[AMRadio] Local neighborhood radio noise

sbjohnston at aol.com sbjohnston at aol.com
Sun Sep 27 12:19:27 EDT 2009

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.

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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 

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 

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

sbjohnston at aol.com
Radio is your best entertainment value.

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