Wednesday, January 02, 2008

Radio in Tunnels

You drive through a tunnel, you lose your station. With AM signals the effect is much more pronounced, but an FM can blink out as well depending on tunnel length and location. Some press in 2005 claimed that IBOC for AM would solve "through tunnel fade-out." It won't. To be fair, I should also state that satellite radio fades out too. Physics is a bummer.
Think of water, a wave of water. When moving water encounters an obstacle, it continues to move straight ahead, but it also bends. Radio waves bend a little into the tunnel. This is why it fades out instead of blinking off. In addition to that the demodulation of the signal may become impossible as is weakens.

In an FM signal silence usually means pristine reception but a carrier wave with no data. In absence of an FM signal, indigenous RF noise is demodulated and we call this wonderful noise static. So in the tunnel when you hear silence that's because your radio can detect "no signal" unambiguously and it's AGC (automatic gain control) can clamp down the volume automatically to spare you the noise. These radios "lock" on a signal and adjust volume to improve the listening experience. But the amplifier itself generates some noise so actual silence is difficult to achieve.

In the 1970s a lot of study went into this arena. Coal companies found themselves forced to add safety features to mines to protect the lives of miners. Then President Jimmy Carter signed a set of mine safety laws spurring the U.S. bureau of mines crack down. They got busy making standards, requirements for permits, gear and communications. More here. Transmitting UHF waves in coal mines was difficult despite the fact that the waves were smaller than the tunnel dimensions. Any part of the wave that impinges on the surfaces of the tunnel is refracted back away from the wave guide and represents a power loss.

It's easier to fix the tunnel than the radio. Any short tunnel can be wired for AM and or FM broadcast by feeding a broad-band amplifier into a leaky transmission line. The leaky transmission line will need to be run into or even through the tunnel depending on it's length.

A leaky transmission line is just a single length of wire or flashing running near the ceiling. In this scenario the ceiling and the line are both conductors in an electric circuit. But concrete and rock are conductors. It's an inefficient configuration but simple in design and easy to execute. The "broad-band amplifier" is more complicated than it sounds. Amplifying all frequencies is very difficult. in application we use an amplifier in as narrow a range as possible. Typically separate amplifiers for AM FM and cell. In sum, the problem is not unsolvable, just not cost effective to correct, so it is likely that things will proceed as is.