Parabolic antenna

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main advantage of a parabolic antenna is that it is highly directive; it functions similarly to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, that is they can produce the narrowest beam width angles, of any antenna type.[1] In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave (SHF) frequencies, at which wavelengths are small enough that conveniently sized dishes can be used.

Parabolic antennas are used as high-gain antennas for point-to-point communication, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite and spacecraft communication antennas, and radio telescopes. Their other large use is in radar antennas, which need to emit a narrow beam of radio waves to locate objects like ships and airplanes. With the advent of home satellite television dishes, parabolic antennas have become a ubiquitous feature of the modern landscape.

Contents

Design

The operating principle of a parabolic antenna 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 is a metallic surface formed into a paraboloid of revolution and usually truncated in a circular rim that forms the diameter of the antenna. In a transmitting antenna, radio frequency current from a transmitter is supplied through a transmission line cable to the feed antenna, which converts it into radio waves. The radio waves are emitted back toward the dish by the feed antenna and reflect off the dish into a parallel beam. In a receiving antenna the incoming radio waves bounce off the dish and are focussed to a point at the feed antenna, which converts them to electric currents which travel through a transmission line to the receiver.

Parabolic reflector

The reflector can be of sheet metal, metal screen, or wire grill construction, and it can be either a circular "dish" or various other shapes to create different beam shapes. A metal screen reflects radio waves as well as a solid metal surface as long as the holes are smaller than 1/10 of a wavelength, so screen reflectors are often used to reduce weight and wind loads on the dish. To achieve the maximum gain, it is necessary that the shape of the dish be accurate within a small fraction of a wavelength, to ensure the waves from different parts of the antenna arrive at the focus in phase. Large dishes often require a supporting truss structure behind them to provide the required stiffness.

A reflector made of a grill of parallel wires or bars oriented in one direction acts as a polarizing filter as well as a reflector. It only reflects linearly polarized radio waves, with the electric field parallel to the grill elements. This type is often used in radar antennas. Combined with a linearly polarized feed horn, it helps filter out noise in the receiver and reduces false returns.

Feed antenna

The feed antenna at the reflector's focus is typically a low-gain type such as a half-wave dipole or more often a small horn antenna called a feed horn. In more complex designs, such as the Cassegrain and Gregorian, a secondary reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point. The feed antenna is connected to the associated radio-frequency (RF) transmitting or receiving equipment by means of a coaxial cable transmission line or waveguide.

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, that is a wide bandwidth. All that is necessary to change the frequency of operation is to replace the feed antenna with one that works at the new frequency. Some parabolic antennas transmit or receive at multiple frequencies by having several feed antennas mounted at the focal point, close together.

Dish parabolic antennas

Shrouded microwave relay dishes on a communications tower in Australia.
A satellite television dish, an example of an offset fed dish.
Cassegrain satellite communication antenna in Sweden.
Offset Gregorian antenna used in the Allen Telescope Array, a radio telescope at the University of California at Berkeley, USA.

Shaped-beam parabolic antennas

Vertical "orange peel" antenna for military altitude measuring radar, Germany.
Early cylindrical parabolic antenna, 1931, Nauen, Germany.
Air traffic control radar antenna, near Hannover, Germany.
ASR-9 Airport surveillance radar antenna.
"Orange peel" antenna for air search radar, Finland.

Types

Parabolic antennas are distinguished by their shapes:

Parabolic antennas are also classified by the type of feed, that is, how the radio waves are supplied to the antenna:[2]

Feed pattern

The radiation pattern of the feed antenna has to be tailored to the shape of the dish, because it has a strong influence on the aperture efficiency, which determines the antenna gain (see Gain section below). Radiation from the feed that falls outside the edge of the dish is called "spillover" and is wasted, reducing the gain and increasing the backlobes, possibly causing interference or (in receiving antennas) increasing susceptibility to ground noise. However, maximum gain is only achieved when the dish is uniformly "illuminated" with a constant field strength to its edges. So the ideal radiation pattern of a feed antenna would be a constant field strength throughout the solid angle of the dish, dropping abruptly to zero at the edges. However, practical feed antennas have radiation patterns that drop off gradually at the edges, so the feed antenna is a compromise between acceptably low spillover and adequate illumination. For most front feed horns, optimum illumination is achieved when the power radiated by the feed horn is 10 dB less at the dish edge than its maximum value at the center of the dish.[4]

