Antennas are an essential part of radio telecommunications equipment, bridging the gap between electronic and electromagnetic signals. The shape and size of an antenna is a strong clue as to its type, as the design dictates the antenna’s purpose. The antenna’s length, for example, corresponds to the length of the radio waves the antenna receives or transmits. The shape affects whether it receives radio waves from different directions or a single direction.
Monopole and Dipole
A Dipole antenna consists of two long pieces of wire or metal tubing, forming a straight line but not connected to each other, feeding into a pair of wires leading to a radio receiver. A monopole resembles a dipole but substitutes a thin, flat conductor called a ground plane for one of the dipole’s pieces. The ground plane is at right angles to the rest of the antenna. Mobile radios, such as for vehicles, use trunk- or roof-mounted monopole or dipole antennas. You also see monopole antennas on portable phones, pocket radios and walkie-talkies.
Building on the principles of the dipole antenna, the Yagi has several pairs of metal tubing elements laid parallel to each other on another long tube that serves as a backbone. One pair of elements functions as a traditional dipole antenna; the others reinforce the incoming radio signal, boosting its strength. Engineers select the lengths of each element and their relative spacing to produce the best radio sensitivity, in effect tuning the antenna to desired wavelengths. The Yagi is the familiar TV antenna you see on the rooftops of homes. It has more directional sensitivity than a dipole, so you aim it in the direction of the radio source.
Satellite TV receivers, radar installations and radio telescopes use parabolic antennas for their high gain. The curved parabolic surface of the antenna dish gathers and focuses radio waves into a small feed antenna, which converts them into electronic signals. The larger the surface of the parabola, the greater the antenna’s sensitivity. The largest parabolic antenna is the Arecibo Radio Telescope, located in Arecibo, Puerto Rico. It measures 1,001 feet across.
A microwave horn antenna is an open-ended metal box with sides that flare outward. The microwave source or detector sits at the back of the box on the inside. For a transmitter, the flared sides prevent microwave energy from reflecting back into the antenna. For a receiver, the flaring collects and focuses microwave energy inside the antenna. Police radar guns, microwave telecommunications gear and garage door openers use horn antennas.