. Stepping back and thinking of my overall experience, across the whole band, in all the locations, it would be fair to say I saw as much as a 20 - 30 dB increase in overall noise going from outside to indoors. But even without hard numbers it is very clear that the modern indoor environment is much noisier in the 88-108 MHz range than the nearby outdoor locations. In all these situations I was able to find places to put the listener’s radio and antenna to get better reception. In most cases the listener had installed their radio on a shelf or table quite close to other electronics which were spewing out noise. I moved their radio out from among the noise sources and reception was naturally better. But of course we cannot expect all the listeners to have a broadcast engineer with test equipment come to their home and help their position their radio for best reception! -grin- Better control of this noise at the manufacturing end is vital. Steve WD8DAS [email protected] http://www.wd8das.net/ --------------------------------------------------------- Radio is your best entertainment value. --------------------------------------------------------- - - - - - - - - - ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