History

The idea of using parabolic reflectors for radio antennas was taken from optics, where the power of a parabolic mirror to focus light into a beam has been known since classical antiquity. The designs of some specific types of parabolic antenna, such as the Cassegrain and Gregorian, come from similarly named analogous types of reflecting telescope, which were invented by astronomers during the 15th century.[5]

German physicist Heinrich Hertz constructed the world's first parabolic reflector antenna in 1888. The antenna was a cylindrical parabolic reflector made of zinc sheet metal supported by a wooden frame, and had a spark-gap excited dipole along the focal line. Its aperture was 2 meters high by 1.2 meters wide, with a focal length of 0.12 meters, and was used at an operating frequency of about 450 MHz. With two such antennas, one used for transmitting and the other for receiving, Hertz demonstrated the existence of radio waves which had been predicted by James Clerk Maxwell some 22 years earlier.[6]

Italian radio pioneer Guglielmo Marconi used a parabolic reflector during the 1930s in investigations of UHF transmission from his boat in the Mediterranean.[5] In 1931 a microwave relay link across the English Channel using 10 ft. (3 meter) diameter dishes was demonstrated.[5] In 1937 Grote Reber built the first radio telescope to use a parabolic antenna and did a sky survey with it, one of the events that founded the field of radio astronomy.[5] The development of radar during World War II provided a great impetus to parabolic antenna research, and saw the evolution of shaped-beam antennas, in which the curve of the reflector is different in the vertical and horizontal directions, tailored to produce a beam with a particular shape.[5] During the 1950s dish antennas became widely used in terrestrial microwave relay communication systems.[5]

The first parabolic antenna used for satellite communications was constructed in 1962 at Goonhilly in Cornwall, England, UK to communicate with the Telstar satellite.

Gain

The directive qualities of an antenna are measured by a dimensionless parameter called its gain, which is the ratio of the power received by the antenna from a source along its beam axis to the power received by a hypothetical isotropic antenna. The gain of a parabolic antenna is:[8]

G = \frac{4 \pi A}{\lambda^2}e_A = \frac{\pi^2d^2}{\lambda^2}e_A

where:

It can be seen that, as with any aperture antenna, the larger the aperture is, compared to the wavelength, the higher the gain. The gain increases with the square of the ratio of aperture width to wavelength, so large parabolic antennas, such as those used for spacecraft communication and radio telescopes, can have extremely high gain. Applying the above formula to the 25-meter-diameter antennas used by the VLA and VLBA radio telescopes at a wavelength of 21 cm (1.42 GHz, a common radio astronomy frequency) yields an approximate maximum gain of 140,000 times or about 50 dBi (decibels above the isotropic level).

Aperture efficiency eA is a catchall variable which accounts for various losses that reduce the gain of the antenna from the maximum that could be achieved with the given aperture. The major factors reducing the aperture efficiency in parabolic antennas are:.[9]

For theoretical considerations of mutual interference (at frequencies between 2 and c. 30 GHz - typically in the Fixed Satellite Service) where specific antenna performance has not been defined, a reference antenna based on Recommendation ITU-R S.465 is used to calculate the interference, which will include the likely sidelobes for off-axis effects.

