Parabolic antenna
A high-gain dish antenna that focuses radio waves into a narrow beam.
A parabolic antenna uses a curved reflector shaped like a parabola to focus or collect radio waves. Most people know it as a dish antenna. It works much like the reflector in a flashlight or searchlight, sending radio waves into a tight beam or picking up waves coming from a single direction. Its biggest strength is high directivity, and it offers some of the highest gain of any antenna type, which means it can produce extremely narrow beams.
To get those narrow beams, the reflector must be much larger than the radio waves’ wavelength. That’s why parabolic antennas are used at higher frequencies—UHF and microwave (SHF)—where wavelengths are small enough that a reasonably sized dish works. They serve as high-gain antennas for point-to-point links, such as microwave relay towers carrying phone and TV signals between cities, wireless WAN/LAN data links, satellite communications, and spacecraft antennas. Radio telescopes also rely on them. Another major use is radar, where a narrow beam helps locate ships, planes, and missiles, as well as for weather detection. Home satellite TV receivers have made dish antennas a common sight in modern countries.
German physicist Heinrich Hertz invented the parabolic antenna in 1887 while discovering radio waves. In his experiments, he used cylindrical parabolic reflectors with spark-excited dipole antennas at their foci for both transmitting and receiving.
**Design**
The basic principle: a point source of radio waves placed at the focal point in front of a conductive parabolic reflector will bounce off into a collimated plane wave beam along the reflector’s axis. Conversely, an incoming plane wave parallel to that axis will be focused to a point at the focus. A typical setup has a metal parabolic reflector with a small feed antenna suspended at its focus, aimed back at the dish. The reflector is a metallic surface shaped as a paraboloid of revolution, usually cut off at a circular rim that sets the antenna’s diameter. For transmitting, current from a transmitter travels through a cable to the feed antenna, which converts it into radio waves. Those waves hit the dish and reflect into a parallel beam. For receiving, incoming waves bounce off the dish and concentrate at the feed antenna, which turns them into electric currents sent to the receiver.
- Inventor
- Heinrich Hertz
- Year of invention
- 1887
- Frequency range
- UHF and microwave (SHF) frequencies
- Typical gain
- some of the highest gains of any antenna type
Lore & Background
The parabolic antenna was invented by German physicist Heinrich Hertz during his discovery of radio waves in 1887. He used cylindrical parabolic reflectors with spark-excited dipole antennas at their foci for both transmitting and receiving during his historic experiments. The operating principle is that a point source of radio waves at the focal point in front of a paraboloidal reflector of conductive material will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point. A typical parabolic antenna consists of a metal parabolic reflector with a small feed antenna suspended in front of the reflector at its focus, pointed back toward the reflector. The reflector can be constructed from sheet metal, a metal screen, or a wire grill, and can be either a circular dish or various other shapes to create different beam shapes. To achieve maximum gain, the shape of the dish needs to be accurate within a small fraction of a wavelength, around one sixteenth wavelength. Large dishes often require a supporting truss structure behind them to provide the required stiffness.
Reader's Guide
Parabolic antennas are used as high-gain antennas for point-to-point communications, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite communications, and spacecraft communication antennas. They are also used in radio telescopes. The other large use of parabolic antennas is for radar antennas, which need to transmit a narrow beam of radio waves to locate objects like ships, airplanes, and guided missiles. They are also often used for weather detection. With the advent of home satellite television receivers, parabolic antennas have become a common feature of the landscapes of modern countries. An advantage of parabolic antennas is that most of the structure of the antenna (all of it except the feed antenna) is nonresonant, so it can function over a wide range of frequencies. All that is necessary to change the frequency of operation is to replace the feed antenna with one that operates at the desired frequency. Some parabolic antennas transmit or receive at multiple frequencies by having several feed antennas mounted at the focal point, close together. Parabolic antennas are distinguished by their shapes, including paraboloidal or dish, shrouded dish, cylindrical, and shaped-beam antennas. They are also classified by the type of feed, such as axial, prime focus, or front feed.
Did You Know?
- A metal screen reflects radio waves as effectively as a solid metal surface if its holes are smaller than one-tenth of a wavelength.
- A shiny metal parabolic reflector can focus the sun's rays, so solid reflectors are always given a coat of flat paint.
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