Brad Hudack, age 14, of Gadsden, Ala., for his question:
HOW DOES A TELEVISION ANTENNA WORK?
An antenna or aerial is the structure used to transmit and to receive television, radio and radar signals. The size, shape and complexity of the antenna assembly depends on the frequency band used and the directional properties to be achieved.
Television antennas must be placed so that they can pick up high frequency waves. Waves used to transmit TV broadcasts travel only in straight lines. For this reason, they do not travel past the horizon.
Good television reception requires an unobstructed path between the transmitter and the receiver. In large cities, tall buildings sometimes interfere with TV reception.
Many kinds of antennas may be used for television reception. The most widely used type is the dipole antenna. A dipole antenna consists of a metal rod or wire that is split in the center. The two sections are one to two inches apart.
Dipoles range from about four to 50 inches in length. Most TV antennas have two dipoles, one short and one that is long. The two are used to receive the various frequencies.
A common form of indoor television antenna is called rabbit ears. This antenna is placed on the TV set and may be moved and rotated easily.
A folded dipole is somewhat like a hairpin, the long ends of which are connected together. One of the straight sections is split at the center, like an ordinary dipole. The overall length is about the same for the ordinary dipole as for the folded dipole.
Often a reflector, a straight metal rod, is mounted a short distance behind the dipole. If the length and spacing of the reflector are proper, the signal is stronger than it would be for the dipole alone.
For commercial broadcasting, transmitter towers may vary in height from 250 to 500 feet. Often a group of towers provides a directional array that transmits greater power.
Radio receiving antennas often consist of a wire suspended between supports outdoors. For best reception, the antenna should have the same orientation in space as the transmitting antenna.
To provide good reception from both vertical and horizontal transmitting antennas, receiving antennas have both horizontal and vertical sections.
Some receiving antennas consist of spirally wound wire inside the radio set. Such loop antennas receive a stronger signal when the loop points toward the broadcasting station then when it is at right angles to the station.
Loop antennas are used in many direction finding devices.
Radar waves are of higher frequency than television waves. The antenna on a radar set is short, but the reflector is large.