Unexpected Tunnel Static

June 7, 2006 11:24 AM

I use an FM transmitter for my MP3 player. When I went through a tunnel in Pittsburgh this past weekend, I thought that the reception would have come in crystal clear since most of the radio interference would be missing. Instead it fuzzed out the same way a radio station in a tunnel does. Why?
posted by Hubajube to technology (12 comments total)

It could have been interference from electrical in the tunnel.
posted by nathan_teske at 11:28 AM on June 7, 2006

It's gotta be something about electrical interference. I use the same type of device and I get static when I pass by large transformer boxes or under high voltage wires.
posted by NationalKato at 11:31 AM on June 7, 2006

Does the tunnel have an emergency broadcast system that overrides all the radio bandwidth?
posted by neustile at 11:41 AM on June 7, 2006

Pittsburgh, being the home of Westinghouse and all, has for a long time prided itself on pumping radio into its tunnels.

Since the 70's your AM and FM radio continues uninterrupted in many of the tunnels. I'm betting that that is why, there is probably more radio interference in the tunnels.
posted by StickyCarpet at 11:42 AM on June 7, 2006

I know that here in MD, when we go through the Ft. McHenry tunnel, we pass under what look like antennae... and our FM tuner goes static momentarily, as well as the speakers that are connected to the tv. I'm not sure what it is, but there are these antennae at each end of the tunnel.

FWIW, I've never experienced any emergency broadcasting in the tunnel. I dont' know if it exists.
posted by TuxHeDoh at 11:47 AM on June 7, 2006

For what it's worth, I get the same thing when driving through the Squirrel Hill tunnel here in the 'burg. So it's not just you. Actually, I just got a new car with an Aux plug so I don't use the transmitter anymore but when I did, I got the same effect. They do pump FM radio through that tunnel so it's probably what sticky carpet said.
posted by octothorpe at 11:54 AM on June 7, 2006

I'd suspect an RF field of some sort, sufficiently close to the FM band (88-108 MHZ) to cause your FM radio automatic gain control (AGC) to drastically desensitize your radio's "front end", resulting in an apparent loss of signal.

Why there would be such field, I am at a loss for explanations. It would have to be a lot stronger than your FM transmitter (which is probably 1 to 3 milliWatts). Perhaps the tunnel operator has something using RF that has a harmonic in the FM band, or at least something that fools the radio into thinking it's in a strong field.

There is a balance between selectivity and sensitivity in radio receivers and gain adjustments are necessary. AGC is used to compensate for varying signal strengths.

Perhaps it's part of the explanation. To find out, a spectrum analyzer would be used to show a visual representation of the signal strengths at various frequencies.
posted by FauxScot at 1:56 PM on June 7, 2006

The quality of signal that you receive is determined by the amount of signal as compared to the noise and the amount of interference. Or in mathematical terms Signal Quality ~ C/(I+N) where C is Signal (Carrier), I is Interference, and N is Noise. The ratio is typically measured logarithmically (in dB), and if C, I and N are experessed logarithmically (in dBm), then simple math can be used for the calculations (substraction for division, addition for multiplication)
So if without a tunnel you have a Signal Quality of ... let's say C=-85 dbm and (I+N) =-110 dBm = -85 - (-105) = 20 dB, when you go through a tunnel (assuming in this case there are no transmitters in the tunnel), both C and I are decreased by approximately the same amount. N typically does not decrease being (somewhat) fixed. So it might end up being something like C=-105 dbm (I+N)=-115dbm = -105-(-115) = 10 dB. A significant decrease in signal. This is because the interference was reduced, but not the noise
... I waved my hands on the I+N figure, because I don't have time to do the math ... but trust me the concept is accurate. What this means is that even though interference is reduced in a tunnel, the signal is reduced an equal amount, and even if you eliminate interference you still have to overcome the noise floor. Thus your signal quality will always be worse in tunnels (unless someone builds a transmitter in there).

Likewise if you pass close to an interfering source, the I factor can become the dominating factor and your signal quality can suffer.

I'd work on making this easier to understand, but have to run now.
posted by forforf at 2:05 PM on June 7, 2006

Oh, nevermind ... I misunderstood the question ...
posted by forforf at 2:05 PM on June 7, 2006

I remember reading about this a while ago so here you go, probably more info than you want, but here's the story. Radio=fuzz. Still, it's pretty nice for those of us listening to the radio who have to go thru the tunnels on a regular basis.
posted by nnk at 6:29 PM on June 7, 2006

hmm. The link's not working. I'll try again
posted by nnk at 6:34 PM on June 7, 2006

Bingo....

The nice folks in the article are re-transmitting locally acquired signals inside the tunnel. Your front end is probably getting desensitized, as I suspected.

I'd relocate the iPod transmitter as close as possible to the radio antenna and see if the problem goes away next time you go through the tunnel, (if ever). If it improves, then that's the culprit.
posted by FauxScot at 6:09 AM on June 8, 2006