I noticed the FM reception as I go through a tunnel here varies some. It seems to get worse in proportion to the number of vehicles transiting through. I would have thought the signal might improve some from reflections, rather than decrease from absorption. This also isn’t quite a precise relationship, as the traffic density is greatest at rush hours, when the time of day, relative humidity and other factors might have more of an effect. Any antenna gurus out there with a thought on this ?
This is just a rough guess, but there are several factors that would interfere with reception in a tunnel. The biggest is probably the fact that each car in that tunnel is absorbing some of the wave from your preferred station. Also, you have to remember that cars emit a good deal of random energy in all kinds of frequencies as well. So much that most decent CB’s contain an RF filter not only for the power supply in the car (i.e., the cig lighter), but also for the antenna. Most people assume that the antenna filter is there to filter out radio stations, but their main goal is to filter out random emissions from nearby vehicles (mainly the explosions inside your engine that vibrate all sorts of metal in your car, causing all sorts of frequencies to be emitted).
As an example, pick your favorite CB channel on a cheapy set. Figure out where one of the broadcasters is at (usually a freeway near you), then have your friend park his car a couple feet from you, and rev the engine. Try it with his car off as well, and compare the two. (The effect is more noticeable if the broadcaster is farther away.) To see the metal absorbtion effect, take any receiver (radio, cell phone, etc.) and wrap a piece of steel around it. You’ll still get the station, but you’ll also notice a lot of static.
It has more to do with being inside a conducting cage (which blocks the radio waves) than anything else. The presence of RF emissions from the other cars is reasonable. Probably what you’re experiencing is a low signal-to-noise ratio – you’re radio filters out the noise before boosting the signal. If there’s a lot of noise or a weak signal then you will get crummy sound. FM is supposed to be relatively immune to this – it’s more likely to fade than to get staticky.
The main culprit is that the tunnel blocks out the radio waves and leaves you with a weak signal, susceptible to noise sources (such as the other cars) that otherwise are too weak to matter.
The cars aren’t absorbing an appreciable amount of the radio energy. That’s kind of like saying you can’t hear well when there are lots of people in the room because their ears are absorbing all the sound.
Or perhaps your radio filters out the noise.
There are a few road tunnels in the UK,for example the Dartford Tunnel under the Thames, where you can enjoy perfect FM reception.This is because a cable is installed ,called a “Leaky Feeder”, which re-radiates FM signals from outside.A similar system is to be installed on the London Underground rail system but in this case it is so that emergency 2-way and cell-phones can be used all over the network.
It is because a tunnel creates a Faraday cage.