Beamwidth

The angular width of the beam radiated by high-gain antennas is measured by the half-power beam width (HPBW), which is the angular separation between the points on the antenna radiation pattern at which the power drops to one-half (-3 dB) its maximum value. For parabolic antennas, the HPBW θ is given by:[4][10]

\theta = k\lambda / d \,

where k is a factor which varies slightly depending on the shape of the reflector and the feed illumination pattern. For a "typical" parabolic antenna k = 70 when θ is in degrees.[10]

For a typical 2 meter satellite dish operating on C band (4 GHz), like the one shown at right, this formula gives a beamwidth of about 2.6°. For the Arecibo antenna at 2.4 GHz the beamwidth is 0.028°. It can be seen that parabolic antennas can produce very narrow beams, and aiming them can be a problem. Some parabolic dishes are equipped with a boresight so they can be aimed accurately at the other antenna.

It can be seen there is an inverse relation between gain and beam width. By combining the beamwidth equation with the gain equation, the relation is:[10]

G = \left ( \frac{\pi k}{\theta} \right )^2 \epsilon_A

See also

References

  1. ^ Straw, R. Dean, Ed. (2000). The ARRL Antenna Book, 19th Ed.. USA: American Radio Relay League. pp. 19.15. ISBN 0872598179. 
  2. ^ a b c d e f Lehpamer, Harvey (2010). Microwave transmission networks: Planning, Design, and Deployment. USA: McGraw Hill Professional. pp. 268–272. ISBN 0071701222. http://books.google.com/?id=-kiH5WZy88UC&pg=PA263&dq=%22beamwidth%22+%22parabolic+antenna%22#v=onepage&q=%22beamwidth%22%20%22parabolic%20antenna%22&f=false. 
  3. ^ A. David Olver (1994) Microwave Horns and Feeds, p. 61-62
  4. ^ a b Straw, R. Dean, Ed. (2000). The ARRL Antenna Book, 19th Ed.. USA: American Radio Relay League. pp. 18.14. ISBN 0872598179. 
  5. ^ a b c d e f Olver, A. David (1994). Microwave horns and feeds. USA: IET. pp. 3. ISBN 0780311159. http://books.google.com/books?id=soJiuUwevRIC&pg=PA9&dq=how+horns+work+impedance&hl=en&ei=-V7DTpyxEILJiQKS_InyCw&sa=X&oi=book_result&ct=result&resnum=2&sqi=2&ved=0CDIQ6AEwAQ#v=onepage&q=how%20horns%20work%20impedance&f=false. 
  6. ^ Love, Allan W.. "Large Space Antenna Concepts for ESGP" (PDF). Rockwell International. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009937_1990009937.pdf. Retrieved 2009-07-31. 
  7. ^ Drentea, Cornell (2010). Modern Communications Receiver Design and Technology. USA: Artech House. pp. 369. ISBN 1596933097. http://books.google.com/?id=9juUwbKP-58C&pg=PA369&dq=Arecibo+observatory+%22radio+telescope%22+gain#v=onepage&q=Arecibo%20observatory%20%22radio%20telescope%22%20gain&f=false. 
  8. ^ Anderson, Harry R. (2003). Fixed broadband wireless system design. USA: John Wiley & Sons. pp. 206–207. ISBN 0470844388. http://books.google.com/?id=r-o3SmNsvD8C&pg=PA205&dq=parabolic+antenna+design#v=onepage&q=parabolic%20antenna%20design&f=false. 
  9. ^ Pattan, Bruno (1993). Satellite systems: principles and technologies. USA: Springer. pp. 267. ISBN 0442013574. http://books.google.com/?id=0GJWEro9ea4C&pg=PA267&dq=aperture+efficiency#v=onepage&q=horn%20antenna&f=false. 
  10. ^ a b c Minoli, Daniel (2009). Satellite Systems Engineering in an IPv6 Environment. USA: CRC Press. pp. 78. ISBN 1420078682. http://books.google.com/?id=4yJi1UQDPp8C&pg=PA80&dq=%22beamwidth%22+%22parabolic+antenna%22#v=onepage&q=%22beamwidth%22%20%22parabolic%20antenna%22&f=false. 

External links

Antenna types
Isotropic
Omnidirectional
Directional
Application-specific