Antennas with the best conical horns
The measured radiation characteristics of conical horns using waveguide excitation are discussed in this paper. Gray and Schelkunoff’s theoretical finding are in excellent agreement with the experimentally induced gains. For conical horns of arbitrary proportions, the gain and effective area are given, and the radiation patterns are included for horns of optimum design. All dimensional data has been normalised in terms of wavelength and is displayed in a nomographic format for ease of use.
There is a flare angle that gives minimum reflection and maximum gain for a given frequency and horn length. Internal reflections in straight-sided horns originate at two points along the wave path where the impedance suddenly changes: the horn’s mouth or aperture, and the throat, where the sides begin to flare out. The amount of reflection at these two positions varies depending on the horn’s flare angle and the angle the sides create with the axis. The bulk of reflection occurs at the mouth of the horn in narrow horns with slight flare angles. The antenna’s gain is poor due to the small mouth’s similarity to an open-ended waveguide. The reflection at the mouth decreases rapidly as the angle is raised, and the antenna gain increases. In big horns with flare angles reaching 90 degrees, however, the majority of the reflection occurs at the throat. Since the throat resembles an open-ended waveguide, the horn’s gain is low once more. The amount of reflection at this location decreases as the angle decreases, and the horns increase again. This discussion illustrates that there is a flare angle between 0° and 90° that offers the most benefit and the least reflection. This is referred to as the “ideal horn.” The majority of functional horn antennas are optimised horns. The dimensions of a pyramidal horn that give an ideal horn are: an E = 2 L E a H = 3 L H displaystyle a E=sqrt 2lambda L Eqquad a H=sqrt 3lambda L Eqquad The dimensions of an ideal conical horn are: d = 3 d = 3 d = 3 d = 3 d = 3 d = 3 d = 3 d = 3 d = 3 d = 3 d = 3 d L d=sqrt 3lambda L d=sqrt 3lambda L d=sqrt 3lambda L d=sqrt 3lambda L where E is the aperture width in the E-field direction a H is the aperture width in the H-field direction. L E is the side’s slant length in the E-field direction. In the H-field direction, L H is the slant length of the foot. The diameter of the cylindrical horn aperture is denoted by d. L is the cone’s slant length from the apex. is the wavelength of light The maximum benefit for a given aperture size is not reached by an ideal horn. This is done by using a long, aperture-limited horn. The best horn produces the most benefit for a given horn length. Microwave handbooks contain tables that display the measurements of the best horns for different frequencies.
Since horns have very little loss, their directivity is approximately equal to their gain. G = 4 A 2 e A displaystyle G=frac 4pi Alambda 2e A The gain G of a pyramidal horn antenna is the ratio of the radiated power intensity along its beam axis to the intensity of an isotropic antenna with the same input power. The gain for conical horns is G = d 2 e A displaystyle G=leftfrac pi dlambda right2e A where A is the aperture area and d is the conical horn’s aperture diameter is the wavelength, e A is the aperture efficiency, which is a dimensionless parameter between 0 and 1, and In practical horn antennas, the aperture efficiency varies from 0.4 to 0.8. e A = 0.511 for optimal pyramidal horns, and e A = 0.522 for optimal conical horns. As a consequence, a figure of 0.5 is commonly used. The aperture efficiency increases with the length of the horn and is roughly unity for aperture-limited horns.
A Hogg-horn, or horn-reflector antenna, is a type of antenna that combines a horn with a parabolic reflector. It was invented by Alfred C. Beck and Harald T. Friis in 1941 and further developed by David C. Hogg at Bell labs in 1961. Owing to its distinctive shape, it is also known as the “sugar scoop.” It consists of a horn antenna with a reflector positioned at a 45 degree angle in the mouth of the horn such that the radiated beam is perpendicular to the horn axis. The reflector is a part of a parabolic reflector, and the reflector’s target is at the apex of the horn, making the system similar to an off-axis fed parabolic antenna. The horn protects the antenna from radiation coming from angles outside the main beam axis, resulting in a radiation pattern of very small sidelobes. This design has an advantage over a regular parabolic antenna in that it shields the antenna from radiation coming from angles outside the main beam axis. Furthermore, unlike ordinary front-fed parabolic dishes, the aperture is not partially obstructed by the feed and its supports, allowing it to achieve aperture efficiencies of 70% versus 55–60% for front-fed dishes. The downside is that it is much larger and heavier than a parabolic dish for a given aperture area, and it must be placed on a large turntable to be completely steerable. During the 1960s, this design was used for a few radio telescopes and communication satellite ground antennas. The AT&T Long Lines microwave network used it mainly as fixed antennas for microwave relay connections. Since the 1970s, shrouded parabolic dish antennas have largely replaced this design, which can achieve similar sidelobe efficiency while being lighter and more compact. The Holmdel Horn Antenna at Bell Labs in Holmdel, New Jersey, is perhaps the most photographed and well-known example. It was with this antenna that Arno Penzias and Robert Wilson discovered cosmic microwave background radiation in 1965, for which they received the Nobel Prize in Physics in 1978. The cass-horn, which combines a horn with a cassegrain parabolic antenna using two reflectors, is a more recent horn-reflector style.
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