I ran across this intriguing article about a passive TV repeater. The construction is extremely simple: connect two ordinary television antennas (pointed in opposite directions) with a length of coaxial cable. (I once saw a fairly complex installation just like this, although I didn’t realize what it was at the time.) The idea behind it is equally simple: the signal enters one antenna, travels along the coaxial cable, and then is retransmitted through the second antenna. The purpose of a passive repeater is to extend the reach of a signal beyond a line of sight obstruction. Unlike a powered repeater, the signal is rebroadcast on the same frequency.
Apparently a passive repeater can even use a small amplifier without requiring a FCC license, as long as it doesn’t exceed the power limits.