Radio in the tunnel

[Aaron Read]

A. Joseph Ross wrote:
> Air America is in the top 20?
>  
>> Anyways, the article was about the minor bit of controversy caused
>> because someone decided to bump one station in favor of putting WFNX
>> on the system, which wasn't in the Top 20 of the ratings (presumably
>> the Arbitron 12+ rankings).   
> 
> I thought only AM stations couldn't penetrate tunnels.  I don't 
> usually find FM stations fading in tunnels.
> 

Well, being in a tunnel means you've got dirt, steel, power & 
communications cabling, concrete, etc. either providing signal 
attenuation or increasing the noise floor.  Different frequencies are 
more or less susceptible to attenuation by different materials, and AM & 
FM technologies are different on how much signal (above the noise) is 
required to successfully decode an audio signal (true for analog or 
digital).

AM signals are more susceptible to tunnel signal attenuation than FM 
signals are, but parts of the Big Dig are 120ft below street level 
(that's the area under Dewey Square)...unless you've got a very powerful 
signal hitting the street level to begin with...that 120ft is going to 
provide some serious shielding/attenuation.  Ditto for the Ted Williams 
which is roughly 90ft below sea level.

Concordantly, in the area near Government Center, where the Big Dig 
tunnel goes over the Blue Line tunnel, so it's so close to the surface 
that the roof of the tunnel is practically the street itself above. 
Many FM's can penetrate that.

The Sumner and Callahan Tunnels do have an added wrinkle in that they're 
(VERY loosely speaking) more-or-less in line with FM RF coming off the 
Pru...so you might not see much fading of Pru-based stations when you're 
in those tunnels; the walls act as a natural conduit for the RF and the 
Pru stations are all pumping out some hefty ERP to begin with.  You will 
see fading of most non-Pru stations, though.


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--------------------------
Aaron Read
readaaron@friedbagels.com
www.friedbagels.com
Boston, MA 02246

Fried Bagels - Broadcast Radio & Web
Engineering & Operations Consultant