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My lady and I were driving on the interstate recently when we noticed an unusual phenomenon. We were listening to the radio (an FM signal), and only over a couple of bridges did we entirely lose our signal. I had some ideas explaining this but I'm not sure, so I am asking for your help. What caused this?

To clear things up a bit:

  • It happened over three bridges in a thirty mile stretch. Each was over a river (not sure if that matters for reasons of over-water architecture or aquatic radio effects), but we crossed several other bridges that did not cause this phenomenon.
  • Our signal was excellent the whole drive except on these few bridges, when it suddenly disappeared (white noise) and reappeared immediately after crossing.
  • The bridges were concrete with no supports from above, no street lights, etc. There was no difference in obstructions above the vehicle for the entire drive.
  • I have spent a lot of time driving with the radio on and have never experienced such a loss of signal with no explainable obstruction. I can only assume that the construction of these few bridges is unique.
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  • $\begingroup$ One possibility: en.wikipedia.org/wiki/Tunnel_transmitter $\endgroup$ – Esteban Jul 15 '15 at 18:11
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    $\begingroup$ Were the bridges about the same height off the water? If so, you might have gotten a near-null from interference. You might ask over on the Ham SE site. $\endgroup$ – Jon Custer Jul 15 '15 at 18:12
  • $\begingroup$ @JonCuster There's an SE for AR? Hmm.. time to wander that way... $\endgroup$ – CoilKid Jul 15 '15 at 19:26
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    $\begingroup$ I misread the question initially and now note that it is over, not under, bridges. That's weirder. $\endgroup$ – Kevin Reid AG6YO Jul 16 '15 at 1:30
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    $\begingroup$ There could be strong radio noise generated by some system associated with the bridges. Lighting perhaps? Some high-intensity ("high efficiency") lighting emits a lot of VHF noise. (I have this problem near certain buildings in my town.) He would notice the problem more on weaker FM stations. He could check to see if all stations were equally affected. $\endgroup$ – Martin Ewing AA6E Jul 16 '15 at 15:32
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The likely source of this problem is not that the FM signal went away, but that it was overwhelmed by a local interference (RFI) source. Certain electrical systems, like high-intensity lighting, can generate a lot of "white noise" in the VHF range, including the FM broadcast band. It is quite possible that the bridges' lighting system (or some other electrical system) was active when you drove over.

You did not say what time of day was involved. You might notice whether the lights were on. The RFI problem would be worse for weaker FM signals. You might try tuning in other, more local (stronger) FM stations to see if they can get through the radio noise.

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    $\begingroup$ I wrote that there were no street lights, but I should also say that I highly doubt there were lights under the bridges, since it was day time and they were over rivers, not roads. Our FM signal sounded strong otherwise, but if I make the drive again, I'll try different stations. The bridges are a little too short to monitor multiple stations; the white noise lasted only a few seconds. $\endgroup$ – Sam Jul 16 '15 at 16:15
  • $\begingroup$ If the interruption was very brief, you might just have encountered a "null" zone, where direct and reflected signal paths happen to cancel out. This can happen near big reflecting objects, as in downtown "concrete canyons". $\endgroup$ – Martin Ewing AA6E Jul 20 '15 at 21:16
  • $\begingroup$ That was my suspicion, so if it's our best guess, I'll go with it. I know I'm not supposed to make a "Thanks!" comment, but the site wouldn't let me up vote your answer (due to lack of reputation) so... thank you. $\endgroup$ – Sam Jul 21 '15 at 16:26
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You said that you just hear white noise and not man-made interference, so I think the most likely explanation for what you are seeing is that you are in a multipath null.

These occur when a signal arrives at your receiver via a direct path and a dominant reflected path of almost the same amplitude. When the total distance travelled by the reflected wave is a multiple of one-half wavelength different from the distance travelled by the direct path, the reflected wave will be 180 degrees out of phase and will cancel the direct path. It is possible that the reflection is coming from a body of water, which would explain why this is occurring only on a bridge.

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Faraday cage example. Without an external antenna to pick up the modulation you will not have reception. Another good example is cell phone reception in a home with a metal roof.

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It might have to do with the actual construction of the bridge itself.The spacing of the rebar in the concrete if is less than a wavelength it could very efficiently kill signal. The weird thing with VHF/UHF signal reception is sometimes if you are X distance away from a metal object zero signal and then if you move 6" inches left or right of X distance the result is great signal.

A similar effect is noticed in bridges with metal superstructures, FM broadcasts are passed no problem but AM broadcast transmission are blocked.

FM broadcast band wavelength is about 3 meters in length whereas AM broadcast band the wavelengths range from 175 meters to 560 meters in length. I suspect this the cause of your reception issue.

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