Beam tilt
As I understand it, you tune an antenna array (typically a vertical colineanar of some type) to a higher frequency. When you feed it a frequency below it's design signal, the phasing of the elements causes the main lobe to tilt down (I may have this backwards, ie lower tune for down tilt). The effect helps fill in low spots, or improve coverage closer to a very high antenna. Tilt of 4-5 degrees are common. You don't see this much below UHF though. If the coverage area is on one side of the antenna, then a simple tilting of the antenna accomplishes the same thing
I'm sure that I'll be corrected if my memory is off....
tom