[reboot]

It's a matter of frequency. A short wave will travel longer, and through stuff easier than a long wave. For example, a 5w output at 27mhz, can go about 30 miles. You need a ton more power to go any further, and it's "line of sight". A 500mhz signal at 5w will go much farther, and penetrate buildings and such much easier.
I would assume they're using a low wattage transmitter, but at a high frequency, the same rule applies, it must be line of sight, basically, NOT over the horizon.
Think of your car radio. The AM band is low freq, Amplitude Modulated, and doesn't go very far, the FM band is much higher freq, and at the same power, travels much farther, but still line of sight. Now think of a cordless phone (not cell), which runs at VERY low power, but goes through walls with ease, then think of a microwave oven, which uses reasonably low power, but at an extremely high frequency, and it goes right through just about everything except the metal shielding.
Clear as mud?