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In chapter 4 of the text we will see that the gross

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In chapter 4 of the text, we will see that the gross performance of an antenna is different far away (in the so-called Fraunhofer zone) than it is up close (in the Fresnel zone). The separatrix between these zones is determined by the physical size of the antenna and by the wavelength. In most radar applications, the object being observed is safely in the Fraunhofer zone. This simplifies the interpretation of the echoes that are received considerably. Below, the Fraunhofer- zone approximation is assumed to hold. Fresnel-zone effects are discussed in chapter 4, but a comprehensive treatment of the effects on the radar equation is beyond the scope of this text. Radiation pattern What separates one antenna from another is the distribution of power radiated into different directions or bearings. There is no such thing as an antenna which radiates isotropically (equally in all directions). The radiation pattern of an antenna is a diagram (usually in polar coordinates) showing where the RF power is directed. It represents a surface, or sometimes planar cuts through a surface, with a radial distance proportional to radiated power density along the given bearing. By convention, radiation patterns are normalized to a maximum value of unity. They are usually drawn with linear scaling, although logarithmic scaling can be used if care is taken to avoid problems at the nulls of the pattern. We will soon see that the radiation pattern for an elemental dipole antenna is given by P r sin 2 θ , where θ is the polar angle if the antenna is aligned with the ˆ z axis. This pattern forms a torus in three dimensions. Cuts through two representative planes like those shown below usually suffice in conveying an impression of the three-dimensional shape. If not, 3D rendering software may help with visualization. Bulbs in the radiation pattern are referred to as beams or lobes, whereas zeros are called nulls. Some lobes have larger peak amplitudes than others. The lobes with the greatest amplitudes are called the main lobes, and smaller 10
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z x y θ φ Figure 1.3: Elemental dipole radiation pattern. (left) E-plane. (right) H-plane. The overall pattern is a torus. The dipole is aligned parallel to the z axis. (A dipole does not radiate off its ends, which we shall see later.) ones are called sidelobes. If there are several main lobes, they are referred to as grating lobes. The nomenclature can be ambiguous and subjective. In this example, there is essentially a single lobe at the equator ( θ = π/ 2 ) that wraps around all azimuths φ . Several measures of the width of the lobes in the patterns are in common use. These include the half-power full beamwidth (HPFB or HPBW), which gives the range of angles through which the radiation pattern is at least half its maximum value in some plane. Engineers often refer to the E- and H-planes, which are orthogonal planes tangential to the electric and magnetic lines of force, respectively. In this example, the E- and H-planes are shown at left and right, and the HPFB in the E-plane is 90 . Since the radiation is uniform in the H-plane here, the HPBW is not defined
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