of oscillating voltage in the rods. The antenna can have a smaller or larger number of rod elements; in general, the more elements the higher the gain and the more directional. Another design, used mainly for UHF reception, is the reflective array antenna, consisting of a vertical metal screen with multiple dipole elements mounted in front of it. The television broadcast bands are too wide in frequency to be covered by a single antenna, so either separate antennas are used for the VHF and UHF bands, or a combination (combo) VHF/UHF antenna. A VHF/UHF antenna is really two antennas feeding the same feedline mounted on the same support boom. Longer elements which pick up VHF frequencies are located at the "back" of the boom and often function as a log-periodic antenna. Shorter elements which receive the UHF stations are located at the "front" of the boom and often function as a Yagi antenna. Since directional antennas must be pointed at the transmitting antenna, this is a problem when the television stations to be received are located in different directions. In this case, two or more directional rooftop antennas each pointed at a different transmitter are often mounted on the same mast and connected to one receiver; for best performance filter or matching circuits are used to keep each antenna from degrading the performance of the others connected to the same transmission line. An alternative is to use a single antenna mounted on a rotator, a remote servo system that rotates the antenna to a new direction when a dial next to the television is turned. Sometimes television transmitters are deliberately located such that receivers in a given region need only receive transmissions in a relatively narrow band of the full UHF television spectrum and from the same direction hence allowing the use of a higher gain grouped aerial. Installation Antennas are commonly placed on rooftops, and sometimes in attics. Placing an antenna indoors significantly attenuates the level of the available signal. Directional antennas must be pointed at the transmitter they are receiving; in most cases great accuracy is not needed. In a given region it is sometimes arranged that all television transmitters are located in roughly the same direction and use frequencies spaced closely enough that a single antenna suffices for all. A single transmitter location may transmit signals for several channels. CABD (communal antenna broadcast distribution) is a system