. Airborne radar. Airplanes; Guided missiles. 516 ANTENNAS AND RF COMPONENTS c Typical Ray Central Axis " Antenna Feed Point and Focus of Parabola. Antenna Aperture Plane Fig. 10-2 Properties of a Parabolic Re- flector, Showing a Typical Ray with All Path Lengths abc Equal and the Path be Parallel to the Central Axis. The Aperture. With a paraboloidal reflector, it is convenient to project energy from the end of a waveguide, which is small compared with the final aperture size required. The feed, being small, is effectively a point source of radiation which is placed near the focus of the
. Airborne radar. Airplanes; Guided missiles. 516 ANTENNAS AND RF COMPONENTS c Typical Ray Central Axis " Antenna Feed Point and Focus of Parabola. Antenna Aperture Plane Fig. 10-2 Properties of a Parabolic Re- flector, Showing a Typical Ray with All Path Lengths abc Equal and the Path be Parallel to the Central Axis. The Aperture. With a paraboloidal reflector, it is convenient to project energy from the end of a waveguide, which is small compared with the final aperture size required. The feed, being small, is effectively a point source of radiation which is placed near the focus of the parabola (dish) and with its peak radiation aimed toward the center of the dish. The final transmitting aperture can be considered as the plane containing the outer diameter of the reflector. The use of such a reflector is based on two independent and distinct properties of the parabola. First, all rays striking the dish surface from the focus reflect from the dish parallel to its central axis. Second, all ray paths from the focus to the dish to the aperture plane are of the same length. These are shown in Fig. 10-2, where lengths abc for all paths are equal and be is parallel to the central axis. Thus, a wave front on all ray paths from a point source, at the final crossing of the aperture, are automatically of equal phase. Properties of a Parabolic Reflector. The relative phase and power density of the wave at various points in the aperture plane is called the aperture illumination. The constant-phase characteristic provided by the parabola is quite universally desirable. With constant phase, it can be shown that maximum directivity, is achieved by constant amplitude The amplitude characteristic of the aperture, however, is usually controlled in a prescribed manner by the beamwidth of the feed. If the illumination were so adjusted that constant illumination were achieved, the directivity would be maximized and the antenna physical aperture would be equal to th
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