Parabolic antenna

[1][2] In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used,[2][3] so parabolic antennas are used in the high frequency part of the radio spectrum, [4]: p.302  at UHF and microwave (SHF) frequencies, at which the wavelengths are small enough that conveniently sized reflectors can be used.

[2] With the advent of home satellite television receivers, parabolic antennas have become a common feature of the landscapes of modern countries.

[2] 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 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.

It only reflects linearly polarized radio waves, with the electric field parallel to the grill elements.

Combined with a linearly polarized feed horn, it helps filter out noise in the receiver and reduces false returns.

Since most dishes could concentrate enough solar energy on the feed structure to severely overheat it if they happened to be pointed at the sun, solid reflectors are always given a coat of flat paint.

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.

However, maximum gain is only achieved when the dish is uniformly "illuminated" with a constant field strength to its edges.

Therefore, 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.

In a home satellite dish, these are received by two small monopole antennas in the feed horn, oriented at right angles.

The ability of an antenna to keep these orthogonal channels separate is measured by a parameter called cross polarization discrimination (XPD).

If the antenna system has inadequate XPD, cross polarization interference cancelling (XPIC) digital signal processing algorithms can often be used to decrease crosstalk.

In the Cassegrain and Gregorian antennas, the presence of two reflecting surfaces in the signal path offers additional possibilities for improving performance.

This involves changing the shape of the sub-reflector to direct more signal power to outer areas of the dish, to map the known pattern of the feed into a uniform illumination of the primary, to maximize the gain.

However, this results in a secondary that is no longer precisely hyperbolic (though it is still very close), so the constant phase property is lost.

Applying the above formula to the 25-meter-diameter antennas often used in radio telescope arrays and satellite ground antennas 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 52 dBi (decibels above the isotropic level).

There is also usually a backlobe, in the opposite direction to the main lobe, due to the spillover radiation from the feed antenna that misses the reflector.

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:[8][14] where k is a factor which varies slightly depending on the shape of the reflector and the feed illumination pattern.

The electric field pattern can be found by evaluating the Fraunhofer diffraction integral over the circular aperture.

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.

After World War I when short waves began to be used, interest grew in directional antennas, both to increase range and make radio transmissions more secure from interception.

Italian radio pioneer Guglielmo Marconi used parabolic reflectors during the 1930s in investigations of UHF transmission from his boat in the Mediterranean.

[17] In 1931, a 1.7 GHz microwave relay telephone link across the English Channel was demonstrated using 3.0-meter (10 ft) diameter dishes.

[17] The development of radar during World War II provided a great impetus to parabolic antenna research.

This led to 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.

During the 1960s, dish antennas became widely used in terrestrial microwave relay communication networks, which carried telephone calls and television programs across continents.

[19] The Voyager 1 spacecraft launched in 1977 is currently 24.2 billion kilometers from Earth, the furthest manmade object in space, and it's 3.7 meter S and X-band Cassegrain antenna (see picture above) is still able to communicate with ground stations.

A 28.5 meter parabolic satellite communications antenna at Erdfunkstelle Raisting ( Raisting Earth Station ), Bavaria , Germany , the biggest facility for satellite communication in the world. It has a Cassegrain -type feed, transmits at 6 Ghz and receives at 4 Ghz with a gain of 64.2 dB
Parabolic antennas are based on the geometrical property of the paraboloid that the paths FP 1 Q 1 , FP 2 Q 2 , FP 3 Q 3 are all the same length. Thus, a spherical wavefront emitted by a feed antenna at the dish's focus F will be reflected into an outgoing plane wave L travelling parallel to the dish's axis VF .
Wire grid-type parabolic antenna used for MMDS data link at a frequency of 2.5-2.7 GHz. It is fed by a vertical dipole under the small aluminum reflector on the boom. It radiates vertically polarized microwaves.
Main types of parabolic antenna feeds
Array of multiple feed horns on a German airport surveillance radar antenna to control the elevation angle of the beam
Effect of the feed antenna radiation pattern (small pumpkin-shaped surface) on spillover. Left: with a low gain feed antenna, significant parts of its radiation fall outside the dish. Right: with a higher gain feed, almost all its radiation is emitted within the angle of the dish.
Radiation pattern of a German parabolic antenna. The main lobe (top) is only a few degrees wide. The sidelobes are all at least 20 dB below (1/100 the power density of) the main lobe, and most are 30 dB below (if this pattern was drawn with linear power levels instead of logarithmic dB levels, all lobes other than the main lobe would be much too small to see).
The angle theta is normal to the aperture.